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Title: Our Nuclear Future
Facts, Dangers and Opportunities
Author: Edward Teller
Albert L. Latter
Release Date: September 27, 2021 [eBook #66408]
Language: English
Produced by: Tim Lindell, Stephen Hutcheson, and the Online Distributed
Proofreading Team at https://www.pgdp.net (This book was
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*** START OF THE PROJECT GUTENBERG EBOOK OUR NUCLEAR FUTURE ***
OUR NUCLEAR FUTURE ...
_FACTS DANGERS AND OPPORTUNITIES_
BY Edward Teller AND Albert L. Latter
CRITERION BOOKS • NEW YORK
Copyright © 1958, by Criterion Books, Inc.
Library of Congress Catalog Card Number 58-8783
_Designed by Sidney Feinberg_
MANUFACTURED IN THE UNITED STATES OF AMERICA
BY AMERICAN BOOK-STRATFORD PRESS, INC., NEW YORK
Preface
This book has been written for the layman who has no knowledge about
atoms, bombs and radioactivity. He knows that the world is made of
atoms, that bombs might destroy it and that radioactivity could make it
a place much less agreeable to live in.
We should like to give some advice about the use of the book: Each
chapter can be read by itself. The chapters need not be taken in the
order in which they are printed. To read them all will give a more
complete understanding—and if you have time it is best to read them in
the order they are arranged. Some of the earlier chapters perhaps
overflow with facts. In some later chapters we wish that more facts were
available. These latter the reader will probably understand and remember
quite easily. He may not agree with all of their contents. On the other
hand the more scientific chapters (II to VIII) will not be questioned
but may be harder to read and to remember. It will be a help to keep
this in mind: No chapter _follows_ from another but most chapters are
related and support some other part of the book.
Our knowledge about fallout is increasing rapidly. Some questions which
are raised in the book may already have been answered. With this added
knowledge we might have been more quantitative in some of our
statements. But we believe the main conclusions would not be altered.
This book was completed before the Sputniks. In their present form these
have little to do with the subject of nuclear energy. However, to our
mind, the urgency has become greater for the non-scientist to understand
those parts of science and technology which may affect his safety and
well-being, and the safety and well-being of his country. We hope that
this book will contribute in some measure to such understanding.
Contents
Preface 5
I. The Need to Know 13
II. Atoms 18
III. Nuclei 26
IV. The Law of Radioactive Decay 37
V. Breakup of the Nucleus 41
VI. Reactions Between Nuclei 49
VII. Fission and the Chain Reaction 58
VIII. Action of Radiation on Matter 68
IX. The Test 80
X. The Radioactive Cloud 87
XI. From the Soil to Man 104
XII. Danger to the Individual 116
XIII. Danger to the Race 127
XIV. The Cobalt Bomb 134
XV. What About Future Tests? 137
XVI. Has Something Happened to the Weather? 146
XVII. Safety of Nuclear Reactors 152
XVIII. By-products of Nuclear Reactors 160
XIX. The Nuclear Age 168
Glossary 175
List of Illustrations
A section of photographs will be found between pages 96 and 97.
1. A shallow underground explosion.
2. An atomic test tower.
3. A tower shot.
4. An air shot.
5. Leg bone of a rabbit after injection of Sr⁸⁹.
6. Leg bone of a woman dead of radium poisoning.
7. Capsules of cobalt⁶⁰.
8. Cobalt irradiation.
9. Smoke-ring cloud from the air-defense atomic weapon.
10. Wilful exposure—an experiment.
11. Condensation trails produced in a Wilson Cloud Chamber.
12. Closely-spaced tracks form a cloud.
13. Cutaway section of a nuclear reactor.
OUR NUCLEAR FUTURE
CHAPTER I
The Need to Know
Our world is changing, and the change is becoming more rapid. The moving
force behind this change is scientific discovery. All of us are deeply
affected by the consequences of science. At the same time, very few
understand the highly technical foundations of our civilization. In this
situation it is natural that scientific and technical progress should
create uneasiness and alarm.
Fear of what we do not know or do not understand has been with us in all
ages. Man, knowing that his life will end, has often been prey to an
even more terrible nightmare—the end of his whole world. In a scientific
age most of the past terrors have turned out to be senseless chimeras.
But one menace remains. It is the great and permanent unknown: what will
we humans do to each other and to ourselves?
The worry about our own actions will continue. It may grow as our power
over nature increases. Against this worry there exist two weapons:
understanding and courage. Of the two, courage is more important but
understanding must come first.
We are frequently alarmed by imaginary dangers, while disregarding risks
which are much more real. There should exist a close interaction between
public opinion on the one hand and technical progress on the other. For
this end an understanding of modern scientific developments is required.
There is an increasingly urgent need to know. Little is done to satisfy
this need. The opinion has gained ground that this need can in fact not
be satisfied.
At the same time, more and more people believe that the scientists and
technical people themselves are responsible for the changes which their
ideas and inventions have brought about. The scientist is put in the
position where his voice is heard, not only in the highly specialized
fields in which he is an expert, but also in the much more general
matters which are affected by his discoveries. The real source of
important decisions in our country is the people. We believe that this
is rightly so, and we believe that it is not proper if scientists take
over any essential part of these decisions.
The responsibility of a technical man certainly includes two important
functions. One is to explore nature and to find out the possible limits
of our power over nature. The other is to explain what he has found in
clear, simple, and straightforward terms, so that essential decisions
can be made by all the people of our country—to whom the power of
decision properly belongs, and whom the consequences of these decisions
will ultimately affect.
To explain scientific and technical matters is not easy, and to become
familiar with all science might actually be impossible. In the
specialized field of physics there have been revolutionary developments
in the twentieth century like the theory of relativity discovered by
Einstein and the theory of the atom originated by Niels Bohr. These new
discoveries are not easy to understand, and every good physicist has
spent years of his life trying to get thoroughly acquainted with their
meaning. All of us who have done so feel that we are well rewarded by
the better understanding of nature which we have acquired. But it is not
necessary to talk of these matters here.
What we have to discuss in this book is connected with parts of atomic
and nuclear physics which are much more elementary. The facts which we
shall present in a simple fashion are sufficient to give the reader an
orientation in the seemingly bewildering fields of nuclear energy and
atomic explosions.
We shall have to start by describing atoms and nuclei. These are rather
small objects, but this circumstance need not particularly bother us;
and it is not necessary to frighten ourselves with the idea that we are
talking about “unimaginably” small objects. Our minds adapt themselves
quite readily to new dimensions; and while we are talking about nuclei,
we can temporarily forget that any bigger objects exist. Real
difficulties arise only when science discovers laws which seem to
contradict common sense. This does not happen frequently, and we shall
not need to dwell upon such subjects.
The difficulties of explaining science are increased by the fact that
scientists have developed a language of their own which they practice
and perfect by talking to each other. One sometimes has the impression
that they talk to each other exclusively. The authors feel that their
own “native tongue” is this scientific language; this book is an effort
at a translation.
A further difficulty is connected with the special subject:
radioactivity. The great practical importance of this subject has dawned
upon the public in connection with the explosion at Hiroshima. This was
a frightening occasion, and the subsequent developments and prospects
are no less frightening. It is not necessary that everything connected
with nuclear explosions should be equally frightening; and it is
important that we should approach the subject with an open mind and with
as few emotions as is humanly possible. The emotions have their
necessary place when we get to the stage in which we want to decide our
actions. We suggest to the reader that he should delay this stage until
the time when he has finished reading the book.
The greatest difficulty in discussing the radiation hazards arises
because the working of living organisms is involved. Basically, we are
in the dark about the question how such an organism works. We are
equally in the dark about the question how such an organism is affected
by radiation. It would therefore seem that we must remain in doubt
whether or not radioactivity is dangerous, except for those cases where
obvious damage has been done. Since the immediate effects of
radioactivity are not perceived by our senses, we are faced with the
thought of an invisible menace of unknown extent. Some of the harmful
consequences may show up years later, and therefore even the absence of
any observed damage will not reassure people.
Fortunately, our practical knowledge is by no means as deficient as
these statements would suggest. Radioactivity, and processes similar to
radioactivity, surround us and have surrounded our ancestors for as long
as life has existed on earth. We do not know what life is, and we do not
know in what detailed manner life is affected by radioactivity; but we
have broadly based and certain knowledge that artificial radioactivity
will produce similar effects to those produced by the natural background
of radioactivity. This background, therefore, provides us with a
yardstick to which all man-made contaminations can be compared.
There is a final obstacle to the explanation of matters connected with
radioactivity. This is the secrecy which has been associated with the
development of nuclear energy, and in particular with the military
applications of nuclear energy. The arguments for keeping information
concerning weapons secret are strong, proper, and generally understood.
There is, however, no such strong argument, and in fact no possibility
for secrecy connected with the widely dispersed radioactivity which
originates from the weapons. In recognition of this fact, secrecy has
been completely and properly removed from this field. It is not
surprising that it took some time to do so. Administrative decisions
have been involved, and these are never taken in a very great hurry.
Even though world-wide radioactive contamination has been since 1955
open to general scientific discussion, the time does not seem to have
been sufficient to insure a wide dissemination and explanation of the
results. There may also remain some lingering doubts whether all
relevant information has been made available. In actual fact, the
scientific information on this important topic is completely and freely
available at the present time.
Information concerning the peaceful applications of nuclear energy is
also completely and freely available. Even in the field of military
applications, much of the essential information has been published.
We are therefore in a position to put before the reader the most
important facts about the peaceful and military applications of nuclear
energy—of the possible dangers and of the eventual benefits. If we do
not succeed, we cannot blame either secrecy or the difficulty of the
subject. It is true that the subject is involved, but only in the same
way as are those subjects of everyday experience with which all of us
have to struggle once in a while. No greater intellectual effort is
needed than is involved in the understanding of the income tax form or
the racing form, to mention two analogies of rather diverse emotional
content. Many of the ideas will be unfamiliar, but they are not complex.
Furthermore, their bearing on our safety, well-being, and the possible
improvement of our lives is great. Therefore we hope that the reader
will give as much of his attention to this matter as he is accustomed to
devote to other subjects which are connected with his necessities or his
amusement.
CHAPTER II
Atoms
All matter is composed of atoms, which are very tiny objects. We cannot
see them because waves of light wash over them like ocean waves over a
pebble. An atom is about as big in comparison to a human cell, which can
be clearly seen under an ordinary microscope, as a human cell is in
comparison to a billiard ball. Somewhat more precisely, a hundred
million atoms laid side by side would be about an inch in length.
Despite its Greek name, which means indivisible, the atom is made up of
parts. It consists of a central nucleus, which carries a positive
electrical charge, around which one or more negatively charged electrons
are distributed. One frequently hears of the electrons revolving in
orbits around the nucleus, somewhat as the planets revolve around the
sun in our own solar system. This is not quite a correct picture,
however. For one thing the electrons are more elusive than the planets.
They do not revolve in definite orbits as the planets do. Also the
orbits are more delicate. One would destroy the atom by the attempt to
find out precisely what the electron orbits are.
[Illustration: This is how an atom does _not_ look. The electrons do
not move along well-defined paths. It is more difficult to convey
the idea of an atom by a picture than it is to make a drawing of
last night’s dream.]
The planets do not fly away from the sun because of the gravitational
attraction which the sun exerts. The electrons and the nucleus, however,
are held together because positive and negative electrical charges
attract each other. The gravitational attraction between the electrons
and the nucleus is incredibly weak compared to the electrical
attraction.
Most of the atom’s weight comes from its nucleus. Even the lightest
known nucleus weighs about 1840 times as much as an electron. In spite
of this, the nucleus occupies only a tiny portion of the total volume of
the atom. In fact, the nucleus is about as big in comparison to the
whole atom as the atom is in comparison to the human cell. Twenty
thousand nuclei laid side by side would be about equal in length to the
diameter of the atom. If matter were composed of nothing but nuclei
densely packed together, an object the size of a penny would weigh
approximately forty million tons.
Later we are going to see that the size of the nucleus has a great
effect upon the ways in which nuclei react with each other. For that
very reason the size of the nucleus is a well-defined measurable
quantity. It is much harder to say precisely what one means by the size
of the electron. It seems acceptable to say that it is somewhat less
than the size of the average nucleus. In any case it is certain that
both the electrons and the nucleus are small compared to the size of the
whole atom. Consequently, the atom must consist mostly of empty space.
This means, of course, that when you look at solid matter, what is
before your eyes is empty space with a slight addition of real
substance. What lends strength to solids is the interplay of electric
attractions and repulsions inside atoms and between atoms.
When a charged particle, such as an electron or a nucleus, happens to
move through solid matter, it is constantly acted on by large electric
forces. To such a particle matter does not seem to be very transparent.
But if there were such a thing as an electrically neutral particle,
comparable in size to the nucleus, it would be able to move around
freely inside matter, without experiencing electric forces, and only now
and again bumping into a nucleus or maybe an electron. As a matter of
fact, there is such a particle and it can pass right through an inch or
two of solid matter without bumping into anything. Later on in this book
we shall be very interested in this particle, which is called a neutron.
Although the electrons and the nucleus are charged particles, the atom
as a whole is electrically neutral; this means that the positive charge
of the nucleus must be equal in magnitude to the total charge of all the
negative electrons. All electrons have precisely the same charge, which
is the smallest charge that has ever been observed. What is particularly
strange and not yet explained is the fact that all other charges are as
big as the electron charge, or twice as big, or three times as big, or a
million times as big. But we never find a charge which, expressed in
terms of the electron charge, is fractional. No object ever carries
three and a half electron charges. The electron charge therefore may be
used conveniently as a standard unit of charge.
Every atom can be distinguished by the charge of its nucleus. The
simplest atom one can imagine would clearly be one with a single
electron revolving around a nucleus having one unit of positive charge.
Such an atom exists and is called hydrogen. An atom with a nucleus of
charge two and two electrons revolving around it, is called helium;
three, lithium ... six, seven, eight; carbon, nitrogen, oxygen ... 92,
uranium. Atoms with almost all charges from one to 92 are found in
nature, and practically none above 92 are found. Some odd charges—43,
61, 85, and 87—are missing. The reason for these missing atoms is
connected with the properties of the nucleus. The nucleus will soon
become our main object of interest.
The most surprising fact about atoms is their similarity, indeed their
identical behavior. If two atoms have the same kind of nucleus and have
the same number of electrons revolving around these nuclei, then these
two atoms are apt to be encountered in a condition which is most
precisely the same for the two. One could imagine that the various
component parts of the atom would be arranged in different ways and
found in different states of motion, in a variety without limit. Whence
the complete similarity? The answer to this question is not only most
surprising, but it is even in apparent contradiction to common sense.
For this very reason it is difficult to explain. The hardest things to
understand are not those which are complicated but those which are
unexpected.
Fortunately for our purpose we need not go into this more intricate
portion of atomic physics. It is sufficient to say that there is one
arrangement or pattern of motion of the electrons which is preferred and
which leads to the greatest stability of the atom. If the electrons are
in this particular state of motion, which is called the ground state,
they have less energy than they would have if they were in any other
state of motion. There are other less stable, but not less sharply
defined, states of atoms which we call “excited” states. When an atom is
in such an excited state, it tends to be unstable and tries to get into
the ground state as soon as possible. Since the ground state contains
less energy than any other state, the atom must release energy in the
process of adjustment. The released energy manifests itself in the form
of electromagnetic radiation—often as a little pulse of visible light.
The color of this light depends upon the amount of energy released,
going progressively through the rainbow from red toward blue as the
amount of energy increases.
There are very few states in which the excitation energy is small. But
of strongly excited states there is a great abundance. In the region of
this high excitation small additional changes are possible. Thus we
approach a situation more in accordance with experience and common
sense: the pattern of motion can be changed by any small amount.
The description we have just given is of course incomplete. We must
avoid here the crucial questions why only some patterns of motion are
possible, why one lowest level is stable and why the electrons never
descend into decreasing states of energy, following the attraction of
the nucleus. At the same time one should emphasize that a complete
explanation of these facts has been given. This explanation makes
precise predictions about many of the properties of matter, and we can
have complete confidence that, but for the involved mathematical
procedure, all ordinary properties of materials could be precisely
predicted. The atom has been explained as completely as Newton has
explained the motion of planets.
To form an idea what an atom is or why two atoms of, let us say,
hydrogen are precisely the same, it is not necessary to search for
intricate reasons or deep meanings. Two atoms of the same kind are alike
as two pawns are for the chess player, except for one little point: in
the case of the pawns we do not _care_ about the difference; in the case
of the atoms there _is_ no difference. This is a simple statement and it
honestly describes a simple situation. The beauty of science is due to
the fact that the correct answers to our most interesting questions have
turned out to be surprising by their simplicity.
In order to understand an atom one must consider the distribution of
electrons around one nucleus. In order to understand a molecule one has
to consider the distribution of electrons around two or more nuclei. The
_chemical_ behavior of an atom is the manner in which it interacts with
other atoms, and that means the precise way in which the electrons
rearrange themselves when two or more atoms approach each other. The
interaction between atoms occurs mainly between their outermost
electrons. It may happen that two quite different atoms, containing
nuclei of different charges and different numbers of electrons, may
nevertheless be similar in the structure of their outermost electrons.
In this case the two atoms exhibit similar chemical properties. Examples
are lithium with charge 3 and sodium with charge 11; also helium, charge
2 and neon, charge 10. A most important example for our purpose is the
set of three chemically similar atoms: calcium, charge 20, strontium,
charge 38; and radium, charge 88.
When two or more atoms approach each other, whether they are similar or
different, their electrons—particularly the outermost ones—find new
states of motion instead of those that were available to them when there
was only one nucleus in the vicinity. It may now happen that amongst
these new states of motion there are some that are even more stable than
the state of the separated atoms. In this event the atoms will tend to
stick together, and the electrons will adopt whatever new state of
motion now corresponds to maximum stability. The composite system of the
atoms is called a molecule, and its state of maximum stability, the
ground state of the molecule.
There are atoms of particularly great stability which cannot increase
their stability by combining with other atoms. Examples are helium,
neon, and argon. These atoms tend to remain single, retain their
independent motion in a rather “permanent” gaseous state, and are
generally unsociable. They are called therefore the noble gases.[1]
An especially simple example of the formation of a molecule is the
combining of sodium and chlorine to form ordinary table salt. The sodium
atom happens to have a rather loosely bound outer electron. The chlorine
atom possesses a convenient niche for an extra electron. Consequently
the energy spent in prying the outer electron loose from the sodium atom
is largely repaid by adding it to the chlorine atom. The remaining
sodium “atom,” deprived of one of its electrons, now has a net positive
charge.[2] The chlorine “atom” with its extra electron has a net
negative charge. The two “atoms” therefore attract each other to make a
molecule of sodium chloride. Actually matter will continue to aggregate.
A great number of positive sodium “atoms” and negative chlorine “atoms”
will arrange themselves into a beautiful and regular lattice which is
the sodium chloride crystal.
The simplest molecule which does not tend to grow into a bigger
aggregate is made up of two hydrogen atoms. Around two hydrogen nuclei a
particularly stable pattern of two electrons can be formed. Because of
this fact hydrogen atoms associate pairwise so that this pattern should
become possible.
The ways in which atoms can be joined are incredibly manifold. They can
form metals in which the outer electrons roam freely and carry electric
currents with the greatest of ease. They can form liquids in which atoms
or molecules are tied together in a loose and disorderly fashion. They
can move independently making occasional encounters, which is what
happens in a gas. And they can form long spiraling molecules where
groups of atoms are strung together without an apparent simple order,
but in a way which is somehow related to the processes of life.
[Illustration: Arrangement of sodium and chlorine “atoms” in a
crystal of common salt.]
We all know in how many forms matter can appear and how changeable these
forms are. That the stone and the spray, the air and an insect, and even
the human brain should be composed of the same few kinds of atoms, and
that these atoms should be subject to laws which are subtle and simple
and precisely described—this certainly is the most remarkable fact that
we have learned since Newton proved that the same science applies to the
earth and in the heavens.
CHAPTER III
Nuclei
Up to now we have regarded atoms as being divisible into electrons and
nuclei. Electrons and nuclei, however, we have regarded as indivisible
entities. This point of view is perfectly adequate to account for all
the facts of chemistry and most of the facts of physics. Even in
physics, it has not been necessary to ascribe an internal structure to
the electron.[3] The electron is a truly elementary particle in this
sense. However, to understand some physical phenomena, and radioactivity
is one of these, it is necessary to recognize that the nucleus is not
indivisible but consists of parts. The parts of the nucleus are called
protons and neutrons.
The simple statements of the previous chapter apply to these smaller
particles also. All electrons are equal—precisely equal. All protons are
equal and all neutrons are equal. There are methods which would have
shown up exceedingly small differences between these particles. No such
differences have been discovered. As far as we know these particles are
_always_ the same. We cannot pour energy into them and excite them as
was the case with the atoms. When we come to consider these small
particles, the complex structure of the world has an end. Instead what
we find is simple.
A proton and a neutron have almost exactly the same weight. The proton
has one unit of positive charge, which means that its charge is the same
as that of the electron except that it is opposite in sign. The neutron,
as its name implies, is an electrically neutral particle. Hence the
charge of the nucleus is equal to the number of protons it contains, and
is independent of the number of neutrons. The weight of the nucleus,
however, taking the proton (or the neutron) as a unit of weight, is
equal to the number of protons _plus_ the number of neutrons.
Imagine that we have two atoms whose nuclei have the same number of
protons but a different number of neutrons. Such atoms exist in nature
and are called isotopes. The point about these isotopes is that since
they have the same number of protons, they have the same nuclear charge,
the same electron structures, and hence they have almost the same
chemical properties. Their nuclei have somewhat different volumes. But
the nucleus is small in any case. It is almost as though we tried to
look for the difference between nothing and twice-nothing. The
difference in the weights of isotopes due to the difference in their
numbers of neutrons, has only a negligible influence on their chemical
behavior. An important consequence of this fact is that molecules which
differ only in that one isotope has been substituted for another are
biologically indistinguishable. They taste the same and smell the same.
They are ingested in our bodies in the same way, and they are deposited
or excreted in the same way.
The simplest isotopes are the isotopes of hydrogen. Most of the hydrogen
atoms we find in nature have a nucleus which is a single proton. This is
the common hydrogen or light hydrogen. A few hydrogen atoms, however,
have nuclei which consist of a proton and a neutron. This is the heavy
hydrogen, found in heavy water. In all natural sources of water these
two kinds of hydrogen are mixed in a ratio which is practically the same
for every sample. The electron circulating around the nucleus behaves
almost exactly the same way whether the extra neutron is present or not.
On the state of that electron depend most properties of the atom and the
molecules which contain it. Of course, heavy hydrogen has twice the
weight of common hydrogen, and heavy water is somewhat more dense than
light water. But otherwise there is little difference.
The story of the discovery of the hydrogen isotopes is amusing. About
half a century ago—before the discovery of any isotope—two scientists
tried to measure the density of water. They purified the water by
boiling it and condensing the vapor. But the more they purified, the
lighter it became—slightly but perceptibly. Finally they gave up: water
seemed to have no density!
What really happened was this: Light water boils a little bit more
easily than heavy water. Without realizing it, these scientists had
started to separate isotopes.
Many years later Harold Urey—on the basis of some mistaken experiments
of other people—concluded that heavy hydrogen must exist. He looked for
it and found it, but found much less than he had expected. There was so
little heavy hydrogen that on the basis of correct experiments Urey
never would have guessed its presence. It seems that an unfounded idea
is much more fruitful than the absence of an idea.
Almost all naturally occurring elements are found to consist of more
than one isotope. Uranium, for example, is composed mainly of two, one
having 143 neutrons and the other having 146. Since both of these
isotopes have 92 protons, their weights are 92 + 143 = 235 and 92 + 146
= 238 respectively. It is customary to refer to these isotopes as U²³⁵
and U²³⁸. The U²³⁵, which is valuable in atomic reactors and in the
manufacture of atomic bombs, is comparatively rare, occurring as only
one part in 140 of natural uranium. The separation of this rare isotope
from the common 238 was one of the major undertakings of the two billion
dollar Manhattan Project during World War II.
We come now to a most important question, one that will lead us to the
idea of radioactivity: What is it that determines which isotopes a given
element will have? For example, uranium has isotopes weighing 235 and
238. Small amounts of U²³⁴ and U²³⁶ are also found in nature. Why do we
not find U²³², U²³³, U²³⁷ or U²³⁹? Evidently only certain numbers of
neutrons will hang together with 92 protons.
Consider another example, this time of the lightest known element,
hydrogen. We have already mentioned two isotopes of hydrogen: light
hydrogen with weight 1 (symbolized H¹), having a nucleus consisting of a
single proton and no neutrons, and heavy hydrogen (also called
deuterium) of weight 2 (H²), having one proton and one neutron. The
latter isotope occurs as only about one part in 5,000 of natural
hydrogen. There is also a slight trace of tritium (H³), having one
proton and two neutrons. But here the sequence stops. What has happened
to H⁴, H⁵, H⁶, etc?
This question is related to the earlier one: why there are no atoms in
nature of charge 43, 61, 85, and 87, and why there are none with charges
greater than 92. To answer these questions requires a little knowledge
about the laws which govern the motion of neutrons and protons within
the nucleus, and the nature of the forces which are exerted by a neutron
on a neutron, a neutron on a proton, and a proton on a proton.
The motion of neutrons and protons within the nucleus is governed by the
same laws which govern the motion of electrons within the atom. For both
the nucleus and the atom there is a ground state of motion which has
more stability (less energy) than any other state. Of course the
arrangement and motion of electrons in the atom depend not only on this
general rule but also on the specifically electrical nature of the
forces which act between the electrons and the nucleus. In the same way
the arrangement and motion of the neutrons and protons within the
nucleus depend upon the nature of the forces which act between neutrons
and protons.
These forces are definitely not of gravitational origin. Gravitational
attraction is extremely weak compared to the attraction between neutrons
and protons, and is utterly negligible in the realm of nuclear
phenomena. Neither can the nuclear forces be electrical in origin. The
neutrons are electrically neutral; and the protons actually repel each
other by virtue of their electrical charge. The nuclear forces are
something entirely new. They are the strongest forces yet encountered,
and they are without a counterpart in the macroscopic world.
Nuclear forces are not yet completely understood. But to understand
nuclear stability we need to know only one peculiar fact governing the
behavior of neutrons and protons (and incidentally also electrons): They
want to be different. To each particle a state or pattern of motion can
be assigned. When any two neutrons are compared, their pattern of motion
must be essentially different. The same holds for any two protons. A
neutron and a proton, however, may be found in similar patterns since
they differ anyway in their charge.
Now among the possible patterns of motion some have lower and some have
higher energies. Individual neutrons and protons will first occupy the
lowest energy states, in accordance with the rule of least energy for
maximum stability. Then the demand for a difference will force
subsequent particles into patterns of higher and higher energies.
Since a neutron does not exclude a proton from being in the same
pattern, the lowest energy state may be occupied simultaneously by one
neutron and one proton.[4] If another neutron or proton is added, it
must be put into the next state of higher energy. For this reason we
would expect that nuclei are most stable when they contain an equal or
nearly equal number of neutrons and protons. For nuclei which are not
too heavy, this is indeed the case. For example, nitrogen, which has
seven protons, has two stable isotopes, N¹⁴ and N¹⁵, with seven and
eight neutrons respectively. For heavy nuclei, however, the situation is
a little different.
The nuclear force between neutrons and protons acts only over a very
short range—the particles must almost be in contact with each other in
order to experience a sizeable attraction. Consequently a neutron or a
proton interacts only with its immediate neighbors in the nucleus. The
electrical repulsion between the protons, however, acts over a much
longer range. A proton is repelled by all the other protons in the
nucleus. For heavy nuclei this repulsion is sufficient to reduce the
number of protons relative to the number of neutrons. Lead, for example,
with 82 protons, has four stable isotopes, with 122, 124, 125, and 126
neutrons.
We have said that seven protons will combine stably with seven or eight
neutrons. What happens if seven protons are combined with six or nine
neutrons (to make N¹³ or N¹⁶)? Our rule does not prevent them from
sticking together; it says only that these combinations would be _more_
stable if a proton could be converted into a neutron (in the case of
six) or a neutron into a proton (in the case of nine).
Actually seven protons and nine neutrons _do_ stick together, but such a
nucleus is not stable and does not continue to exist indefinitely. The
reason is quite simple and a little surprising: The conversion of a
neutron into a proton is actually a physically realizable process, and
furthermore it releases some energy. Similarly a nucleus containing
seven protons and six neutrons will have an existence of only finite
duration because the conversion of a proton into a neutron can also
occur. Of course the proton is charged and the neutron is not. What
happens to the charge during these transformations? Actually the neutron
is transformed, not into a proton, but into a proton plus an electron.
The proton is transformed likewise into a neutron plus something else.
This something else is called a positron and is identical with the
electron in every respect except in having a positive instead of a
negative charge.
The changes just described occur spontaneously. They are examples of
radioactivity. More specifically they are called “beta decay” processes
because an electron (or a positron) when emitted by a nucleus is called
a beta ray. Such beta-radioactive substances are produced whenever
nuclear energy is used in an explosion or in a power plant. Many of the
difficulties and worries concerning nuclear energy are connected with
these beta activities. We shall be concerned with them often as harmful,
sometimes as helpful agents.
When a neutron is converted into a proton and an electron inside a
nucleus, the electron escapes immediately, but the proton remains in the
nucleus. Similarly, when a proton is converted into a neutron and a
positron, the positron escapes and the neutron remains in the nucleus.
Since the electron and the positron have a negligible weight compared to
a proton or a neutron, the process of beta decay leaves the weight of
the nucleus nearly unchanged. Since the electron and the positron are
charged, the process of beta decay increases or decreases the charge of
the nucleus by one unit.
After beta decay a nitrogen nucleus with seven protons and six neutrons
(N¹³) becomes a nucleus with six protons and seven neutrons—carbon with
weight 13 (C¹³), which is a stable combination. Similarly a nitrogen
nucleus with seven protons and nine neutrons (N¹⁶) becomes a nucleus
with eight protons and eight neutrons, oxygen with weight 16 (O¹⁶),
which is ordinary stable oxygen.
Sometimes after a beta decay the residual nucleus finds itself with a
“correct” number of neutrons and protons but with an excess of energy.
That is, the residual nucleus is not in its ground state but is excited.
This happens in about two thirds of the known cases of beta decay. It
happens, for instance, when N¹⁶ decays to O¹⁶.
In this situation the excited nucleus will behave like an excited atom.
An excited atom, the reader will recall, gets rid of its excess energy
by emitting electromagnetic radiation, usually visible or near-visible
light. The excited nucleus will get rid of its excess energy in exactly
the same way. The only difference is that the amount of energy carried
by the electromagnetic radiation from the nucleus is approximately a
million times greater than that carried by the electromagnetic radiation
from the atom—an indication of the large quantity of energy stored up
inside the nucleus. Such energetic electromagnetic radiation emanating
from a nucleus is usually called a gamma ray. Gamma-ray emission, or
gamma decay, like beta decay, is an energy-releasing process which
changes an unstable nucleus into a stable one, or at least into a more
stable one. More generally, any spontaneous energy-releasing process
(which tends to stabilize the nucleus) is called radioactivity. Beta and
gamma decay are two examples. Later on we shall consider a third example
called alpha decay. An alpha particle is the nucleus of the helium atom
and consists of two neutrons and two protons.
The decay of a neutron and the decay of a proton appear to be quite
analogous processes. Actually there is an important difference between
the two. A free neutron—one not confined inside a nucleus—will decay
into a proton and an electron; but a free proton will not decay into a
neutron and a positron. This difference is due to the fact that the
proton has a slightly lower weight than the neutron and therefore has
less energy. For the proton to decay, it must be inside a nucleus where
it can absorb some energy from the other protons and neutrons.
One sometimes finds pairs of nuclei which could transform into each
other by a proton-neutron (or neutron-proton) conversion; nevertheless
neither of these conversions can occur in the way we have just
described. The reason is that in a proton-neutron or neutron-proton
conversion an additional electron or positron has to be emitted. Now
according to Einstein the mass of the electron or positron corresponds
to some energy (E = mc²), and it may happen that neither the
neutron-proton transformation or the proton-neutron transformation
releases enough energy to make an electron or a positron.
In such cases one of the innermost electrons of the atom may combine
with a proton to make a neutron. Such an electron-capture process will
always release energy provided that the reverse process—the
transformation of a neutron into a proton and an electron—is connected
with an energy deficit. Thus, excluding the possibility of a really
exact coincidence of two energies, one of the two transformations from
neutron to proton or proton to neutron will always be possible.
It is one of the most firmly established laws of nature that energy is
always conserved. One would therefore expect that the energy of a beta
ray would be exactly equal to the difference between the energy of the
nucleus before the beta decay and the energy of the nucleus after the
beta decay. As a matter of fact the energy of a beta ray is found never
to be as great as this amount. Frequently it is much less. Some of the
energy has apparently disappeared and the suspicion has been voiced that
energy may not be conserved after all. It has turned out, however, that
the missing energy is smuggled out of the nucleus, and the smuggler (who
has only recently been caught) is called the neutrino.
The neutrino is an electrically neutral particle, like the neutron, but
its weight, like the weight of a ray of light, is equal to zero. Like
such a ray, it moves with the velocity of light.
The energy released by the nucleus in the beta-decay process is shared
more or less equally between the neutrino and the beta ray. We shall see
later that the electron gives rise to a number of effects. Some of these
are harmful. The neutrino, however, is not in the least dangerous. Like
an ideal smuggler it passes unnoticed and practically without a trace.
It interacts so slightly with matter that several billion of them may go
right through the whole sphere of our earth before a single collision
occurs.
Very recently this strange little particle has upset one of our most
unquestioned concepts about symmetry. We have always believed that
nature made no distinction between her right hand and her left hand;
that for every natural process that exists, there exists also the mirror
image of this process. The neutrino, however, is an exception. It has a
definite symmetry, like a screw.[5] This fact may turn out to be most
important in the development of science. It has no bearing, however, on
the questions to be discussed in this book.
Neutrinos reach us from some distant and hidden places like the interior
of our sun and of exploding stars. It may become possible to use
neutrinos as messengers to reveal the kind of nuclear reactions from
which the energy of the stars is derived.
Neutrinos are also emitted every time we release some nuclear energy.
Among all the remarkable practical consequences of nuclear energy, the
neutrinos have a unique distinction: they are never useful, and they are
never harmful. They have not even been suspected of any mischief.
CHAPTER IV
The Law of Radioactive Decay
A radioactive nucleus is one that will eventually disintegrate and
release some energy. But when?
One might imagine that a radioactive nucleus would begin to “age” from
the moment of its birth, and that after the passage of a predetermined
time, the disintegration process would take place. This is how
radioactivity _might_ work in a deterministic universe. What actually
happens to a radioactive nucleus, however, is much more interesting.
At any instant of its life, the radioactive nucleus has some probability
of disintegrating in the next second. This probability is unaffected by
its age. No matter how long the nucleus has lived, its chance of
disintegrating in the next second is always the same. It is as if a game
of roulette were being played. The wheel spins, and if its number comes
up, the nucleus disintegrates in the first second. If not, the wheel
spins again. Each time the wheel spins there is some probability of its
number coming up. The precise value of this probability is a
characteristic of each particular radioactive species. The higher the
probability, the more rapidly the nucleus may be expected to
disintegrate. But a given nucleus need not do at any particular time
what is expected of it.
The notion of probability (or chance) has meaning only when applied to a
large number of cases. To say that a given nucleus has one chance in a
hundred of decaying in the next second means that out of some large
number (say 100 million) of such radioactive nuclei, one per cent (one
million) will decay in the next second. But it is absolutely impossible
to say beforehand which nuclei will be the ones to decay. A particular
nucleus may decay immediately or only after some very long time. The
collection as a whole, however, will always do the expected thing. (This
is the principle on which insurance companies operate.)
The situation is best described in terms of a time span which is called
the half-life of the radioactive species. The half-life is defined as
the amount of time which is required for one half of a large number of
identical radioactive nuclei to disintegrate. It makes no difference
what this large number is, provided only that it is large enough.
If the number is not large enough, fluctuations will occur, and instead
of 50 per cent of the nuclei decaying during the period of a half-life,
it may be 40 per cent or 60 per cent. As a matter of fact the 40 per
cent to 60 per cent limits correspond to a sample size of about 100
nuclei. For 10,000 nuclei, the limits will be 49 per cent to 51 per
cent. The number of radioactive nuclei with which we customarily deal,
is about 10²³ (100,000,000,000,000,000,000,000). This is the number, for
example, of radioactive nuclei in about an ounce of radium. For such a
large number of nuclei the deviation from 50-per-cent decay during a
half-life will be utterly negligible. Thus we live in a universe which,
on a macroscopic scale, appears ordered and subject to exact laws; while
underlying these laws, on a microscopic scale, nature plays out a game
of chance, full of randomness and uncertainty in the individual case.
We may draw a graph showing how _N_, the number of the remaining
radioactive nuclei, varies with the time _t_. The graph shows that: in
the first half-life _T_, half of the original number _N_₀ of radioactive
nuclei decay. In the second half-life, half of those remaining decay,
and so on. After the time _T_, one half of the original radioactive
nuclei still remain; after 2_T_, one quarter remain; and so forth.
[Illustration: uncaptioned]
Different radioactive species have different half-lives. Many are only a
small fraction of a second; some are billions of years. N¹⁶ decays to
O¹⁶ (plus an electron and a neutrino) with a half-life of about eight
seconds. A free neutron decays into a proton, an electron, and a
neutrino with a half-life of 13 minutes. Strontium with weight 90 (Sr⁹⁰)
undergoes a beta decay with a half-life of 28 years. (This is an isotope
that is not found anywhere in nature, but is made in fairly large
quantities in the fission process.) Potassium with weight 40 (K⁴⁰),
which is present in the amount of 0.01 per cent in ordinary potassium,
has a half-life of one billion years. It has presumably been left over
from the time when the primordial elements were formed. Half-lives for
gamma decay are extremely short by comparison to those for beta decay.
They usually amount to a small fraction of a second.
Radioactivity is characterized by the kind of particle emitted from the
nucleus (our examples, so far, have been of beta and gamma particles),
by the energy possessed by this particle, and by the half-life in which
the radioactive decay takes place.
The biological hazard from radioactivity depends on all three of these
characteristics. No matter whether the radioactive nuclei are produced
in an atomic explosion or in an atomic reactor, some time will in
general elapse before a human population can become exposed. If this
time is long compared to the half-life of the radioactive species, most
of the nuclei will have disintegrated, and the hazard will thereby be
reduced. If, on the other hand, the half-life is long compared to this
time, as well as to the life-span of a human being, the rate at which
disintegrations occur will be low, and again the hazard will be reduced.
In short the dangerous half-lives are the intermediate ones, not too
long, not too short. Sr⁹⁰ is an example.
CHAPTER V
Breakup of the Nucleus
The positive electric charges within an atomic nucleus repel one
another. In the most heavily charged nuclei this repulsion becomes so
great that the nucleus can break into two parts, simultaneously
releasing a considerable amount of energy. In the case of _spontaneous
nuclear fission_ the two parts are more or less equal in size. In the
process of _alpha decay_ one of the parts (the alpha particle) is much
smaller than the other.
An alpha particle consists of two neutrons and two protons and is
identical with the nucleus of the helium atom. (The symbol for this
nucleus is He⁴.) Since two neutrons and two protons can simultaneously
occupy the lowest energy state, the alpha particle is an especially
stable nuclear unit. As a result, from time to time in heavy nuclei, two
neutrons and two protons will coalesce into an alpha particle, which may
then attempt to escape.
In attempting to escape from the nucleus, however, an alpha particle
encounters considerable resistance because of the short-range nuclear
attraction of the other neutrons and protons. This resistance which an
alpha particle experiences in trying to leave the nucleus is usually
referred to as an “energy barrier.” If the alpha particle could acquire
a little additional energy, it would be able to overcome the barrier and
get away from the nuclear attraction. Once outside the nucleus, just
beyond the reach of the nuclear attraction, the alpha particle would be
accelerated violently outward by the large electrical repulsion between
its two protons and the other protons in the residual nucleus.
[Illustration: How an alpha particle escapes from the nucleus. From
A to B it goes “uphill,” losing speed. At B its speed is zero and it
almost always turns around. With a small probability it may sneak
through the energy barrier B to C. Beyond C, it is repelled and
emerges with increasing speed.]
The alpha particle needs some extra energy to escape. According to the
laws of older physics there is no possibility for it to obtain this
extra energy and therefore escape is impossible. But the more newly
discovered laws governing the motion of neutrons and protons (the laws
of quantum mechanics) are not so stringent; they permit the alpha
particle to use “borrowed” energy to overcome the energy barrier. Of
course the alpha particle must always repay the loan—which it can easily
do out of the large fund of electric energy that is released when it
gets out of the repulsive range of the residual nucleus. There is no
interest on the loan.
Such energy loans are not automatically granted in nature. There are two
factors which make the loan improbable: if the amount is big or if the
term is long. These restrictions effectively limit the particles which
may apply for an energy loan. Objects of great size and weight are
unable to qualify, but the small particles of the atomic world often do.
The more energy carried off by the alpha particle after the alpha decay,
the less energy must be borrowed in order to overcome the barrier, and
the more rapidly the decay may be expected to occur. So sensitive is the
decay to the energy of the alpha particle, that an alpha particle
carrying twice the energy is emitted a hundred trillion times fester.
Half-lives for alpha decay vary from a fraction of a second to billions
of years. But even the shortest half-life for alpha decay is remarkably
long compared to the time required for the alpha particle to cross the
nucleus. This means that the alpha particle makes a tremendous number of
attempts to escape from the nucleus before it actually succeeds.
According to the older classical theory the alpha process should never
occur, and in fact it occurs with a very small probability.
A single alpha decay is not usually a sufficient process to bring about
stability of the daughter nucleus. A whole chain of radioactive decays
is usually required before stability is achieved. Most nuclei which emit
alpha particles belong to one of these radioactive decay chains.
The heavy nuclei for which alpha decay occurs all contain a large excess
of neutrons. Since the alpha particle carries off exactly two neutrons
and two protons, the ratio of the number of neutrons to the number of
protons is increased in the daughter nucleus. This has an unstabilizing
influence. (Actually, in lighter nuclei stability requires that the
ratio of neutrons to protons be closer to unity.) The daughter nucleus
is thus apt to be beta-active, converting a neutron into a proton (plus
an electron and a neutrino) in order to decrease its ratio of neutrons
to protons. In this way a chain of radioactive decays may occur, more or
less alternating between alpha and beta emissions, with gamma rays being
occasionally emitted also.
There are four radioactive chains. One of them starts with the abundant
isotope of uranium, U²³⁸. This isotope undergoes a few alpha decays and
a couple of beta decays to become radium, which has a charge of 88 and a
weight of 226. All the radium in the world is produced in this manner as
a daughter product in the fifth decay of the chain. After a number of
further decays, stable lead (weight 206) is produced and the chain
terminates.
The other chains are similar to the U²³⁸ chain, though not quite as
long. One chain starts with the rare isotope of uranium, U²³⁵; another
starts with the isotope of thorium that weighs 232. Both of these
terminate in stable isotopes of lead. In all cases the first decay of
the chain has a very long half-life. The half-life of U²³⁸ is 4.5
billion years; of U²³⁵, 710 million years; and of thorium, 14 billion
years.
The fourth radioactive chain has been made in the laboratory but is not
found in nature because its first isotope, neptunium with weight 237,
has too short a half-life. It decays in two million years and all the
other members of the chain live for even shorter periods. Thus the
neptunium chain decayed long ago, whereas the three other chains have
survived from the time when the elements were made.
It is interesting to notice that the lesser abundance of U²³⁵, as
compared with U²³⁸, is connected with its shorter half-life. Assuming
that comparable amounts of both isotopes were present at the beginning
of the universe (and there is good reason to believe that this was the
case), one would expect to find significantly less U²³⁵ than U²³⁸ after
a period of a few hundred million years. After 710 million years (the
half-life for U²³⁵) only one half of the original number of U²³⁵ nuclei
would still exist. But 90 per cent of the original U²³⁸ nuclei
(half-life 4.5 billion years) would remain. From the presently observed
ratio of U²³⁵ to U²³⁸ nuclei (1 to 139), it may be calculated, using the
law of radioactive decay, that 6 billion years ago natural uranium
consisted of equal parts of U²³⁵ and U²³⁸. The age of the universe is
hotly debated. With each passing year the universe seems to be a billion
years older. Right now six billion years does not seem widely off the
mark.
Natural radioactivity occurs mainly among the heavy elements, but there
are a few light elements that are naturally radioactive. Of these,
potassium⁴⁰ is an especially interesting one because it can decay either
by electron emission or by electron capture. The processes are:
potassium⁴⁰ → calcium⁴⁰ + electron + neutrino,
(1.1 billion years)
and
potassium⁴⁰ + electron → argon⁴⁰ + neutrino.
(11 billion years)
Calcium⁴⁰ and argon⁴⁰ are both stable nuclei. The second reaction is
followed immediately by a gamma ray emission from the argon⁴⁰. The one
per cent of argon found in the earth’s atmosphere comes almost entirely
from the second reaction. These radioactivities are also interesting
because appreciable amounts of potassium⁴⁰ are always present in human
tissue.
All nuclei at the heavy end of the periodic system are radioactive alpha
emitters. Uranium, for example, has no stable isotopes; they all undergo
alpha decay. But there is another mode of spontaneous decay of uranium,
which is much less frequent than alpha decay but is of much greater
practical importance. This is the fission process.
The fission process is just like alpha decay in that the nucleus breaks
up into two fragments. The main difference between these processes is in
the relative weights of the fragments. In the alpha decay of U²³⁸, for
instance, one fragment has a weight of four and the other 234. In the
fission process the fragments tend to be more nearly equal in weight.
For example, one may weigh 90 and the other 148.[6] Other weight
combinations are also possible.
The explanation of spontaneous fission is in essence the same as that of
alpha decay. Spontaneous fission, however, is a less probable process
because the two fragments are more strongly bound to each other by the
nuclear forces than they are in alpha decay. More energy must be
borrowed, and it must be borrowed for a longer term in order to
penetrate the energy barrier.
The relative likelihoods of spontaneous fission and alpha decay can be
appreciated from the following fact. In one hour in a gram of U²³⁸ there
occur about 45 million alpha decays but only about 25 spontaneous
fissions.
Once the energy barrier has been overcome, the energy released in alpha
decay or spontaneous fission is proportional to the charges on the two
fragments. For alpha decay, the product of the charges is 2 × 90 = 180;
for spontaneous fission, this product will typically be about 40 × 52 =
2,080. Hence one might expect the fission energy release to be 10 to 15
times greater than the alpha energy release. As a matter of fact the
fission energy release is even greater than this estimate indicates,
being about 30 to 50 times greater than the alpha energy release. That
so large an amount of energy is released, is a very important feature of
the fission process from the point of view of practical utilization of
atomic energy.
Being at the end of the periodic system, uranium requires a large ratio
of neutrons to protons for its greatest stability. The fission
fragments, however, lie in the middle of the system of elements,
requiring a much smaller ratio of neutrons to protons for stability.
This has two consequences.
One is that the fragments themselves may be expected to be unstable.
They will undergo beta decay (electron emission) several times
consecutively before a stable combination of neutrons and protons is
reached. This radioactivity of the fission products constitutes a
potential hazard in any practical application of fission atomic energy.
In later chapters of this book we shall consider particularly the
possible hazard from the fallout of radioactive fission products created
in atomic explosions, and also the hazard associated with the operation
and maintenance of atomic reactors.
The second consequence of the neutron excess is that neutrons may boil
off from the fragments immediately after the fission process has
occurred. This can happen because a lot of disorderly internal motion is
generated by the fission process within the fragments, and these
fragments do not have a particularly strong hold on their neutrons. The
practical value of the released neutrons is something we shall discuss
at length in a later chapter. For the present we mention only that these
neutrons provide the mechanism whereby a chain reaction is made
possible.
Spontaneous fission and alpha decay are responsible for the fact that
elements with charge greater than 92 are not found in nature. There is
little doubt that these elements were made in the beginning. But they
have long since decayed.
An interesting case of spontaneous nuclear fission is californium²⁵⁴
(charge 98), with a half-life of 55 days. This isotope is formed in
large quantities in certain stellar explosions called super-novae. Once
in a millennium one of a collection of a billion stars flares into
incredible brilliance. For a few weeks this single star shines with the
combined energy and luster of a billion ordinary stars—then it fades
away gradually. Such a “new” star (nova), with the greatest power of
radiation, is called a “super-nova.”
We believe that many nuclear reactions take place in a super-nova. It
has been observed that a few weeks after the initial outburst of light,
the intensity of light is reduced almost exactly by a factor of two
every 55 days for a year or so. This is precisely what would be expected
if the energy generated in the star during this time were due to the
spontaneous fission of californium²⁵⁴. Here we see a model of what
happens to naturally radioactive elements. Of these we have retained on
earth only the ones with the longest half-lives, like uranium, thorium,
and potassium.
CHAPTER VI
Reactions Between Nuclei
The alchemists tried to transform one element into another artificially.
They used heat, they used chemicals; they even used witchcraft. They
failed. Their simplest method—to heat the substance in order to
transform it—was really correct. The trouble was that their temperatures
were too low by a factor of more than 10,000. What is needed, is a
temperature of the order of tens of millions of degrees.
At such high temperatures two nuclei may occasionally approach each
other in spite of the electrical repulsion between them. Sometimes they
may even get close enough to each other to undergo a nuclear reaction.
This, of course, happens with least difficulty if the nuclear charge is
small. Hydrogen nuclei, which carry charge 1, participate in such
reactions most easily.
In the interior of stars temperatures range from about 10 to 100 million
degrees, and nuclear reactions do occur. The reaction responsible for
the production of energy in the stars is:
4H¹ → He⁴ + energy
Four protons combine to make an alpha particle with a release of energy.
Actually this reaction does not take place all at once but several steps
are required. That energy should be released, one expects from the fact
that the alpha particle is very stable. Any process in which light
nuclei combine to form a heavier nucleus with a release of energy is
known as “fusion.”
The particular fusion process that goes on in the stars releases its
energy in many forms: as positrons, neutrinos, electromagnetic
radiation, and motion of the reacting particles. The positrons also
carry off the excess charge of the reaction.
The neutrinos fly through the star without interacting, carrying their
energy away into outer space, probably never again to make contact with
the material universe. The remainder of the fusion energy is deposited
within the star’s interior, which is thus kept hot enough so that the
fusion reaction can keep going. The name “thermonuclear” is
appropriately applied to this type of reaction.
A lot of effort and imagination is being devoted to the problem of
making a controlled thermonuclear reaction. The motivation for this
project comes from the fact that good thermonuclear fuels, such as
deuterium (H²), are abundant and cheap. There is enough deuterium in the
oceans of the world to supply man’s energy needs for many millions of
years. One difficulty, of course, is to find a container for the
reaction.
Even under stellar conditions the rate of fusion reactions is not very
great. It takes approximately a billion years for only one per cent of
the nuclei to react. Consequently even higher temperatures than those
found in stars are required to produce large amounts of energy in a
short time. But no known materials can withstand temperatures of more
than a few thousand degrees centigrade. One idea is to keep the
“burning” fuel away from material walls by means of magnetic fields.
Is there a way to make nuclei react without the extreme temperatures
needed in the thermonuclear reactions? What one is really trying to do
is bring two nuclear particles into intimate enough contact so that the
nuclear forces can act between them. There is no reason why one should
not use a _cold_ target material, which is bombarded from the outside by
energetic nuclear projectiles, for example protons or alpha particles.
The projectiles, if they are energetic enough, can overcome the
electrical repulsion of the target nuclei, and they actually can
penetrate. The resulting “compound” nuclei would either be unstable and
disintegrate instantaneously, or else be almost stable (i.e.,
radioactive) and disintegrate after some period of time. In either case
nuclei of new elements would probably be formed in the reaction. This
procedure sounds simple, but it has its difficulties.
[Illustration: Interior of the sun. The thermonuclear reactions take
place mainly in the very hot, very dense central region (shaded).
This region is about 20,000 miles in radius and has a density
approximately 80 times the density of water.]
The main difficulty is that the nucleus is a very tiny target. Its area
is about 100 million times smaller than the area of the atom as a whole.
If a piece of matter is bombarded by an energetic particle, chance alone
will determine whether the particle is directed toward a nucleus. To be
sure, if the particle misses the nucleus of one atom, it still has the
opportunity of hitting the nuclei of other atoms which may lie in its
path. It does not have many such opportunities, however, because, being
charged, it constantly interacts with the atomic electrons, which
gradually absorb energy causing the particle to slow down.
As the particle slows down, its chance of hitting a nucleus decreases,
even if it is heading directly toward one, because of the repulsion
between its charge and that of the nucleus. Unless the particle has
sufficient speed, it cannot overcome this repulsion.
Charged particles may be given the required speeds by accelerating them
through large electric fields. If a unit charge is accelerated through a
potential difference of one volt, it acquires an energy of one
_electron-volt_. The energies required for nuclear bombardment are of
the order of several million electron-volts, which can be provided by
atom-smashing machines such as the cyclotron.
Even at such high energies very few of the nuclear projectiles actually
find their way to a target nucleus. Most of them are slowed down by the
electrons, wasting their energy in heating up the target material.
Perhaps one particle out of a million will be lucky enough to induce a
nuclear reaction.
If the purpose of the nuclear accelerating machines were to produce
cheap energy, they would not be of much value. A nuclear reaction may
typically release five to 20 million electron-volts of energy. But to
obtain this reaction, a million particles had to be accelerated to
energies of several million electron-volts. The recoverable and useable
energy will be only a minute fraction of the total invested.
On the other hand, as a tool for scientific discovery, the atom-smashers
have been of great importance. That one event in a million has given us
much of our knowledge of nuclear physics.
The achievement of nuclear reactions by particle bombardment did not
actually wait on the invention of man-made accelerating machines.
Energetic alpha particles are available from the radioactive decay of
heavy elements. In 1919 Ernest Rutherford used such radioactive elements
as a source of alpha particles. The alpha particles were made to bombard
ordinary nitrogen, causing the reaction:
He⁴ + N¹⁴ → O¹⁷ + proton
(2 protons) (7 protons) (8 protons)
(2 neutrons) (7 neutrons) (9 neutrons)
That is, an alpha particle plus a nitrogen¹⁴ nucleus react to produce a
nucleus of (stable) oxygen¹⁷ plus a proton. Oxygen¹⁷ is a nucleus with 8
protons and 9 neutrons. The ordinary abundant form of oxygen has 8
protons and 8 neutrons. Natural oxygen contains a small amount of
oxygen¹⁷.
Later, in 1934, Irene Curie Joliot (the daughter of the discoverer of
radium, Madame Curie) and her husband, Frederic Joliot, used naturally
available alpha particles to make artificial radioactive nuclei for the
first time. The reaction was:
He⁴ + aluminum²⁷ → phosphorus³⁰ + neutron
(2 protons) (13 protons) (15 protons)
(2 neutrons) (14 neutrons) (15 neutrons)
Phosphorus³⁰ is an unstable nucleus and emits a beta ray (a positron) to
become silicon³⁰ (which is stable). The half-life for this decay is
about 2.5 minutes. The Joliots’ reaction was the first instance in which
man had produced radioactivity and known it. Actually cyclotrons had
been producing radioactivity in good abundance for the preceding two
years—but physicists had been unaware of this fact.
It is amusing that nature has also provided us with an atom-smashing
machine and indeed one that produces far greater energies than any
apparatus yet devised by man. This machine operates on the principle of
fluctuating, turbulent magnetic fields in interstellar space. Cosmic
particles—mainly protons, but also some alpha particles and even heavier
nuclei—are accelerated by these changing magnetic fields and hurled
occasionally into the earth’s atmosphere. The energies of these cosmic
particles are enormous, ranging from billions of electron-volts to
values a million times higher.
When a cosmic particle gets inside the earth’s atmosphere, it does not
go far before colliding with a nucleus of nitrogen or oxygen. Out of
this nuclear event emerge all the fundamental particles mentioned so
far, and some others known as mesons. Mesons are particles which may be
charged or neutral, and which have a weight a few hundred times that of
the electron. Some of these particles are believed to be connected with
the forces that hold the nucleus together.
The nuclear debris from the collision will itself be very energetic and
will further disrupt other nitrogen and oxygen nuclei. There soon
develops a cascade of electrons, positrons, mesons, neutrons, protons,
and electromagnetic radiation moving toward the surface of the earth.
About once a second every square inch of the earth’s atmosphere receives
such an energetic particle from outer space. The cascade that results
carries penetrating radiations to the surface of the earth. All living
organisms are constantly subjected to this radiation background. It is
an important fact that the intensity of this radiation is reduced in its
passage through the air, and inhabitants of Denver or Lima receive more
cosmic radiation than the inhabitants of Los Angeles or New York.
Some neutrons made by collisions of the primary cosmic particles in the
atmosphere may collide with nuclei of nitrogen. When this happens, the
following reaction occurs:
nitrogen¹⁴ + neutron → carbon¹⁴ + proton
(7 protons) (6 protons)
(7 neutrons) (8 neutrons)
Carbon¹⁴ is a radioactive electron emitter with a half-life of 5,600
years. This half-life is long enough so that much of the carbon¹⁴ in the
world today was probably made ten to twenty thousand years ago. Willard
Libby studied this process in a careful and quantitative way, traced the
history of the radioactive carbon from the atmosphere into living
beings, and, by measuring the carbon¹⁴ content in historical remains,
opened up a whole new branch of archeology.
Living organisms breathe in carbon (in the form of carbon dioxide) from
the air. Most of this carbon is ordinary stable carbon¹²; a tiny
fraction is radioactive carbon¹⁴. The organism is unable to distinguish
between the two isotopes, and takes in carbon¹⁴ in the same ratio to
carbon¹² as exists in the atmosphere. This ratio persists throughout the
organism’s lifetime, but when the organism dies and no more carbon is
assimilated, the ratio begins to decrease as the carbon¹⁴ nuclei
gradually disintegrate. By observing the ratio of carbon¹⁴ to carbon¹²
in fossil remains and other archeological objects, the date at which
death occurred can be calculated. In this way the age of ancient
Egyptian mummies has been found, and it has been shown that some sequoia
wood is more than 1,500 years old. By measuring the carbon¹⁴ in trees
that were killed by the last advance of glaciation, and looking into
other remains of life from the last ice age, it has been possible to
show that this last ice age occurred only 10,000 years ago—instead of
20,000 years, as had been previously believed. Carbon¹⁴-dating has
therefore thoroughly revised our ideas about the rapidity with which the
empires known to history have emerged from the most primitive
conditions. A crucial part of the argument is that isotopes of the same
element are chemically indistinguishable.
An alternative reaction which may occur when neutrons strike nitrogen,
is
N¹⁴ + neutron → carbon¹² + H³
(7 protons) (6 protons) (1 proton)
(7 neutrons) (6 neutrons) (2 neutrons)
H³, triton, is also radioactive, undergoing a beta decay to become He³
(2 protons and 1 neutron) with a half-life of 12.25 years. Tritons too
can be used for dating old objects—for example, old wine. The water in
the wine cannot be replenished with cosmic-ray tritons after the wine
has been bottled. Thus fifty per cent of the tritons disappear every
12.25 years.
We have here two examples of nuclear reactions induced by neutron
bombardment. Recalling the disadvantages of charged particles as nuclear
projectiles for alchemists, it must surely seem that neutrons would be
ideal for this purpose. Being chargeless, they are neither electrically
repelled by the nuclei nor constantly slowed down by energy-losing
collisions with the electrons. The fate of almost every neutron moving
in a large piece of matter is eventual collision with a nucleus.[7]
Neutrons are ideal nuclear projectiles, except for one thing: they are
hard to get.
Protons and alpha particles are found abundantly in nature as the nuclei
of hydrogen and helium atoms. Neutrons, however, are not found in
nature, and in the past have been made in nuclear reactions that were
themselves initiated by charged particles. For example,
He⁴ + beryllium⁹ → C¹² + neutron
(2 protons) (4 protons) (6 protons)
(2 neutrons) (5 neutrons) (6 neutrons)
But now we encounter again the difficulty associated with charged
particles. Only one alpha particle in a million undergoes a nuclear
reaction to produce a neutron. The neutron, of course, makes a nuclear
reaction every time. Over-all, then, we obtain two nuclear reactions per
million nuclear projectiles, instead of one per million. With such
methods we are not so much better off than the old alchemists. A cheap
and plentiful source of neutrons would, however, put the alchemist in
business. In this way one could make rare elements and radioactive
isotopes, and what is more important, he would be able to utilize
concentrated nuclear energy.
CHAPTER VII
Fission and the Chain Reaction
Neutrons are ideal projectiles for nuclear bombardment because they
carry no charge, can approach nuclei easily, and interact with them
strongly. These neutral particles, discovered by James Chadwick in 1932,
were used soon afterward by Enrico Fermi and his collaborators to
bombard most of the elements of the periodic table. Very often in these
experiments a nucleus would capture a neutron and become unstable with
too much weight for its charge. Stability would then be restored by a
beta decay, leaving the nucleus with one more unit of charge than it had
to begin with. In 1934 Fermi tried this experiment with uranium, charge
92, the most highly charged element known at that time. He hoped to make
a transuranic element with charge 93.
Throughout the experiments the uranium was observed with radioactive
counters and found to become far more radioactive than uranium
ordinarily is in its natural state. There was no way to account for all
this radioactivity except to assume that new elements had been formed in
the process of neutron bombardment. A chemical analysis revealed no
elements with charges between 86 and 91. From this evidence Fermi
concluded that no elements of charge less than 92 had been made and
therefore the radioactivity must be due to charges greater than 92. He
concluded that transuranic elements had been made in the laboratory.
Neither Fermi nor anyone else, however, was happy with this conclusion.
There was far too great a variety of radioactivity for comfort. It had
to be assumed that not only was the element with charge 93 being made,
but also elements with charges 94, 95, and many more. This was very hard
to understand. Ida Noddack,[8] a chemist, published a paper proposing an
alternative explanation of the experiment: that a nucleus of uranium,
when it captures a neutron, might break up into two fragments that could
have any of various weights and charges. In other words, she suggested
that Fermi had produced nuclear fission.
Fermi, however, believed that the fission process was an impossibility.
He had a convincing proof, based on the measured values of the weights
of nuclei and the formula of Einstein, E = mc². From this formula Fermi
calculated the energy liberated when uranium breaks into two pieces;
then he took into account the energy of electric repulsion between the
pieces and found that the energy barrier was so large that the fission
process could not take place. This proof was absolutely correct. The
only trouble was that the measured values of the weights of nuclei
happened to be inaccurate at that time!
But for this accident, fission would have been discovered in 1934
instead of 1938. If it had been, Nazi Germany might easily have been the
first country to make the atomic bomb. At that time some German
scientists were active in the field of military applications. The
American physicists had not yet turned much attention to the subject.
An important feature of Fermi’s experiment is the large amount and
variety of radioactivity that he found. The reason for this variety, as
we now know, is that the fission process does not take place in a unique
manner. The two primary fission fragments are very rarely of equal
weight and charge. On the average the lighter fragment weighs about 90,
and the heavier one about 140. Sometimes the lighter fragment will weigh
as little as 75, and the heavier one as much as 160. As the weight
varies, of course, so also does the charge. The charge of the lighter
fragment averages 38, which is strontium, and the heavier one 54, which
is xenon. All in all there are more than a hundred different species of
nuclei represented among the primary fission fragments.
Practically all of these nuclei are radioactive and undergo three or
four disintegrations before reaching stability. Overall therefore,
several hundred distinct radioactive species are created by the fission
process in uranium. Elements with charges 43 and 61 (which are not found
in nature) have been identified as fission products in fairly
appreciable quantities. Most of the fission products are short-lived
electron and gamma emitters that can contribute only to the local and
immediate radioactive hazard. Two of the long-lived products are
abundant and important. These are cesium¹³⁷ and strontium⁹⁰.
Cesium¹³⁷ has a half-life of 30 years and emits a gamma ray with an
energy of 0.6 million electron-volts. Strontium⁹⁰ has a half-life of 28
years and emits an electron with an average energy of 0.22 million
electron-volts. The daughter nucleus in this process is yttrium⁹⁰, which
emits another electron with an average energy of one million
electron-volts. The half-life of yttrium⁹⁰ is 64 hours. In effect,
therefore, strontium⁹⁰ emits two electrons, each with an average energy
of 0.6 million electron-volts. For the long-term radioactive hazard,
particularly the world-wide fallout associated with atomic explosions,
the two isotopes cesium¹³⁷ and strontium⁹⁰ are the most significant.
Strontium⁹⁰ is the more dangerous to living organisms because it is
deposited in the bones and retained in the body for long periods.
Besides radioactivity there is another feature of the fission process
which is so conspicuous that it may seem hard to understand how Fermi
failed to notice it—namely the large amount of energy released. The
fission of a single nucleus of uranium releases an energy of 200 million
electron-volts as contrasted with ordinary radioactive decay energies of
5 to 10 million electron-volts. (The energy released from the burning of
one atom of coal is only 4 electron-volts.)
Of the 200 million electron-volts released in fission, about 10 million
go into gamma rays and neutrons created in the fission process itself.
This energy contributes to the immediate and local radiation danger.
Another 24 million electron-volts go into radioactivity of the fission
products, and of this, about half go into neutrinos, which are neither
dangerous nor useful; the other half is carried by electrons and gives
rise to the delayed radioactive hazard. But the bulk of the energy, over
160 million electron-volts, goes into kinetic energy of the two primary
fission fragments. Of this amount, 100 million, on the average, go to
the lighter fragment.
One hundred million electron-volt fission fragments should certainly
have been noticed by Fermi’s radioactive counters—if they had been able
to reach the counters. The fragments were not able to reach the
counters, however. The reason is that Fermi was a careful worker. He
knew that his sample of uranium would emit some radioactive particles
even before neutron bombardment. This natural radioactivity he did not
want to get mixed up with the radioactivity that would be produced in
the experiment. So he put an absorbing foil between the uranium sample
and the radioactive counters. The fission fragments could not get
through the foil.
It is amusing that shortly afterward another noted physicist repeated
Fermi’s experiment, but this time without the foil. He reported that he
was unable to get any significant results because his counter, for
reasons unknown, started to spark.
Thus fission remained a secret. But in England Leo Szilard obtained
patent papers on the nuclear chain reaction. He pointed out that in some
nuclear reactions free neutrons might be released. These neutrons might
then succeed in producing further reactions which would produce more
neutrons. Provided that at least one neutron made in each reaction were
able to induce a reaction in another nucleus, a chain reaction would
take place.
The main problem, of course, was to avoid excessive neutron losses.
There are two ways in which the losses mainly occur. One is by wasteful,
nonreproductive capture in the nuclei; the other, by neutron leakage
from the material surface. This second loss, Szilard showed, could be
minimized by using a sufficiently large amount of chain-reacting
material.
The point is that a neutron born in a nuclear reaction must travel on
the average a certain distance before it can produce another reaction.
If the size of the chain-reacting material is much less than this
distance, practically all of the neutrons produced will be able to
escape through the material surface, and no chain reaction will be
possible. If the size of the material is large compared to this
distance, the leakage loss becomes negligible, and the possibility of a
chain reaction depends entirely on the magnitude of the first kind of
loss, the wasteful captures in nuclei. If this loss is not too great,
and a chain reaction is possible, there will be a _critical_ size of the
material at which on the average exactly one neutron per reaction will
be able to induce another reaction. A just critical chain reaction of
this kind is what is needed for an atomic reactor.
If the size of the material is greater than the critical size, on the
average more than one neutron per reaction will cause another reaction
and the chain reaction will run away. If, for example, two neutrons can
cause another reaction, there will be two neutrons after the first
generation, four after the second, eight after the third, and so forth.
This is the principle of the atomic bomb.
After about 80 generations, an appreciable fraction of all the nuclei in
the material will have undergone a nuclear transformation and so much
energy will have been released that the material will not stay together
even for the short time needed to produce the next generation. The whole
material begins to fly apart, the system becomes sub-critical, and the
chain reaction stops. The entire process lasts only a fraction of a
microsecond.
Thus even before fission was discovered, Szilard laid the basis for
constructing the atomic bomb and the nuclear chain reactor. As materials
in which a chain reaction might conceivably be made to occur he named
thorium, uranium and beryllium. On beryllium he was wrong because the
mass of this atom was incorrectly known. On thorium, his guess was good.
On uranium, he hit the bull’s eye.
Finally in December 1938 the secret broke. Hahn and Strassmann in
Germany made a chemical analysis of a uranium target that had been
exposed to neutrons. They were far more thorough than previous
investigators had been, and they found barium, charge 56, which had not
been present in the target material before the experiment. The only
possible explanation was the fission process. Within a few weeks the
violent kicks caused by the fission products in counters were found, and
in the following days this experiment was repeated around the world.
There was no doubt that neutrons could induce fission in uranium nuclei.
A few more weeks, and it was ascertained that the fission process
released neutrons which might lead to more fissions.
The chain reaction, however, was still far from a reality. Niels Bohr
and John Wheeler proved that a neutron could not cause fission in U²³⁸
unless its energy were greater than about one million electron-volts.
When the neutrons are first made in the fission process, many of them do
have energies greater than one million electron-volts. But before they
can cause a fission, they usually make a few nonfission collisions with
uranium nuclei, giving part of their energy to the nuclei and escaping
with the remainder. The nuclei are then left with too little energy to
undergo fission and the neutrons with too little energy to cause
fissions in their next encounters. Thus too few neutrons reproduce
themselves and no chain is possible.
Bohr and Wheeler suggested, however, that the rare isotope of uranium,
U²³⁵, can undergo fission when any neutron, even a slow neutron, hits
it. Thus a chain reaction is possible in U²³⁵. This was confirmed
experimentally shortly afterwards by John Dunning and Alfred Drier and
their co-workers at Columbia University.
Why the isotopes 235 and 238 behave so differently, is not difficult to
understand. The 235 is more explosive and more prone to undergo fission
than 238 because it is smaller and therefore its protons repel each
other more strongly. More important still, when a neutron is captured by
235, it acquires a greater kinetic energy by virtue of the short-range
nuclear attraction than a neutron acquires when it is captured by 238.
This happens for the simple reason that nuclei tend to be more stable
when they have an even number of neutrons (or protons) than when they
have an odd number. U²³⁵, having an odd number of neutrons, is more
eager to receive an additional neutron than 238, which already has an
even number of neutrons. Consequently, the capture of a slow neutron by
235 almost always eventuates in the fission process; while in 238, the
excess energy, introduced by the neutron, is merely ejected from the
nucleus in the form of a gamma ray, and U²³⁸ becomes U²³⁹.
A chain reaction is possible in U²³⁵ , but it is necessary to separate
this rare isotope from the abundant U²³⁸. The separation process is
anything but simple since isotopes of the same element are chemically
indistinguishable. Even the weight difference in this case, is little
more than one per cent. Bohr rejected the idea of a large-scale
separation with the remark: “You would have to turn the whole country
into a factory.” Of course it is now a matter of history that the job
was actually done under the Manhattan project during World War II.
During the war Bohr (alias Nicholas Baker) again visited the United
States and was shown the separation plants. He said: “You see I was
right. You _did_ turn the country into a factory.”
Natural uranium contains U²³⁵ in the ratio of 1 part to 139 of U²³⁸. It
was hoped at first that this concentration would be sufficient to make a
chain reaction, and that the expensive enrichment processes could be
avoided. This seemed possible because at energies of a fraction of an
electron-volt the neutrons are much more easily caught by U²³⁵ than by
U²³⁸, which compensates for the low concentration. Actually neutrons are
slowed down until their energy is as low as the energy of all other
particles participating in the general agitation caused by the
temperature. This energy is low enough for the purpose.
However, the neutrons are made in the fission process with an energy of
about a million electron-volts. Before they slow down sufficiently, they
must pass through a stage in which their energy is about 7
electron-volts. In the neighborhood of this energy, it happens that the
U²³⁸ has an extremely high probability for capturing a neutron and
changing into U²³⁹. Near some other energies, similar though smaller
absorption hurdles must be passed. Therefore natural uranium by itself
cannot be used to make a chain reaction. In 1940, Fermi and Szilard,
working now in the United States, found a way around this difficulty.
Their trick was to mix the natural uranium with a material whose nuclei
are so lightweight that they suffer a big recoil when struck by a
neutron and thus absorb a large fraction of the neutron energy. The
neutron is thus _moderated_ down to a low energy, rapidly and in big
energy jumps, so that either it does not spend much time at the
unfavorable energies where it can be caught by U²³⁸ or else it misses
these energies altogether. By imbedding the uranium in lumps in the
moderating material instead of making a homogeneous mixture of the two,
the absorption can be circumvented even better.
For the purpose of making a _controlled_ chain reaction, one may use the
method of enrichment, or the method of moderation, or both. But to
produce a _violent_ chain reaction, an atomic bomb, only the enrichment
method will work. The reason is that all the energy of the bomb must be
generated in a time that is as short as the time it takes the bomb to
fly apart, which is a fraction of a microsecond. If natural uranium were
used, the reaction would be slow and sluggish and would be extinguished
before a substantial fraction of the nuclei could have reacted.
It is interesting to consider that chain-reacting substances could have
been obtained easily six billion years ago, before the U²³⁸ had time to
decay and become a rare isotope. (The U²³⁵ was then about as abundant as
U²³⁸.) A chemical separation would still have been necessary and so we
do not need to imagine that chain-reacting mixtures accumulated
spontaneously on the young earth.
On the other hand, six billion years from now U²³⁵ will have become so
rare that it will be impossible to get a reactor going by moderation. At
the same time the isotope separation will have become most expensive
since the isotope to be separated will be present in an abundance of
less than 100 parts in a million. For those who like to worry about the
distant future we should hasten to add that other methods of obtaining
atomic energy will remain possible. And in any case there is good reason
to believe that some stellar explosions produce fresh supplies of U²³⁵
which space merchants could undoubtedly make available.
As to our present terrestrial supplies: uranium, like other heavy
elements, is quite rare. But the earth is divided into layers of which
the topmost 10 miles, forming something of a slag or scum, contain quite
a few rare compounds. In particular almost all of the uranium in our
planet is conveniently collected right under our feet, for us to use as
we see fit.
CHAPTER VIII
Action of Radiation on Matter
When an energetic particle moves through matter (living or nonliving),
what happens is a question of chemistry. Chemistry is the subject that
deals with the arrangement and rearrangement of electrons in atoms and
molecules. A chemical rearrangement generally requires an energy in the
neighborhood of a few electron-volts. (As we have seen, an electron-volt
is the energy released when an electron moves through a potential of one
volt, i.e., a little less than one per cent of the driving force in a
standard electric outlet.) An energetic particle, such as might be
emitted in a radioactive decay, typically has an energy of a few million
electron-volts. Thus a single such particle has the potentiality of
about a million chemical rearrangements.
Energetic particles may be charged or neutral, light or heavy, or
electromagnetic in nature. Because of this diversity one might think
there would be no common grounds for comparing the action on matter of
different particles. Each particle might conceivably make its own
inimitable variety of chemical rearrangements. Actually this is not the
case.
Unlike some chemical poisons, which seek out specific molecules in our
body, the energetic particles strike at whatever atoms or molecules
happen to get in their way. They act, in this sense, like a sledge
hammer. Their effects can be measured directly from the strength (or
energy) of the blow. Which particle delivers the blow is of little
consequence provided the same amount of energy is delivered and provided
the same tissues are affected (in the case of living matter). After the
blow, however, some specific chemical effects may occur. When water or
some other molecule in the body is broken up by radiation, the fragments
produced may themselves be chemical poisons and attack the biologically
important large molecules in a secondary way. In fact, it seems probable
that a considerable part of the radiation damage caused in living
systems, both healthwise and genetically, occurs in this manner.
Although the energetic particles are all similar in their ultimate
action on matter, namely in producing wholesale destruction of atoms and
molecules, they differ somewhat in the way in which they bring about
this destruction. Charged particles act in one way, gamma rays in
another, and neutrons in still another. It is simplest to begin our
discussion with the charged particles.
The most important charged particles are those connected with the
natural background of radioactivity and cosmic rays, and the fission
process. These include alpha rays, beta rays, mesons, and fission
fragments. For review, a table of the weights and charges of these
particles, as well as a few others, is shown. As usual, we have used the
weight and charge of the proton as units.
_Particle_ _Weight_ _Charge_
proton 1 1
alpha 4 2
electron beta rays 1/1840 -1
positron ” 1/1840 1
deuteron 2 1
triton 3 1
meson 1/8 1, -1
average light fission fragment 97 20
average heavy fission fragment 138 22
If the fission fragments were completely stripped of their orbital
electrons, they would have charges even greater than the values
indicated in the table. The reader will recall that the average charge
of the nucleus of the light fission fragment is 38, and of the heavy,
54. But such highly positively charged particles exert an enormous
attraction on electrons. Some of these remain attached even during the
fission process itself. As the fission products lose their speed during
passage through matter, they pick up more electrons and gradually lose
their charge.
When any of these energetic charged particles moves through matter, it
interacts with electrons in the atoms. As a result of this interaction,
the electrons may be dislodged from their usual states of motion. If the
interaction is gentle—either because the charged particle passes the
atom at a considerable distance or else because the particle is moving
so rapidly that the interaction lasts for only a short time—the electron
may be left undisturbed. If the interaction is more violent, however,
the electron may be excited to a more energetic state of motion while
still remaining in the same atom or molecule; or it may actually be
ejected, ending up at some other atomic site. In this latter event the
original atom is left with a residual positive charge and is said to be
_ionized_. At the same time the displaced electron is apt to unite with
whatever atom or molecule happens to be nearby, creating in this way a
negative ion. The whole process may be described as forming an ion pair.
In the wake of the charged particle one finds, therefore, ionized and
excited atoms and molecules. A rearrangement of atoms will now ensue
which leads to new chemical compounds. The important thing for us is,
however, that these chemical changes do not depend very much on the type
of particle which produced the ionization; the proportion between
ionization, excitation, and eventual chemical reaction remains more or
less the same. Roughly speaking, the more ion pairs that are formed in
living cells, the greater is the extent of biological damage.
To make an ion pair requires the expenditure of a certain amount of
energy. It might seem as though this amount should depend crucially on
the weight, charge, and energy of the particle, and also on the medium
through which the particle is moving. This is not so. There is some
dependence, of course, but only slight. Any charged particle,
irrespective of its energy, moving in any medium—air, water, soil, or
living tissue—creates ion pairs at the rate of about one per 32
electron-volts. A one-million-electron-volt particle produces about
30,000 ion pairs before losing all of its energy. (When it does lose its
energy, if it is a positively charged particle, it will pick up enough
electrons to become neutral. An alpha particle, for example, will become
an ordinary helium atom; a proton will become an atom of hydrogen.)
We have said that two charged particles having the same energy, produce
the same total number of ionizations. There is an important respect,
however, in which charged particles of the same energy may differ. That
is, in the density of ionization along their paths. In particular, the
more slowly the particle is moving and the greater its charge, the more
ionization and damage it will produce in a given distance. At the same
time it will lose energy at a greater rate. If we compare two charged
particles of the same energy plowing into matter, the one which leaves
the deeper furrow will be stopped more quickly.
For a greater charge it is easy to understand that the electrical
interaction is increased and hence each atomic electron is more strongly
disturbed. If, on the other hand, the particle moves more slowly (which
is usually the case if it is heavy) it spends a longer time in the
neighborhood of the atomic electrons. The electrical interaction thus
has a longer duration and is more effective in ejecting an electron. For
this same reason the density of ionization along the path of a
particular charged particle should tend to become greater and greater as
the particle slows down. Actually this tendency is opposed in the case
of a fission fragment by the increased likelihood of the particle’s
picking up electrons and reducing its charge. As a result, the
ionization density for these fragments is rather uniform. If a heavily
charged, slow particle moves through matter it leaves so many disturbed
and disrupted molecules behind that now these molecules may react with
each other. Therefore heavy ionization may lead to peculiar effects.
Nevertheless all ionizing particles give rise to roughly similar
chemical change and destruction.
Except for the beta rays, all the charged particles are very heavy
compared to the electron. Consequently, as they move through matter and
interact with the atomic electrons, their paths are not perceptibly
deflected from the original direction. The beta rays, on the other hand,
having the same weight as the atomic electrons, are appreciably affected
by their encounters and are frequently forced to change direction. Their
paths are thus winding and random.
Because the beta ray does not travel in a straight line, its ability to
penetrate matter must not be measured by its total path length. As a
rule of thumb, the _range_ of a beta particle, being the distance it
travels along the line of its original direction, is about one half of
its total path length. For heavier charged particles, however, no
distinction need be made between range and actual distance traveled.
The most important fact about the ranges of charged particles is that
they are small. An alpha particle, for instance, with a typical
radioactive energy of a few million electron-volts, has a range in water
(or living tissue) of a few thousandths of an inch. Such a particle
could not penetrate a sheet of paper. A fission fragment, despite its
great energy, is even less penetrating than the alpha particle. The
proton has a somewhat greater range than the alpha particle. But the
beta ray, because of its low weight, has by far the greatest range of
any of the charged particles. Even it, however, goes only a fraction of
an inch in solid or liquid materials.
The following table shows the ranges (in inches) of some of the charged
particles in air and water as a function of energy (in millions of
electron-volts):
_Range_
_Air_ _Water_
(Same as living tissue)
_Energy_ _5_ _1_ _2_ _5_ _5_ _1_ _2_ _5_
alpha 0.1 0.2 0.4 1.4 0.0001 0.0002 0.0004 0.0014
proton 0.3 0.9 2.8 13.4 0.0005 0.001 0.003 0.014
beta 49. 130. 300. 770. 0.063 0.16 0.38 1.0
The table shows that charged particles travel only short distances in
matter. For this reason these particles are not a serious external
radiation hazard. The protons and the alpha rays are usually stopped by
less than a foot of air. Ordinary clothing or even the outer layer of
our skin (which is composed of nonliving cells) will stop them
completely.
Beta rays are stopped by less than seventy feet of air or an inch or
less of solid material. (Actually most of the beta rays produced in the
fission process have energies less than a million electron-volts or so,
and hence their ranges are even smaller.) Radioactive contamination of
beta emitters directly on one’s clothes or body could cause trouble; but
a good scrubbing soon after exposure will eliminate this problem. The
interior of a house or building should be quite safe from any outside
source of charged particles emitted by radioactive substances except
possibly the most energetic beta rays. Only if the source of charged
particles is inside the body so that in spite of their limited ranges
the particles can find their way to sensitive tissues, is there any
danger. In this case, as we shall see in a later chapter, the danger may
be considerable.
Charged particles of one type stand pretty much by themselves. These are
the mesons found in cosmic rays. These particles move as fast as
energetic beta rays and, like the beta rays, carry unit charge. Their
biological effects are therefore the same as the biological effects of
beta radiation, with one important difference. The cosmic ray mesons
carry much more energy and therefore have a much greater range. Whereas
the beta rays are stopped in the skin, the mesons can cause damage
throughout the entire body. The mesons produce the same effects as a
substance which emits beta radiation uniformly in the whole body. This
fact is important. It puts us in the position to compare effects of
man-made radioactivity with effects of the cosmic rays to which we are
constantly exposed.
Not all the energy in cosmic rays is carried by mesons. We also find
showers of electrons. These are almost the same as beta rays except that
they have more energy and arrive frequently in fairly sizeable numbers
traveling along nearly parallel tracks. Their effects, however, are the
same as the effects of the mesons.
We have been talking now about the interactions between charged
particles and the atomic electrons. No mention has been made of
interactions between the charged particles and nuclei. Nuclear
interactions do occur sometimes, but by and large they have only a
negligible influence in slowing down the charged particle. They do
affect, however, beta rays.
When a beta ray collides with a highly charged nucleus, the beta
particle is violently deflected. The violence of this process is due to
the heavy charge of the nucleus and the small mass of the beta particle.
In the sudden change of velocity which occurs, part of the electric
force field which surrounds the electron breaks loose; the result is
high-frequency radiation called X-rays. The importance of such
electromagnetic radiation is that it can penetrate more deeply into
matter. In our bodies, for typical beta-ray energies, only a small part
of the beta-ray energy is converted into X-rays. But in many radioactive
processes gamma rays (which are physically the same as X-rays) are
produced quite abundantly. These rays may carry as much or more energy
than the beta rays.
Unlike charged particles, which constantly interact as they move through
matter, gamma rays can go for long distances without having a single
encounter. The actual distance depends on the energy of the gamma ray,
the medium in which it moves, and pure chance. On the average, a
one-million-volt gamma ray goes about six inches in water before
anything at all happens to it. A four-million-volt gamma ray goes about
a foot. In living matter the distances are approximately the same. Thus
gamma rays from an external source can find their way deep inside the
body.
Of course living matter is not injured by the mere presence of a gamma
ray. There is a small probability that the gamma ray could go right
through the body without a single encounter. If so, there would be no
biological effect. An effect is produced only when the gamma ray
interacts with the matter. There are three most important ways in which
such an interaction may occur.
One way is simple _absorption_ of the gamma ray by one of the atomic
electrons. The gamma ray disappears in this process, and the electron
acquires all of its energy. A tiny bit of this energy is used for the
electron to break its bond with the atom. The remainder goes into
kinetic motion of the electron. The electron is now on the loose and can
cause biological damage by exciting and ionizing other atomic electrons.
In fact it is now the same thing which we used to call a beta ray.
A second way in which the gamma ray may interact with matter is by
_scattering_. In this case the gamma ray does not disappear but merely
loses a part of its energy to the atomic electron. Again the electron is
free to cause biological damage, while the gamma ray goes on to its next
encounter.
The third way requires that the gamma ray be near a nucleus and have an
energy greater than a million electron-volts. (Ordinary X-rays such as
are used in medical practice are not energetic enough for this process
to occur.) Under these conditions the gamma ray may disappear, with the
simultaneous appearance of an electron and a positron. This is an
example of the creation of matter out of pure energy. In accordance with
the formula E = mc², a part of the gamma-ray energy is consumed in
producing particles with definite masses. This amounts to about one
million electron-volts. The remainder of the gamma-ray energy goes into
kinetic motion of the two particles. Again biological damage results
from the subsequent ionization due to the charged particles. After the
positron has expended its kinetic energy in the ionization process, it
will join with an electron in a disappearing act. The energy reappears
in the form of two or three gamma rays (each having less energy than the
original gamma ray).
In no case is the gamma ray directly responsible for any biological
damage. The damage is always made by electrons (or positrons) to which
the gamma ray has transferred some or all of its energy. But this only
makes gamma rays the more dangerous. They can first penetrate to the
sensitive tissues of the body, and then cause ionization.
We have already mentioned that X-rays are the same as gamma rays. The
latter are produced by an excited nucleus, the former in the collision
of an electron (or a beta ray) with a nucleus. The man-made X-rays are
obtained by first accelerating a stream of electrons and then letting
them impinge on a target containing highly charged nuclei.
The usefulness of X-rays is, of course, due to their power of
penetration; that is the same property which renders X-rays dangerous.
One can use X-rays to find out what happens to be inside the human body.
But this cannot be done without producing some disruption and
rearrangement in the tissues which lie in the path of the X-rays. The
damage is of the same kind as that caused by radioactivity or cosmic
rays.
The effects of neutrons on matter are rather similar to the effects of
gamma rays. Like gamma rays, neutrons can travel long distances in
matter without interacting. On the average, a million-volt neutron goes
a few inches in water before having a collision of any kind. Also like
the gamma rays, the neutrons are not themselves directly responsible for
any biological damage. Being neutral, they interact only with the atomic
nuclei to which they are strongly attracted. By far the most important
of these interactions is with the nuclei of hydrogen. There are a great
number of these in living tissue in the form of protein and water
molecules.
The collisions with hydrogen nuclei (i.e., protons) are important
because a large fraction of the neutron energy is transferred in the
process. This happens because the neutron and the proton have very
nearly the same weight. If the neutron hits a heavy nucleus, it loses
only a small fraction of its energy in the impact.[9] After colliding
with hydrogen or a heavier nucleus, the neutron continues on to other
such collisions. The nucleus, however, being charged and energetic, now
causes excitation and ionization of atomic electrons. Thus, like gamma
rays, energetic neutrons are exceedingly dangerous, because they can
first penetrate and then cause ionization.
Neutrons are dangerous even when they are not energetic. A nonenergetic
neutron may react with nuclei of living matter in a number of ways of
which two are particularly probable. Either the neutron may be captured
by a proton to form a deuteron, in which case the excess energy will be
emitted in the form of a two-million-volt gamma ray that will cause
further damage. Or the neutron may react with a nucleus of nitrogen¹⁴
(abundantly present in living matter) to produce a nucleus of carbon¹⁴
and an energetic proton. Thus a nonenergetic neutron will have a
biological effect equivalent to an energetic gamma ray, or to an
energetic proton plus an energetic carbon¹⁴ ion.
In summary, all particles, charged or not, have a similar action on
matter. Directly or indirectly, they produce excited atoms, molecules,
and ion pairs. These processes always occur in practically the same
proportions, and therefore the number of ion pairs formed can be used as
a measure of the radiation effects. The more ion pairs produced in
living matter, the greater the extent of biological damage. For this
reason it is customary to describe radiation effects in terms of the
number of ion pairs created per gram of living tissue in various parts
of the body. Since each ion pair corresponds to an energy transfer of
about 32 electron-volts, an alternative description may be given in
terms of the amount of energy deposited. The unit in common usage for
this purpose is the _roentgen_, which means specifically an energy
equivalent to lifting the body (in which the radiation is deposited) by
one twenty-fifth of an inch. This is equivalent to about 60 million
million ion pairs in each ounce. It is less exact but more significant
to say that one roentgen deposits in a cell of our body a few thousand
ion pairs.
Of course the amount of ionization within individual cells is not a
quantity that is easily measured. What one usually knows instead, is the
roentgen dosage to a piece of tissue, which consists of many cells. If
the charged particles inducing the ionization are electrons (as they are
when the primary radiation is a beta ray or a gamma ray), the ionization
will be distributed more or less uniformly among the cells in the
affected neighborhood. If the charged particle is heavy—a proton or an
alpha ray—the density of ionization which it produces is much greater,
so that some cells receive a good many more ion pairs, while others
nearby may receive none. For this reason it is sometimes important to
specify not just how many roentgens the tissue has been exposed to, but
also which kind of radiation has been responsible.
In a later chapter we shall discuss the biological effects of various
amounts of radiation. We may mention here, however, that 1000 roentgens
of X-rays or gamma rays delivered more or less uniformly over the whole
body of a human being in a time less than a few hours or so, will lead
to almost certain death. And it is a remarkable fact that nature has not
provided us with a warning. Radiation does not hurt. The greater is the
need that we understand this process which affects our well-being but
not our senses.
CHAPTER IX
The Test
Testing of atomic explosives is usually carried out in beautiful
surroundings. There is a good reason for this: the radioactive fallout.
Because of the fallout, the test site must be isolated. The presence of
human population does not improve nature (with exceptions which are
quite rare and the more notable). Also, to keep the site clean, tests
must be carried out in the absence of rain. Therefore, at the site one
usually finds sunshine and solitude.
For the participants the beauty of nature forms the back-drop to
preparations of experiments which are difficult and exciting to everyone
involved. At the end, the atomic explosion is always dwarfed by its
setting. But the work that culminates in the detonation is rewarded by
something quite different from a flash and a bang.
The really important results of a test consist in marks on photographic
plates. Most of the apparatus that produced the plates has been
destroyed in the explosion. But enough is saved so that one can conclude
what has happened in the short fractions of a second that pass between
the pressing of the button and the knowledge in the observer: this was
it. In those fractions of a second another stone was added to the
structure which we may call astrophysical engineering. What happens and
what is observed in nuclear explosions are closely related to the
behavior of matter in the interiors of the stars.
The details of the nuclear explosion cannot be described here for three
reasons. First, the details are secret. Second, the size of this book
and the forbearance of the reader set limitations. And third, we
understand only a small part of the process. Within these limitations,
this is what happens:
The actual nuclear reaction takes only a fraction of a microsecond (one
microsecond = one millionth of a second). All the energy of the bomb is
released in this short period. At the end of this period, the main body
of the nuclear material is moving apart at a rapid rate and by this
motion further nuclear reactions are stopped. In addition to the more or
less orderly outward motion, considerable portions of the energy are
found in the disorderly temperature motion, which has stripped most of
the electrons off the nuclei and has transformed the atoms into a freely
and chaotically moving assembly of charged particles. By this time many
of the original nuclei have been transformed into nuclei of radioactive
species, partly by the fission process and partly by the capture of
neutrons in all sorts of atoms which had been originally present in the
bomb materials.
Still another portion of the energy is present as electromagnetic
radiation. This radiation closely resembles light except that it is of
shorter wave length and is therefore not actually visible; but it can be
absorbed and re-emitted by all sorts of materials, and is in a violent
exchange of energy with the exploded bomb fragments.
All this perturbation spreads outward from the region where the nuclear
reaction has taken place into the surrounding components of the bomb.
During the outward spread, more atoms and more space get engulfed. The
agitation and the radiation become somewhat less hot.
This hot region tends to be limited by a sharply defined boundary which
is called a shock front and which is moving outward at a speed of
several hundred miles per second. This front finally reaches the limits
of the more or less dense material in which the whole bomb structure was
originally encased. It then breaks through into the surrounding air. The
air heats up in the immediate vicinity, and this is the beginning of the
fireball.
From this point on, the energy spreads due to the push of the
high-temperature air. A sharp shock front forms and keeps moving outward
at a speed greatly surpassing ordinary sound speed. The radioactive
material is contained within this hot and expanding sphere.
As the fireball expands and the temperature falls, more and more visible
radiation is emitted. Actually, the surface is growing less brilliant as
the structure expands and cools, but its greater size and the longer
time that is available for the emission of radiation overcome this
disadvantage. Finally, at a radius of perhaps a few hundred feet for a
small bomb and a mile for a big one, the fireball expansion halts. This
happens because the shock front is no longer strong enough to make the
air luminous. The luminosity not only stops advancing but is actually
partly dimmed by absorbing substances formed by the badly mistreated air
molecules.
The time which has elapsed to reach this stage of the explosion depends
on the bomb energy. If two explosions are compared, and the bigger one
has a thousand times the explosive power of the smaller one, then the
time needed to reach the extreme expansion of the fireball will be
approximately ten times greater for the more violent event. In any case,
a reasonably close observer has to use strongly absorbing glasses during
this time if he is not to be blinded. For small bombs, the expansion of
the fireball is too short to register. For the really big ones, you can
see the expansion developing and you wonder when it will stop. To the
unprotected eye the small bombs are almost as dangerous as the big ones,
because there is not enough time to blink.
In the meantime, the shock wave, now separated from the fireball,
travels through the air and carries with it a considerable fraction of
the original explosive power. An important part of the damage which a
bomb can cause is due to this invisible pressure wave which spreads with
a speed close to that of sound, over a distance of miles, before it
settles down into harmless rumbling.
The rest of the energy is still sitting in the fireball near the point
where the explosion occurred and the hot air now commences to ascend,
breaking up into a turbulent mushroom as it goes. The hot interior
portions get occasionally exposed and the object gives the appearance of
an enormous flaming mass, at least when seen in a motion picture which
slows down the action and reduces the size. The radiant tongues are too
big and too fast for any ordinary flames.
During this stage the display gradually pales sufficiently so that it
can be viewed with the naked eye. The originally hot masses have now
emitted enough energy in the form of light and mixed with a sufficiently
great mass of cool air that they no longer glow violently. This mass of
central and rising gas contains practically all the radioactivity, not
only that originally formed in the explosion but also some produced by
neutrons which leaked out of the bomb and got captured by a variety of
nuclei in the air, water, or ground within the neighborhood.
And now the aftermath of the explosion is turning into a display growing
rapidly and yet in a measured manner so that not only the eye of the
observer but his mind and his feelings can follow the events. The
mushroom which has been formed by the first updraft develops into a
column with more and more agitated boiling masses added on the top and
with slanting skirts of a snowy appearance descending toward the sides.
What is this white mass that looks just like a cloud of peculiar shape
and that has grown up to the high heavens (or as the meteorologists call
it: the stratosphere) in a few minutes before our eyes?
It is actually a cloud: a collection of droplets of water too small to
turn into rain but big enough to reflect the white light of the sun. And
it is formed in a similar way to the cumulus clouds of a thunderstorm.
Indeed it is a beautiful example of a many-storied castle of cumulus
upon cumulus. But strangely enough what makes this cloud is not the heat
of the bomb. It is the cooling of the air masses that have been sucked
in as the remnants of the fireball rush upward like a giant balloon.
Under this balloon air is drawn upward. As this air rises, it cools and
water vapor contained in it condenses into droplets: precisely the same
mechanism which gives rise to thunderheads on a hot summer day.
The white skirts (which are not always present) do not consist of any
material that is falling out of the cloud. On the contrary, a moist
layer of air is sucked up into the cloud from the side and the droplets
which form in this layer give rise to a cloud-sheet with the appearance
of a skirt.
In big bombs near the top a particularly smooth and white cap is seen.
This is again condensation, not into droplets but into fine crystals of
ice. In some explosions more than one of these caps are present.
Finally the cloud has gained its full height. Depending on the size of
the bomb it may have grown to 20,000 feet, to 100,000 feet or more. Then
the wind blowing at various levels in various directions tears the
structure apart sweeping some of it to the east, some to the west. The
radioactive debris in the cloud has started on its travel.
What this radioactivity will do, how it can affect living beings, how
dangerous it actually is, we shall discuss in succeeding chapters. But
one thing is clear and remains present in the minds of all participants
in an atomic test: The danger of the test is nothing compared to the
catastrophe that may occur if great numbers of these weapons should be
used in an unrestricted nuclear war.
It has been frequently asserted that our present atomic explosives can
wipe out the cities and industries of the greatest countries. Why
continue with further development and testing?
The answer is simple: The main purpose of a war is not to destroy the
enemy’s civilian centers but rather to defeat his armed forces, and for
this purpose we need flexible refined weapons of all kinds and sizes. We
also need weapons with which to defend our own cities. We need weapons
with which to defend our allies and in particular we need weapons which
will do their job against an aggressor and will do the least possible
damage to the innocent bystander.
In this last respect, in particular, notable progress has been made. We
are developing clean weapons which are effective by their blast and
their heat, but which produce little radioactivity. Of course, blast and
heat will do damage only near the point of detonation. Radioactivity may
be carried by the winds and escape the control of man to a considerable
extent.
It is clear that war is and always has been terrible. We refuse to
believe that wars will always be with us but we cannot disregard the
danger of war as long as the world is half free and half slave.
An atomic war, limited or even unlimited, need not be connected with
more suffering than past wars. However, such a war would probably be
more violent and it would be shorter.
The story is told that a war which turned out to be perhaps the most
dreadful in the history of mankind was started with this message: “Thou
hast chosen war. That will happen which will happen and what is to be we
know not. God alone knows.” Perhaps the only possible path for a free
people is to be well prepared for war but never to choose war as long as
the choice is free. But what will happen God alone knows.
CHAPTER X
The Radioactive Cloud
In February 1954 preparations were made on Bikini Atoll for the
explosion of a hydrogen bomb. March 1 was the “ready” date. It did not
seem probable that the shot would actually be fired on that date because
the shot could be fired only under quite favorable wind conditions.
Large amounts of radioactivity, especially fission products, were
expected from the explosion. The shot could be fired only if no
inhabited places lay in the downwind direction.
Bikini is an oval-shaped coral reef, an atoll. It is one of several such
atolls belonging to the group called the Marshall Islands. If you look
at the map, you will see that west of Bikini at a distance of 200 miles
lies Eniwetok, on which our people were making preparations for further
tests.
To the east of Bikini, a hundred miles or so, is Rongelap Atoll. At that
time 64 people were living there. They lived primitively in palm houses
on the southern part of the atoll. The northern part was uninhabited.
On nearby Ailinginae Atoll 18 of the Marshallese islanders were on a
fishing expedition, while farther to the east on Rongerik 28 American
servicemen were stationed. The servicemen lived and worked in aluminum
huts. Their main job was to collect weather data.
[Illustration: Map of the Marshall Islands]
Much farther to the east, 300 miles from Bikini, is Utirik. One hundred
and fifty-seven Marshallese people lived on this atoll.
Early on the morning of March 1, a Japanese fishing boat lay somewhere
to the north of Rongelap. Her name was Fukuryu Maru, which means in
English the Fortunate Dragon. There were 23 men on board. Actually she
was in a patrolled zone but had not been sighted by the patrol aircraft.
Operations for the test were being directed from ships of Joint Task
Force 7. For several days prior to the morning of March 1, the
weathermen had been mapping the winds. A wind to the west would be bad
for Eniwetok. A wind to the east might hurt Rongelap and Rongerik. A
wind to the south could affect Kwajalein. The ideal direction would have
been due north, but this probably would not happen for months. On “shot”
morning the wind was blowing to the northeast. The meteorologists gave
their “O.K.” It was at dawn, the first of March, 1954.
The firing crew of nine people led by a man of considerable experience,
Jack Clark, were responsible for the final arrangements. They were in a
blockhouse on the south side of the atoll 20 miles from the bomb.
Others, more than 1000 people, watched from shipboard under the
direction of Al Graves, who was responsible for the technical phases of
the operation. The ships lay south and a little east of Bikini.
The firing mechanism was set into operation in the blockhouse. One after
another signals indicated that the various experiments and observations
were set to work. Finally a red light went off and a green light
appeared on the panel. This meant that the bomb had been detonated.
The men on shipboard watched the enormous fireball through darkened
glasses. The firing crew, sealed off in the blockhouse, saw nothing. A
couple of long seconds and Graves’ voice announced over their radio: “It
was a good shot.” A quick estimate indicated 15 megatons.
Some more slow seconds and the expected ground shock arrived. It was
like a big earthquake. A bad moment passed. The blockhouse rocked but
held.
Another minute or so and the air shock passed over. One could hear the
hinges groan—but this was no longer frightening.
Would the water wave pour over the blockhouse? Everything was
watertight. After fifteen minutes a porthole was opened—no water came
in. The men in the blockhouse emerged to look at the drifting atomic
cloud.
While they watched, Jack Clark’s radiation instrument began to show a
reading. The firing crew was called back into the blockhouse. There, in
the lowest corner shielded by a considerable amount of sand, they were
safe. Outside, the evaporated and condensing coral came down in pellets
carrying more and more radioactivity.
In the meantime there was fallout on the ships too. The wind had
definitely veered after shot time. Quickly the activity was washed down.
No one got a dangerous exposure. But it was wiser to sail away. A
message was sent to the blockhouse: “We will come back for you in the
evening.”
After a little more than an hour the activity around the blockhouse
started slowly to decrease. The firing crew waited patiently inside
without communication, without light for the rest of the day.
Finally the ships came back. At sundown a helicopter went out to the
island using the last of daylight and allowing as much time as possible
for the activity to decay. Clark and his friends rushed out of the
blockhouse wrapped in sheets to stop the beta rays and keep off the
radioactive dust. They moved as fast as possible to avoid unnecessary
exposure.
It was a hard experience but they got no more than two roentgens—no more
reason to worry than if they had had a medical X-ray. Toward the east,
however, some people were in real trouble.
Six or seven hours after the shot the American servicemen on Rongerik
noticed a mistlike fallout of highly radioactive dust. The wind had
veered enough to carry the atomic cloud over the occupied islands of
Ailinginae, Rongelap, and Rongerik. In the anxious hours which followed
no one could say how much damage had been done.
The Americans on Rongerik had had some education in the dangers of
radioactivity. They washed themselves, put on extra clothes, and
remained inside of the aluminum huts as much as possible. These actions
helped to protect them against beta ray burns on the skin. The
Marshallese on Rongelap and Ailinginae knew nothing of the danger and
took no precautions. Many of them suffered quite severe skin burns.
All of the exposed persons were evacuated to Kwajalein as soon as the
Task Force facilities would permit. But it was not until a week or so
after the explosion that arrangements could be made for men with
radiation measuring instruments to tour the atolls and determine what
the levels of exposure had been.
On the southern tip of Rongerik they measured the activity and
calculated that the American servicemen had received approximately 78
roentgens. This was good news because a dosage of 50 to 100 roentgens is
not lethal and only in rare cases leads to any sickness. In any event
full recovery could be expected within a few days.
As they prowled around Rongerik atoll, the measuring crew found places
where the radiation levels had been much higher. At the northern end a
person would have received more than 200 roentgens.
On Ailinginae the measured values were comparable to those on Rongerik.
The estimated dosage to the Ailinginae people was 69 roentgens.
On Rongelap the situation was much worse. Measurements in the southern
part of the atoll showed that the Rongelap people had gotten a dose of
about 175 roentgens. Such a dose would not be fatal, but at least some
of the people would probably be sick.
The crew then went on to explore the rest of the atoll. As they moved
north, the dose levels rose higher and higher. In the middle of the
atoll, only ten or fifteen miles from the inhabited part, a person would
have received 400 roentgens of radiation. At this level he would have a
fifty-fifty chance of surviving.
On the northern tip of the atoll, about thirty miles away, the dose
would have been over a thousand roentgens. Such a dose means certain
death in less than a month.
The following table contains a summary of what happened:
_Number _Time of _Time of _Dose
of fallout evacuation (roentgens)_
persons_ after after shot
shot (hours)_
(hours)_
Rongelap 64 4 to 6 51 175
Ailinginae 18 4 to 6 58 69
Rongerik 28 7 32 78
Utirik 157 22 65 14
Fortunate Dragon 23 4 200
On Kwajalein the Marshallese were cared for and underwent medical
observation. As soon as possible their skin and hair were scrubbed with
soap and water. The coconut oil in their hair made decontamination
difficult.
During all this time the presence of the Japanese fishing boat in the
area was not even suspected. Not until two weeks after the explosion,
when the little boat returned to Yaizu harbor, did the world find out.
By this time the 23 fishermen were pretty sick. We do not know precisely
what dose the fishermen received, but the best guess is about 200
roentgens. Unhappily, one of the fishermen died, presumably from
complications associated with the exposure to radiation.[10] The other
22, however, are in good health and back at work.
Our medical information on the Marshallese islanders is complete. After
staying three months on Kwajalein they were removed to Majuro atoll,
where homes were built for them and where they have been cared for and
under continuous surveillance since the incident. Frequent and thorough
medical examinations have been conducted, handicapped somewhat by the
problem of communicating through an interpreter.
In the first twenty-four hours some of the victims complained of nausea,
fever, and stomach-ache. But these symptoms abated promptly in every
case without treatment. There was also some complaint of skin itching
and a burning sensation, but these symptoms also lasted only a couple of
days. Then followed a week or so of comfort and no complaint. After that
skin lesions and loss of hair began to occur.
Fifty to eighty per cent of the beta rays during the exposure period had
an average energy of 0.3 million electron-volts. Much of this energy was
stopped in the outer layer of skin, which is two thousandths of an inch
thick. The remainder of the beta rays had an average energy of 0.6
million electron-volts; these beta rays could easily penetrate into the
deeper layer of live skin. The most important fact, however, was that
clothing of any kind, even a thin cotton fabric, provided protection
against all the beta rays. Lesions developed only on the exposed parts
of the body and in a few other places such as the armpits and the
creases of the neck where material tends to accumulate. Bare feet were
especially bad. During the acute period some of the people walked on
their heels.
At the end of six months lost hair had grown out again unaltered in
texture and color, and the skin lesions had healed. Everyone appeared
healthy and normal with no apparent after effects.
There had been four pregnancies amongst Rongelap women at the time of
the exposure. One baby was born dead, but the other three were quite
normal. There was no evidence that the stillbirth had been due to
radiation effects. In fact the percentage of stillbirths amongst the
Rongelapese is normally high. Statistically, one in four is not an
unusual ratio.
Today, more than three years since the accident, all of the Marshallese
and American victims seem to be fully recovered. No malignancies or
leukemias have shown up, but these long-term effects are still being
carefully watched for by an AEC medical group.
All in all some serious but limited harm has been done. It was a close
shave. To see how close, one only needs to glance at the map below,
which shows the roentgen dosage for 48 hours of exposure. At the
southern tip of Rongelap, where the inhabitants lived, the dosage was
175 roentgens. But at the northern tip, less than thirty miles away, the
dosage was more than a thousand roentgens. If the wind had veered just a
little bit farther to the south, probably all of the people on
Ailinginae, Rongelap, and Rongerik would have been killed.
[Illustration: Dosage in First 48 Hours After Fallout Began]
This shot proved what had been argued for many years: that radioactivity
is not just an incidental part of an atomic explosion. The people on
Rongelap were far outside the area of danger from blast and thermal
effects. But they received a sizeable dose of radiation. In fact, a
person could have stood unprotected at a distance of thirty miles from
the explosion and been perfectly safe from the blast and thermal
radiation. But at that same distance in a downwind direction he would
have accumulated a lethal dose of radiation within a matter of minutes
after the fallout began.
Because of the radioactive fallout, the test sites must be located in
remote parts of the world. It would be desirable if sites could be found
which are so remote from populous areas that the tests could be
conducted without regard to the direction of the winds. Unfortunately
the bombs are too big and the planet is too small.
As a result the winds must be watched before every test; and the tests
must be delayed until the winds are favorable. What happened to the
Marshallese was an accident which might have been avoided if the winds
had been blowing more directly toward the north at shot time. Since this
accident the wind requirements for the tests have become far more
stringent, our knowledge of the danger has increased, and the rules of
safety have in all respects improved. Many large yield weapons have been
tested since March 1, 1954, but no other accidents have occurred. We can
be confident that accidents of this kind are now very improbable.
At the U. S. test site in Nevada there has been no instance of a major
fallout on a populated area. Probably the most worrisome situation which
has occurred there was in the spring of 1953 during the Upshot-Knothole
test series. After the ninth shot of the series the cloud drifted
eastward over St. George, Utah, a town of about 5000 people. Some
fallout occurred shortly before nine o’clock in the morning. About
nine-thirty AEC officials issued a warning advising the residents to
stay indoors. By noon the warning was withdrawn and people were allowed
to continue with their normal affairs. The incident left everyone a
little bit scared, but no one had received a radiation dose greater than
two or three roentgens.
We have been talking about the local fallout which occurs within a few
hundred miles of the test site. Not all the radioactivity which is made
in the explosion goes into this fallout. Some of it travels for really
long distances, not hundreds but actually thousands of miles from ground
zero. This part of the radioactivity is disseminated world-wide and
completely escapes the control of man. To be sure, by the time this
radioactivity is distributed over a large fraction of the earth’s
surface, the dosage levels of radiation are very tiny, less than a ten
thousandth of a roentgen for a megaton explosion. There is no danger
whatever that a person would die or even become mildly sick from this
amount of radiation. There is, however, the possibility of long-range
effects such as bone cancer, leukemia, and genetic mutation.
The world-wide danger is, of course, primarily due to the big bombs. The
little ones, such as are tested in Nevada, release about ten kilotons
(TNT equivalent) of fission energy. Some of the big ones in the Pacific
release a few megatons of fission energy. Since the amount of
radioactivity is proportional to the fission energy released, one big
bomb is equivalent to several hundred or possibly a thousand little
ones. Altogether in Nevada, to date, there have been only sixty or
seventy shots. It may be desirable to minimize the world-wide fallout
from the big shots in the Pacific. But for the little shots in Nevada,
it is probably more important to minimize the local fallout. How much
radioactivity goes into the local fallout, how much into the world-wide,
and how these relative amounts can be controlled, are the main topics
for the remainder of this chapter.
Not all the radioactivity which is made in the explosion contributes to
the fallout, either local or world-wide. Some of the radioactive fission
fragments (gamma emitters) have such short half-lives[11] that they
actually disintegrate before the bomb has disassembled. A great many
others disintegrate in the first few minutes while the atomic cloud is
rising. The energetic beta and gamma rays released in these early, rapid
disintegrations are stopped in short distances and merely add to the
havoc at the scene of the explosion.
[Illustration: _USAEC—Joint Office of Test Information_
1. A shallow underground explosion. The radioactivity and the ground
dirt are thoroughly mixed.]
[Illustration: _USAEC—Lookout Mountain Laboratory, USAF_
2. An atomic test tower—five hundred feet high.]
[Illustration: _USAEC_
3. A tower shot. Ground dirt rises along the stem, but very little
actually mixes with the fireball.]
[Illustration: _Elton P. Lord—USAEC_
4. An air shot—3,500 feet above ground. No dirt.]
[Illustration: 5. Leg bone of a three-month-old rabbit killed ten
minutes after injection of Sr⁸⁹. The darkened areas show where the
strontium has been deposited. Sr⁹⁰ and normal Sr⁸⁸ would be
deposited in the same places. It is an important fact that the
deposition is fairly uniform in the calcified portions of the bone.
_From a chapter by Vaughan, Tutt, and Kidman in the book_ Biological
Hazards of Atomic Energy, _edited by Haddow, published by Oxford
University Press, 1952_]
[Illustration: 6. Leg bone of a woman who died of radium poisoning.
The bright regions show where the radium has been deposited. Hot
spots are clearly visible.
_From an article, “The Late Effects of Internally Deposited
Radioactive Materials in Man,” by Aub et al., in_ Medicine—_a
professional journal, Vol. 31, No. 3, September, 1952_]
[Illustration: _USAEC—Knolls Atomic Power Laboratory_
7. Capsules of cobalt⁶⁰, shielded in a water tank. One hundred and
thirty million dollars’ worth of radium, twice the world’s present
supply, would be needed to equal the rays from this powerful gamma
source.]
[Illustration: _USAEC_
8. Cobalt irradiation.]
[Illustration: 1. The metallic element cobalt is machined into
wafers slightly larger than a dime.]
[Illustration: 2. The wafers are placed edge to edge in aluminum
containers, then inserted into an atomic furnace, or reactor.]
[Illustration: 3. Under bombardment of neutrons, the nuclei of the
cobalt atoms become excited and emit radiation, or rays.]
[Illustration: 4. After “cooking” in the reactor a certain time, the
cobalt is removed and placed in shielded containers for shipment.]
[Illustration: 5. The now radioactive cobalt goes from the Savannah
River Plant to Oak Ridge for re-shipment to medical centers all over
the country.]
[Illustration: 6. At medical centers, it is placed in tele-therapy
machines. Its powerful rays aid medical specialists in the fight
against cancer.]
[Illustration: _NTO—Lookout Mountain Laboratory Photo_
9. The smoke-ring cloud from the air-defense atomic weapon.]
[Illustration: _Wide World Photo_
10.]
[Illustration: _University of California Radiation Laboratory_
11. The streaks are condensation trails produced by charged
particles in a Wilson Cloud Chamber. They appear bright because the
chamber is illuminated and the condensation trails reflect light
just as an ordinary cloud does.]
[Illustration: _University of California Radiation Laboratory_
12. Another picture in the Wilson Cloud Chamber. A large number of
closely-spaced tracks form a cloud. (The tracks are curved because
of the presence of a magnetic field.)]
[Illustration: _USAEC—Argonne National Laboratory_
13. Cutaway section of a nuclear reactor. The heart of the reactor
is a small region at the center where the fission energy is
generated. Most of the weight and volume are needed for cooling
apparatus and shielding material to keep in nuclear radiation.]
For the radioactivity to affect areas at a large distance from the point
of the explosion, considerable time must elapse while the atomic cloud
rises and drifts in the horizontal winds. During this time more
disintegrations occur, due mainly to the short-lived nuclei. The rate at
which they occur keeps diminishing as the short-lived nuclei disappear.
Roughly speaking, the rate diminishes simply in proportion to the time.
More precisely, the rate drops somewhat faster, decreasing by a factor
of ten when the time increases by a factor of seven. A minute after the
explosion the activity is less than one per cent of what it is at a
second. After an hour it is less than one per cent of its value at a
minute. This law for the decrease in activity of fission products is, of
course, quite different from the simple law of radioactive decay. The
latter law applies to a single radioactive species. The fission products
consist at any instant of many different radioactive species. Each one
obeys the simple law of radioactive decay, but the totality follows a
different law.
It should be kept in mind that the product nucleus of a radioactive
disintegration may itself be radioactive with a different half-life. For
example, there is strontium⁹⁰. Only a small amount of this isotope is
made directly in the fission process. The fission process yields large
quantities of krypton⁹⁰, which decays with a half-life of one-half
minute into rubidium⁹⁰. The latter has a half-life of three minutes and
decays into strontium⁹⁰. This is how practically all of the strontium⁹⁰
is made in the explosion. Thus both the intensity and the nature of the
radioactivity keep changing with time.
These facts are important because they determine the magnitude and the
character of the danger when the radioactivity finally falls out of the
cloud and is deposited on the surface of the earth. Those radioactive
particles which disintegrate while still in the cloud need not worry us
since this radiation can have no effect on living organisms that may be
underneath. Provided that the cloud is more than a few hundred feet
above the ground, the beta and gamma rays released in these
disintegrations merely dissipate their energy in ionizing the air.
The time which the radioactive debris spends in the cloud depends most
critically on one factor: the proximity of the explosion to the ground
surface. The nature of the surface, whether it is soil or water, also
plays a role. If the explosion has taken place right on the ground, on a
soil surface, a lot of big, heavy dirt particles become incorporated
into the fireball and begin to fall under the action of gravity even
before the cloud stops rising. This fallout continues for a period of
several hours to perhaps a half day. At the same time some of the
radioactive fission products which have adhered to these dirt particles
also fall out. This is the origin of the so-called close-in or local
fallout, which extends for a distance downwind of the explosion of a few
miles to a few hundred miles, according to the energy of the bomb and
the strength of the winds. Approximately eighty per cent or so of all
the fission products are accounted for by this close-in fallout in the
case of a surface explosion. The shot on March 1, 1954 was of this
variety.
There are several possibilities for influencing the amount of close-in
fallout. One is to explode the bomb over deep water. In this case the
close-in fallout amounts to between thirty and fifty per cent. This is
because many of the water drops to which radioactive particles have
adhered evaporate before they hit the ground. Over shallow water,
however, if the fireball actually touches the bottom, the close-in
fallout resembles the case of a land explosion and is again about eighty
per cent or so. The close-in fallout for underground or underwater
explosions will be even higher than for the surface explosions. In fact
a really deep underground or underwater explosion would be completely
contained and no activity would be spread around.
Another possibility for reducing the close-in fallout is to detonate the
bomb on a tower so tall that the fireball cannot touch the surface. In
this case the amount of close-in fallout is reduced from eighty per cent
to approximately five per cent. Of course, it is not feasible to build
towers for really big bombs whose fireballs may be a mile or so in
diameter. In this case the bomb might be dropped from an airplane to
produce the same effect. The Hiroshima explosion was an example of an
air burst of a small bomb. The close-in fallout in that case was very
small. Such radiation sickness as occurred there was due to the direct
gamma rays and neutrons released in the explosion itself.
In the case of a near-surface explosion, where the fireball almost
touches the ground, the close-in fallout is also only about five per
cent. This is a somewhat surprising fact since in this case photographs
show large quantities of surface material being sucked up into the
cloud, just as they are in a true surface explosion.
This material certainly consists of large, heavy dirt particles which
subsequently fall out of the cloud. Yet most of them somehow fail to
come in contact with the radioactive fission products.
This peculiar phenomenon can be understood by looking at the details of
how the fireball rises. At first the central part of the fireball is
much hotter than the outer part and thus rises more rapidly. As it
rises, however, it cools and falls back around the outer part, creating
in this way a doughnut-shaped structure. The whole process is analogous
to the formation of an ordinary smoke ring. In most of the photographs
one sees, the doughnut is obscured by the cloud of water that forms, but
sometimes when the weather is particularly dry, it becomes perfectly
visible. During the rather orderly circulation of air through the hole,
the bomb debris and the dirt that has been sucked up remain separated.
(See pictures 1-4.)
The close-in fallout accounts for only a portion of the radioactivity,
ranging from less than a per cent for a high altitude shot to almost
complete deposition for some ground shots. For the world-wide fallout we
are interested in what happens to the remainder. This depends on how the
atomic cloud is carried by the upper winds for long distances. In this
connection it is important to distinguish between a big bomb and a
little bomb. It is also important to distinguish between the lower and
higher portions of the atmosphere called, respectively, the troposphere
and the stratosphere.
The atmosphere is heated by the sun in an indirect way. The sun’s rays
pass through air without warming it. They heat up instead the bottom of
the atmosphere, that is, the solid ground. The atmosphere is heated in
the same manner in which a boiling pot is heated on the kitchen range.
The heat is delivered from below and is carried in rising currents to
the top.
Only in the case of the atmosphere there is no sharp upper limit. The
currents rise to an altitude of thirty to fifty thousand feet, then turn
and descend. This boiling part of the atmosphere is called the
troposphere or region of heat. Above it there is less vertical motion.
The upper region is called the stratosphere or stratified region.
For a little bomb the atomic cloud stops rising before it reaches the
stratosphere. For a big bomb, above about a megaton of energy (a million
tons of TNT equivalent), the cloud pokes right into the stratosphere and
keeps going to a height of a hundred thousand feet or so.
The most important fact about the stratosphere is this: It has very
little weather. Most of the weather phenomena such as clouds, rain,
snow, fog, mist, etc., are confined to the lower portion of the
atmosphere, the troposphere. The stratosphere, however, contains
practically no water.
Now suppose a little bomb whose cloud will remain in the troposphere has
been exploded at one of the United States test sites. The Nevada test
site is at a latitude of 37°N and the Pacific test site at 12°N. In
these middle latitudes, in the troposphere, the winds blow mainly from
west to east with an average speed of approximately 20 miles an hour.
There will be a slight southerly or northerly motion on top of this. But
by and large the radioactive cloud will stay in a pretty narrow band
around the latitude at which the explosion took place.
After the first few hours, when the close-in fallout has dwindled, the
radioactive particles remaining in the cloud are too light and too fine
to fall any more under the action of gravity. At this point the weather
becomes important. Rain, fog, or mist captures the radioactive
particles, and returns them to the ground in the rainfall. This results
in the so-called tropospheric fallout.[12] The average time for this
fallout to occur is approximately two weeks to a month. During this
time, while staying more or less in the latitude of the explosion, the
radioactive particles may actually have encircled the earth.
The clouds of the big bombs rise high into the stratosphere. The winds
in the stratosphere do not blow so predominantly in a latitudinal
direction. What is more important, they stay in the stratosphere for
years, in which time the radioactivity is distributed to all areas of
the globe. The fallout from the big bombs is thus really world-wide.
The tropospheric fallout takes about a month. The stratospheric fallout
takes 5 to 10 years. The reason for this difference is the weather, or
rather the lack of it. In the stratosphere there is no rain or fog to
catch the radioactive particles and hence no effective mechanism for
producing the fallout. In fact, since the radioactive particles are too
fine to fall by gravity, they must simply wait until some turbulent
motions impel them downward back into the troposphere. This process
requires a long time.
That rainfall is the most important mechanism for producing the
world-wide fallout has been shown by examining the fallout in certain
dry regions of southern California and South America. In every case the
fallout was found to be considerably sub-normal. In one place in Chile,
where there is never any rain, the fallout was found to be only one per
cent of what might be expected on the basis of the average fallout at
the same latitude.
In regions having at least a few inches of rain per year, the fallout
tends to be proportional to the rainfall on the average. However, the
proportionality to rainfall depends on the nature of the weather so
that, say, twenty inches of rain in one part of the world may not give
as much fallout as the same amount of rain in other weather zones. We
are rapidly learning about this.
Having said what the age is of the various kinds of fallout, we are in a
position to say which radioactive species are still present when the
radioactivity is deposited on the ground. The close-in fallout, being
only a few hours old, still includes many short-lived isotopes, which
disintegrate before there is a possibility of ingestion or inhalation
into the body. Consequently the danger from the close-in fallout results
from external exposure, mainly to gamma radiation on the whole body, and
to a lesser extent to energetic beta rays on the skin. Clothes and
ordinary housing provide relatively little shielding against gamma rays.
Special protective shelters are needed. During a war if the enemy were
to bomb our cities with super-megaton weapons surface-burst, the
close-in fallout would be a far greater agent of destruction against an
unsheltered populace than either blast or thermal radiation.
In the stratospheric world-wide fallout, however, all of the short-lived
radioactivity has disappeared, since a period of many years has elapsed
since the explosion. After a year or so the only gamma emitter which is
left in appreciable quantity is cesium¹³⁷, with a half-life of 30 years.
Its gamma ray, however, is not very penetrating. In spite of this fact
cesium¹³⁷ is considered to be the second most important hazard for the
long term fallout. The first is strontium⁹⁰, which is a beta emitter
with a half-life of 28 years. This is long enough so that most of these
nuclei will still be present even after spending a long time in the
stratosphere. Since strontium is chemically similar to calcium, it
contaminates our foodstuffs and is easily incorporated into our bodies.
Once inside it stays for long periods of time, deposited in our bones.
We shall see in a later chapter how serious this danger may be.
The tropospheric fallout, and to a lesser extent, the stratospheric,
includes some other radioactive species besides cesium¹³⁷ and
strontium⁹⁰, and we shall discuss these in the next chapter. But by and
large they are of little consequence (with the possible exception of
iodine¹³¹) either because they are not easily absorbed in the body or
else because their radiation is not very energetic. The world-wide
hazard is thus narrowed down to just two isotopes, an internal beta
emitter and a weak gamma emitter.
CHAPTER XI
From the Soil to Man
There is a bewildering variety of radioactive products deposited in the
fallout. Given certain conditions all of them could be dangerous to man.
Actually, very few are.
An example of a radioactive isotope which is produced in large quantity
by the fission process and about which there is some reason to worry,
but actually is not dangerous to man, is iodine¹³¹. This isotope in the
fallout is not dangerous because it has a rather short half-life: eight
days.
During the first weeks after a nuclear explosion some radioactive iodine
may fall out of the cloud and contaminate grazing land. A cow eats
hundreds of pounds of grass in a few days time. Now iodine is found in
the cow’s body or in the body of any mammal mainly in one spot. This is
the thyroid gland located in man near the Adam’s apple. The thyroid
gland is important because it secretes a chemical which regulates many
of the body functions. In man, these include how we burn up our food and
in what mood we are. About twenty per cent of all the iodine which is
taken up, whether radioactive or natural, is concentrated in this one
rather small gland. Such a concentration is precisely the kind of danger
for which we must watch.
Shortly after nuclear tests, cows that graze on range land have been
found with abnormally large amounts of radioactive iodine, although not
so large as to be harmful. In human beings, however, the measured levels
of radioactive iodine are less than a hundredth of what they are in the
cows because by the time this radioactive isotope has reached man, it
has mostly decayed into a stable, harmless variety of xenon gas.
There are many potentially dangerous isotopes in the radioactive debris
of a nuclear explosion. But most of them decay too soon to affect man.
Isotopes which live an extremely long time compared to the human
life-span are also not dangerous to man. A radioactive particle in the
body is not harmful unless it disintegrates and releases its energy
while the individual is still alive.
Two examples of long-lived radioactive isotopes, which are used as fuel
in the bombs and which may be left over from the explosion in large
quantities, are: uranium²³⁵ and plutonium²³⁹. Uranium²³⁵ has a half-life
of 710 million years, which is much too long to be dangerous. Plutonium
has a half-life of 24,000 years and is somewhat more dangerous. The
danger from plutonium arises because it emits an energetic alpha ray.
The danger from radioactivity depends on the kind of particle
emitted—alpha, beta, or gamma rays—and whether these rays attack the
body from the inside or the outside. From the outside the gamma rays are
the most dangerous and the alpha rays the least dangerous. From the
inside the order is just reversed.
To cause damage from the outside the radiation must be very penetrating.
Gamma rays can go through the whole body. Beta rays are stopped in the
skin tissue. Alpha rays cannot even penetrate the outer layer of
non-living, protective skin.
On the inside, however, in the sensitive organs, the short range of the
alpha rays makes them exceedingly dangerous. Their energy is
concentrated in a small amount of tissue to which damage is severe. The
beta rays cause a slightly less concentrated damage, and the gamma rays
the least concentrated of all.
Radioactivity may enter the body as contamination in the food we eat or
in the air we breathe. To be dangerous, however, it must remain in the
body, either in the intestines or the lungs or in other vital organs,
long enough for disintegrations to occur, which will ionize and injure
the living cells.
Fortunately, plutonium in our food is easily excreted from the body.
Only a few thousandths of a per cent of what is eaten, is actually
absorbed. If inhaled, large particles are stopped in the nasal passages.
Small particles get into the lungs but are quickly exhaled. Only
intermediate sized particles are absorbed. However, the plutonium which
is absorbed generally gets laid down in the bones, where it stays for a
long period of time. Altogether, plutonium in the small amounts we
usually deal with is not one of the greater dangers to human beings.
Perhaps its most disagreeable property is that, being an alpha emitter,
it is not very easy to detect. Since alpha particles do not penetrate
through the surface of most radiation meters, special instruments are
needed to find them.
Two fission products which are readily absorbed upon ingestion are:
strontium⁹⁰ (Sr⁹⁰) and cesium¹³⁷ (Cs¹³⁷). Depending somewhat on their
chemical form, approximately thirty-five per cent of the Sr⁹⁰ is
absorbed, and all of the Cs¹³⁷ is absorbed. Both of these isotopes are
plentifully made in the fission process. Moreover they have very
“dangerous” half-lives—about 30 years—which is long enough so that decay
is negligible between the explosion and contact with man, but short
enough so that decay is probable after contact.
From such arguments as these one concludes that Sr⁹⁰ and Cs¹³⁷ are the
most important isotopes for the internal hazard from the world-wide
fallout. One can be reasonably sure that there are no others of
importance, because careful and extensive research has not found
significant amounts of any in our bodies. We need not fear that one has
been overlooked, because the beta activity of the fission products is
always easy to detect.
The two main questions which we have to answer are these: In what
precise way will the dangerous elements Sr⁹⁰ and Cs¹³⁷ be distributed in
the body? And after they are distributed, what kind of damage will they
produce?
We know too little about the chemistry of the living body to obtain a
complete answer to the second question. Hence it has to be admitted that
the actual danger cannot be stated in a precise way.
Fortunately, enough is known from direct experience to obtain a good
value for the greatest damage that might be produced. In the present
chapter we shall describe what is known about the uptake of the
dangerous elements into the body. In following chapters we shall turn to
the question of the biological consequences.
We may begin by comparing the danger from Cs¹³⁷ with that from Sr⁹⁰.
Both of these isotopes are made in the fission process in about equal
numbers. (Roughly 2 or 2½ per cent of all the fission products are Sr⁹⁰,
and 3 per cent Cs¹³⁷.) They have approximately the same radioactive
half-lives. But they differ in an important respect: The Cs¹³⁷ is
deposited more or less uniformly throughout the body; the Sr⁹⁰ is
concentrated in the bones.
Cs¹³⁷ emits a large part of its radioactive energy in the form of a
gamma ray, which causes ionization uniformly in the body. Sr⁹⁰, on the
other hand, emits all of its energy in the form of two beta rays, which
have ranges of only a small fraction of an inch in the bone. Thus in the
one case the radioactive disintegration energy is distributed in the
whole body; in the other, the energy is deposited in the bones only.
Since the bones comprise about ten per cent of the total body weight,
they are subjected to ten times the radiation dosage. The bones are
quite sensitive to radiation, and an overdosage can cause bone cancer
and interfere with the production of blood cells that goes on in the
marrow. Thus we are led to the conclusion that Sr⁹⁰ is a far greater
potential hazard than Cs¹³⁷. A further point, which leads to the same
conclusion, is that Cs¹³⁷, after being absorbed, is retained in the body
less than six months and then excreted. Sr⁹⁰ is retained for many years.
On the other hand, Cs¹³⁷ can cause a type of damage which Sr⁹⁰ cannot
cause: namely, damage to the reproductive cells. The effect of Sr⁹⁰ is
indeed limited to the bones and adjacent or nearby bone marrow, and does
not reach the reproductive organs. In a later chapter we shall take up
the question of genetic danger, and then we shall be very interested in
Cs¹³⁷. For the remainder of this chapter, however, we may focus our
attention on Sr⁹⁰.
Since a large fraction of the Sr⁹⁰ which enters the body stays there,
the most important questions which remain are: how it gets there and how
much gets there. The essential fact in this connection is that the Sr⁹⁰
generally occurs in the fallout in a chemical form which is easily
dissolved in water. The water is taken up by plants, by absorption
through the leaves and the roots. Animals graze on the plants. Human
beings eat the plants and drink the milk from the grazing animals, and
thus become exposed to Sr⁹⁰. (See pictures 5 and 6.)
One might worry because Sr⁹⁰ is not a naturally occurring isotope but
has been made for the first time by man in the fission process. Here is
an unfamiliar poison being scattered over the earth. Can we have any
idea how much will be taken up by human beings?
The answer depends on a fact which we have emphasized throughout this
book: that isotopes of the same element are chemically and biologically
indistinguishable. The radioactive variety of strontium will behave
exactly like the stable natural variety. In particular, the ratio of
Sr⁹⁰ to stable strontium in the human body must be the same as this
ratio is in our food. From this premise we can predict how much Sr⁹⁰
will reach the human body.
From the total yield of fission energy released in all nuclear tests to
date, one can calculate exactly how much Sr⁹⁰ has been produced. This
amount turns out to be about 100 pounds.
Approximately one half of this amount has been deposited in and near the
test sites in the close-in fallout. (Most of the radioactivity comes
from the big bombs, and most of these have been burst on the ground or
over shallow water.) A small portion of the 100 pounds has disintegrated
in the cloud. The remainder, roughly 50 pounds, is partially still in
the stratosphere and partially has been disseminated around the world in
the tropospheric and stratospheric fallout. At the present time
measurements show that 25 or 30 pounds have actually been returned to
the surface of the earth. Local values vary from about one third to more
than twice the average world-wide value.
In the northern part of the United States, in the regions of frequent
rainfall, the measured values are about twice the world-wide average. In
the latitudes between 10°S and 50°N the average value is about 50 per
cent greater than the world-wide average. For the rest of the world one
finds, with some variations, about one third the world-wide average.
Most of the Sr⁹⁰ fallout is caught in the top two or three inches of the
soil. It exists there in a water-soluble form that is readily
assimilated by plants. Also in the soil, chemically inseparable from the
Sr⁹⁰, is stable natural strontium. Plants, animals, and human beings
have no way of distinguishing between the two.
It is not easy to determine how much natural strontium is in a form
which is available to the plants. Some of the natural strontium is
insoluble; and some is below the root depth. Our best estimate is that
there are about 60 pounds per acre actually available for uptake by the
plants. This is, of course, an average.
The amount of natural strontium in the human body is a quantity we know
rather well. It has been carefully measured and is about 0.7 gram in the
average adult, with proportionately less in children. Now since we know
how greatly Sr⁹⁰ has been diluted in the soil and how much natural
strontium there is in our bodies, we can calculate the expected quantity
of Sr⁹⁰ in our bones. Considering the many uncertainties in the
calculation one should not expect too good an agreement. The remarkable
fact is that the quantity of Sr⁹⁰ measured in small children does agree
with the calculated amount. For adults the measured value is quite a bit
less than the calculated amount because adult bones have been made for
the most part before there was any Sr⁹⁰ in the environment.
The fact that we can calculate how much Sr⁹⁰ is at present in the body
is most important because it gives us confidence that we understand what
is happening. It is especially important for us to understand what is
happening so that we can predict how nuclear tests which are carried out
today will affect future levels of Sr⁹⁰ in the body.
From arguments such as we have given, plus a record of the Sr⁹⁰ content
of bones over the last several years, it seems unlikely that the level
of Sr⁹⁰ will increase by more than a factor of two or so due to tests
already conducted. Actually this factor may be even smaller both because
of the mixing of the strontium with the deeper layers of the soil, and
because the radioactive strontium which stays in the ground for a long
time tends to become chemically less soluble and mixed more thoroughly
with that part of the natural strontium which is chemically unavailable.
This latter process is called “chemical aging.”
To follow radioactive strontium and normal strontium from the soil into
the food and the bones is not an easy matter. We must worry about the
question of the strontium depth in the soil and the chemical form of the
strontium. The complete identity of Sr⁹⁰ and normal strontium holds only
if both are near the same place and in the same chemical form. A further
difficulty is that until recently little was known about the behavior of
normal strontium and knowledge is accumulating slowly.
Much more is known about calcium. Now calcium and strontium do not
behave in an identical way, but they do behave similarly. In passing
from soil to man the ratio of calcium to strontium does not remain the
same but at least it changes in a more or less definite manner. Actually
most work on Sr⁹⁰ uptake has been done by comparing Sr⁹⁰ with calcium.
In order to use the data on calcium one has to find out how the calcium
to strontium ratio is changed when the material is taken up into the
human body. In the soil there is, on the average, about 1 part of
strontium to 100 parts of calcium. In the human body the ratio is about
1 to 1400.
Thus the strontium is discriminated against relative to calcium in going
from the soil to man by a factor of about 14. This is a factor of
protection.
It is good to double-check this conclusion and to find out how the
calcium to strontium ratio changes step by step in going from the soil
to man. One finds a factor of 1.4 in going from the soil to the plant, a
factor of 7 in going from the plant to the milk, and a factor of about 2
in going from the milk to man. Actually, if we put all these factors
together we should expect that on the way from the soil to man the
calcium to strontium ratio increases by a factor 20. This is in
reasonable but not in excellent agreement with the ratio 14 given above.
Once the factor of protection is established we can get a value of the
expected strontium uptake from the way in which the radioactive material
is diluted by calcium rather than by normal strontium. This is a less
straightforward but, for the time being, a more practical method than
the direct Sr⁹⁰—normal strontium comparison. It is particularly
important when one compares soils of rather different calcium content.
Plants and animals require calcium. When they do not get it, they
develop a calcium-hunger. Since strontium is chemically similar to
calcium, a lack of calcium in the soil is readily substituted by
available strontium. One would expect that plants grown on calcium-poor
soil and animals raised on such land would exhibit abnormally high
natural strontium content and also a proportionately high Sr⁹⁰ content.
The high Sr⁹⁰ content has in fact been verified. Some sheep in Wales,
for example, appear to have about ten times the average amount of Sr⁹⁰
in their bodies.
Fortunately most people derive their food from many areas widely
separated from each other. Soil that is deficient in calcium is not
likely to supply more than a small part of an individual’s sustenance.
However, the possibility of a large fluctuation cannot be ignored. In
this event corrective measures would be needed. One simple measure would
be to fertilize deficient soil with additional calcium.
That soil can be successfully treated in this way is illustrated by the
present situation in Wales. The sheep with the abnormally high Sr⁹⁰
content all come from the steep, poor pastures which are not limed. The
sheep from the lower pastures, which are limed (not because of the
fallout but for economic reasons), show an activity of only one third
the value mentioned above.
The point we have tried to make in this chapter is that the present
human levels of Sr⁹⁰ can be satisfactorily accounted for by simple
arguments based on the chemical similarity of elements and the identity
of isotopes. These arguments give us confidence that we correctly
understand how Sr⁹⁰ and how much Sr⁹⁰ is getting from the soil to the
human body.
At the same time we have seen how many factors influence the eventual
uptake into the human body: geographical latitude, frequency of
rainfall, the chemical form in which strontium is found, the calcium
content of the soil, the method of agriculture. Even though the United
States has pushed this investigation vigorously since 1952 the bulk of
the work is still ahead of us.
For instance, in the United States, dairy products provide most of the
calcium and strontium in our diets. In Japan, however, the situation is
somewhat different. There the main source of calcium and strontium is
rice. As a result, the ratio of strontium to calcium may be passing
differently from the soil to man. Also the fallout strontium might be
washed deeper into the soil and the soluble to non-soluble ratio might
be different.
Considering the complex nature of the Sr⁹⁰ uptake into man, it is
important to keep close track of the actual Sr⁹⁰ levels in the soil, in
our food, and in our own bodies. The following graphs show how these
levels have risen in the last several years due to the bomb tests:
[Illustration: Sr⁹⁰ in the soil—measured in thousandths of a gram
per square mile.]
[Illustration: Average Sr⁹⁰ in U.S. milk—measured in trillionths of
a gram per quart.]
[Illustration: Average radiation doses from Sr⁹⁰ in bones of young
children (U.S.)—measured in roentgens per year.]
The actual amounts of Sr⁹⁰ in the soil, in the milk, and in the bones of
young children are only approximately known. But the main point that we
are trying to illustrate, is that since 1954 the buildup of Sr⁹⁰ has
gone on at a rather steady rate. How far will this buildup continue?
More radioactivity was released in tests in the year 1954 than in all
other years put together. Probably more than one-half of that activity
has already been deposited. Since that time the fission energy produced
in U.S. tests has steadily decreased. Furthermore, we have learned how
to minimize the world-wide fallout by employing ground bursts which
deposit most of their activity in the close-in fallout near the test
site. It is also possible to place chemical additives near the bomb in
order to convert the strontium into a more insoluble form or else into a
form which will more readily fall out in the immediate neighborhood of
the explosion. And what is most important—we are developing clean
nuclear weapons, which produce blast and heat but greatly reduced
radioactivity. In the future these clean weapons may eliminate the
additional radioactivity altogether.
It is hard to make predictions about the plans of all nations. If we
find—and others also find—that clean weapons are the most desirable, the
total strontium contamination is not likely to become more than perhaps
two to four times the present value. We believe that all reasons—respect
for human life, military considerations and simple sanity—lead to one
conclusion. In the development of nuclear explosives we must endeavor to
make them clean. But the real reason for this does not lie in the small
contamination due to tests. The real reason is that war could turn
contamination into a danger to countless people.
CHAPTER XII
Danger to the Individual
How much harm is being done by the atomic tests? Some scientists have
claimed that from past tests alone about 50,000 persons throughout the
world will die prematurely. There is no general agreement on this point.
Some think the number should be smaller. It is possible that
radioactivity produces some effects which prolong life rather than
shorten it. But even if all the biological consequences of radiation
were known many questions would still demand answers. Can tests be
justified if they actually shorten some human lives? Even the
possibility of a health hazard must be taken most seriously. On the
other hand: Are there any reasons which make continued testing
necessary?
We shall return to these questions in a later chapter. First, however,
we shall try to put before the reader the known facts about the fallout
danger to the individual. We shall try to put this danger into
perspective by relating it to other more familiar dangers to which all
of us are exposed. In the following chapter we shall discuss how the
fallout may affect future generations.
The dangers from big doses of radiation are well known. Exposure to a
thousand roentgens over our whole body causes almost certain death in
less than thirty days. Four or five hundred roentgens give a fifty-fifty
chance of survival. At less than a hundred roentgens, there is no danger
of immediate death. Three years ago the Marshallese got a dose of 175
roentgens. None died. Apparently all are in good health.
Over longer periods of time even bigger radiation doses can be
tolerated. A thousand roentgens spread over a lifetime produce no
apparent biological consequences in individual cases. A rough rule
(which is not too well-established) is that five times as much radiation
can be tolerated if one is exposed to only a little radiation at any one
time.
A hundred roentgens all at once, or several times this amount over a
protracted time period, will not cause sickness or death that can be
directly blamed on the radiation. However, such a dose of radiation may
have harmful biological consequences which are more subtle. An exposed
individual may develop an increased susceptibility to certain diseases,
notably bone cancer and leukemia. Leukemia is a fatal disease in which
the white blood cells multiply too rapidly.
A person who receives a hundred roentgens does not necessarily contract
bone cancer or leukemia. Rather, his chance of contracting these
diseases during his lifetime may have been increased. Knowledge of this
kind can be obtained only with the help of statistics.
If, for example, a large number of mice receive a heavy dosage of
radiation over a long period of time, one finds that the incidence of
tumors and leukemia is higher amongst such irradiated animals than the
natural incidence of these diseases.
Direct evidence with human beings—fortunately—is rather scarce.
Statistics exist on the survivors of Hiroshima and Nagasaki, and also on
radiologists. The latter group probably receive several hundred
roentgens during their professional lifetimes. In addition, some
statistics exist on children who have been treated with large doses of
radiation for enlarged thymuses. Persons suffering from ankylosing
spondylitis, which is a painful disease of the spinal joints, have also
been treated with large X-ray doses. The statistics in all these cases
lead to the same conclusion: that large doses of radiation increase the
likelihood that an individual’s life will be shortened by leukemia and
possibly also other cancers. Furthermore, it appears (mainly from the
experiments on animals) that the increased likelihood is simply
proportional to the amount of radiation received, at least for doses in
the neighborhood of several hundred roentgens or so.
This of course sounds frightening. But the radiation doses from the
world-wide fallout are in a completely different class from those we
have been discussing. They are very much smaller. On the average human
bones are getting about 0.002 roentgens per year from the Sr⁹⁰ in the
fallout. In addition the whole body is receiving a roughly equal amount
in gamma rays, mainly from Cs¹³⁷. These figures apply to new bone in
young children who have grown up in an environment of Sr⁹⁰ in the
northern part of the United States. This is a region of maximum fallout.
Adults whose bones were made for the most part before the atomic testing
started are getting about 0.0003 roentgens per year from Sr⁹⁰. None of
these figures appears to be alarming.
At this present rate a lifetime dosage in northern U.S. is only a small
fraction of a roentgen. A rare individual might get several times this
amount. If tests continue at the present rate, radiation levels could
increase by as much as five-fold. However, even in this situation it is
difficult to imagine anyone receiving a lifetime dose of more than five
or ten roentgens from the world-wide fallout. A more reasonable estimate
for the average lifetime dose would be a few roentgens or less.
One might conclude from these figures that there is no danger whatsoever
from the fallout. This conclusion, however, may not be correct.
The danger from such small doses of radiation is not easy to define.
Even the best statistical methods are insufficient. One is looking for
small effects which show up only after millions of cases have been
studied. Animal experiments are extremely difficult to carry out under
these conditions. Direct controlled experience with human beings is, of
course, impossible. As a result, one is forced to draw conclusions from
the effects at higher dose levels, where experimental data have been
obtained.
This may be done in many ways. One way is to assume that the law of
proportionality holds down to the smallest doses. This means that one
roentgen produces one hundredth as many cases of bone cancer and
leukemia as 100 roentgens produce. This law is plausible. It is by no
means proven.
By arguing in this way one finds that for each megaton of fission energy
which escapes from the test site in the world-wide fallout the lives of
approximately four hundred persons would be shortened by leukemia or
bone cancer. Under present conditions of testing, roughly one half of
the fission products are deposited as close-in fallout in and near the
test site. Per megaton of fission energy exploded, therefore, perhaps
200 persons may get leukemia or bone cancer. This figure could actually
be higher, possibly even a thousand persons or more per megaton. It
could also be lower. It could be zero.
It is possible that radiation of less than a certain intensity does not
cause bone cancer or leukemia at all. In the past small doses of
radiation have often been regarded as beneficial. This was not supported
by any scientific evidence. Today many well-informed people believe that
radiation is harmful even in the smallest amounts. This statement has
been repeated in an authoritative manner. Actually there can be little
doubt that radiation hurts the individual cell. But a living being is a
most complex thing. Damage to a small fraction of the cells might be
beneficial to the whole organism. Some experiments on mice seem to show
that exposure to a little radiation increases the life expectancy of the
animals. Scientific truth is firm—when it is complete. The evidence of
what a little radiation will do to a complex animal like a human being
is in an early and uncertain state.
In any event the number of additional cases of leukemia and bone cancer
due to the fallout radiation is certainly too small to be noticed
against the natural incidence of these disorders.
In the next thirty years about 6,000,000 people throughout the world
will die from leukemia and bone cancer. From past tests, which have
involved the explosion of about fifty megatons of fission energy, the
possibility exists that another 50 × 200, i.e., 10,000 cases may occur.
Statistical methods are not able to find the difference between
6,000,000 and 6,010,000. There is no way to differentiate between the
fallout-induced cases of leukemia and bone cancer, and those which occur
naturally.
The possible shortening of ten thousand lives may seem rather ominous.
But mere figures can be misleading. A better way to appreciate the
danger from fallout is to compare it with other more familiar dangers.
Such a comparison can be made with the natural background of cosmic rays
and radioactivity in the earth and in our own bodies.
We are constantly and inescapably exposed to this radiation. Our
ancestors have been exposed to it. The human race has evolved in such a
radioactive environment. Moreover, the biological effects from different
kinds of radiation can be compared in a meaningful way in terms of
roentgens. Therefore the danger from Sr⁹⁰ is not unknown in every
respect. In some ways it is very well-known because we and all living
beings have spent our days in a similarly dangerous surrounding. We live
on an earth which has radioactivity in its rocks, which carries a
similar activity in its waters, and which is exposed from all sides, to
a rain of particles which produce effects identical with the effects of
radioactive materials.
Not all radiations which have the same intensity (the same number of
roentgens) have precisely the same effect. The damage produced also
depends somewhat on the spacing of the ionized and disrupted molecules.
The cosmic rays and the Sr⁹⁰, however, are quite similar even in this
respect.
The reader will recall that the spacing of the ionization depends only
on the charge and the speed of the ionizing particle. The ionizing
particle from the Sr⁹⁰ is an energetic beta ray, which has a charge of
one and a speed close to that of light. A large part of the background
radiation which reaches our bones comes from the cosmic rays. The main
portion of the cosmic rays is due to the mesons. The meson, like the
beta ray, has a unit charge and a speed close to that of light. The two
particles may therefore be expected to produce identical biological
effects. The only difference between their effects is that the beta ray
does not have enough energy to leave the bones, while the meson is so
energetic that it deposits its energy both in our bones and throughout
our whole body. Thus if we compare a Sr⁹⁰ dose with the same dose of
cosmic rays the same effect to the bones must be expected. But the
cosmic rays give rise to additional effects in our bodies.
The total background dose to the bones is about 0.15 roentgens per year
for the average person living at sea level in the United States. Of this
amount, about 0.035 roentgens is due to cosmic rays. At higher altitudes
the cosmic ray dosage increases. In Denver, at an altitude of 5000 feet,
the cosmic rays contribute 0.05 roentgens per year.
The above numbers should be compared with the present level of
world-wide fallout radiation to the bones: about 0.003 roentgens per
year (from Sr⁹⁰ and other sources). The fallout radiation is thus only a
few per cent of the natural cosmic radiation. It is small even when
compared to the variation of cosmic ray intensity between sea level and
5000 feet.
A correlation between the frequency of leukemia and bone cancer, and the
intensity of natural radiation has been looked for. Some statistics for
the year 1947, before weapons testing began, are available. They show
the number of cases of these diseases occurring in that year per 100,000
population.
_Bone Cancer_ _Leukemia_
Denver 2.4 6.4
New Orleans 2.8 6.9
San Francisco 2.9 10.3
The extra radiation that one gets in Denver from cosmic rays is many
times greater than the fallout radiation. But the table shows no
increased incidence of bone cancer or leukemia. On the contrary—the
incidence of these diseases is actually lower in Denver.
Not all of the natural background radiation is due to cosmic rays. Part
of the background comes from natural radioactive elements in the soil
and in the drinking water. These include uranium, potassium⁴⁰, thorium
and radium. Radium behaves like calcium and strontium, and gets
deposited in our bones. All these effects are, to the best of our
knowledge, at least as intensive in the Denver area as in San Francisco
or New Orleans.
One possible explanation for the lower incidence of bone cancer and
leukemia in Denver is that disruptive processes like radiation are not
necessarily harmful in small enough doses. Cell deterioration and
regrowth go on all the time in living creatures. A slight acceleration
of these processes could conceivably be beneficial to the organism. One
should not forget that while radiation can cause cancer, it has been
used in massive doses to retard and sometimes even to cure cancer. The
reason is that some cancer cells are more strongly damaged by radiation
than the normal cells.
In spite of the table, however, there may actually be an increased
tendency toward bone cancer and leukemia that results from living in
Denver. If so—and this is the main point—the effect is too small to be
noticed compared to other effects. We must remember that Denver differs
from New Orleans and San Francisco in many ways (besides altitude), and
these differences may also influence the statistics.
A more thorough consideration of the background radiation gives further
evidence that this radiation is more important than the present or
expected effects of Sr⁹⁰. The radium deposited in our bones from
drinking water has been observed to reach values as high as 0.55
roentgens per year. Furthermore the heavier and slower alpha particles
emitted by radium cause ionization processes which occur in closer
spacing and are therefore more damaging than the ionization due to Sr⁹⁰.
To make things worse radium is deposited in our bones in little nodules
(hot spots). Thus the possibility of local damage is enhanced.
The background radiation to which we are exposed varies for some
unexpected reasons. It has been pointed out recently that brick may
contain more natural radioactivity than wood. The difference between
living in a brick house and living in a wood house could give rise to
ten times as much radiation as we are currently getting from fallout.
(The additional radiation from the brick might be as much as 0.03
roentgens per year.)
Human beings are subject to radiation not only from natural sources, but
also from man-made sources. One of these is wearing a wrist watch with a
luminous dial. Another is having X-rays for medical purposes. Both of
these sources give much more radiation than the fallout.
Of all ionizing radiation to which we are exposed the X-rays are most
important. In some cases medical X-rays have intensities which are
noticeably harmful. Yet this damage is practically always of little
consequence compared to the advantage from correct recognition of any
trouble that the X-ray discloses.
We may summarize in this way. Our knowledge of the effects from the
fallout is deficient. We cannot say exactly how many lives may be
impaired or shortened. On the other hand, our knowledge is sufficient to
state that the fallout effect is below the statistically observable
limit. It is also considerably less than the effect produced by moving
from sea level to an elevated location like Denver, where cosmic
radiation has a greater intensity. It is also less than having a chest
X-ray every year. In other words, we know enough to state positively
that the danger from the world-wide fallout is less than many other
radiation effects which have not worried people and do not worry them
now.
We have compared radiation from the fallout with radiation from other
sources. It is also possible and helpful to compare the fallout danger
with different kinds of dangers. For this purpose it is convenient to
express all dangers in terms of a reduced life-expectancy. For example,
smoking one pack of cigarettes a day seems to cut one’s life-expectancy
by about 9 years. This is equivalent to 15 minutes per cigarette. That
cigarettes are this harmful is, of course, not known with certainty. It
is a “best guess,” due to Dr. Hardin Jones, based on an analysis of
statistical data. A number of Dr. Jones’ statistical findings are listed
in the following table:[13]
_Reduced
Life
Expectancy_
Being 10 per cent overweight 1.5 years
Smoking one pack of cigarettes a day 9 years
Living in the city instead of the country 5 years
Remaining unmarried 5 years
Having a sedentary job instead of one involving exercise 5 years
Being of the male sex 3 years
Automobile accidents 1 year
One roentgen of radiation 5 to 10 days
The world-wide fallout (lifetime dose at present level) 1 to 2 days
The reader will see that the world-wide fallout is as dangerous as being
an ounce overweight or smoking one cigarette every two months.
[Illustration: How people get radiation
Average dose in roentgens per year]
The objection may be raised that the fallout, while not yet dangerous,
may become so as more nations develop and test atomic weapons. On this
point we can only say that the future is not easy to predict. Some
factors, however, justify optimism. We are learning how to regulate the
fallout by exploding bombs under proper surroundings. Development of
clean bombs will greatly reduce the radioactivity produced. Deep
underground tests will eliminate fallout altogether. The activity put
into the atmosphere in 1954 was considerably greater than the activity
released in any other year. It is highly probable that the activity
produced by United States tests will continue to decline.
Finally, we may remark that radiation is unspecific in its effects.
Chemicals are specific. About the effects of a new ingredient in our
diet, in our medicine, or in the air we breathe, we know much less than
we know about radiation. If we should worry about our ignorance
concerning our chemical surroundings as we worry about the possible
effects of radiation, we would be condemned to a conservatism that would
stop all change and stifle all progress. Such conservatism would be more
immobile than the empire of the Pharaohs.
It has been claimed that it is wrong to endanger any human life. Is it
not more realistic and in fact more in keeping with the ideals of
humanitarianism to strive toward a better life for all mankind?
CHAPTER XIII
Danger to the Race
Radiation may hurt the individual. It may also be harmful for our
children and hurt the race. We have seen that the danger from the
radiation due to testing is small compared to many risks which we
habitually take and almost always ignore, which in fact we have to
ignore to continue to live in this civilized world. In addition we are
not even quite sure that the danger to the individual is real.
There can be little doubt, however, that radiation does produce some
harmful changes in our children. What seems even more frightening, is
that these changes may not show up in our children but only in their
children or further progeny. A danger which may lie hidden for
generations might seem more terrifying, especially as it has often been
repeated that all such radiation effects are harmful.
We transmit our properties to coming generations in a most curious and
concentrated fashion. From the mother and the father a child inherits a
number of chromosomes, twenty-four from each.[14] These are structures
along which the actual carriers of the properties—the genes—are strung
up.
We are beginning to understand something about the nature of the genes.
They seem to be very big spiral molecules. They carry the master plan of
our body and even of our character in a strange chemical code.
The laws of heredity are complicated because of the fact that the same
property is influenced by a gene from each parent. Frequently these two
genes dictate different behavior and then the result is a compromise,
sometimes evenhanded, sometimes unbalanced. But of the two genes only
one will find its way to the child of the next generation. The
compromise is temporary and original properties may emerge again. Which
one of any pair of chromosomes (or of the two assemblies of genes)
carries on is a matter of chance. In the world of the cells as in the
world of atoms it is chance that determines the future—not fate.
Of all these facts we need be particularly interested in one. The units
of inheritance are rather constant but not quite immutable. There is a
small possibility that any gene may suffer a mutation. That is, it may
turn into a new chemical, carrying a new code and new properties.
A gene is an extremely finely and precisely constituted object. It must
be so in order to carry all the racial past in so little material. A
mutation due to chance will spoil this order in almost every instance.
The great majority of mutations are detrimental. Many are lethal.
It is an incredible fact that these random mutations, almost always
harmful and never proceeding according to any plan, should have been
responsible in the very long run for all the many beautiful and perfect
living creatures that nature has produced (and this includes the human
race). The thread leading from single cells to cell colonies, worms,
fishes, vertebrates, mammals and human beings does certainly not seem to
be the work of chance. Much less does it seem to be the work of a gamble
taking one chance of a small improvement against a thousand chances of
deformity or death. Nevertheless it is such a terrible game of chance
which has produced both the human body and in some manner also the human
spirit.
Big numbers are strange things and when each member of a huge assembly
must be given individual attention then the numbers are even harder to
appreciate. Billions of contemporary lives in billions of distinct
generations have led to the incredible outcome: the harmony of life
produced by gambling.
Radiation is surely disruptive. It does cause mutations. Since the genes
appear to be single molecules, a single process of ionization or
excitation is likely to result in a change. As has been said before
there is doubt whether or not cancer and leukemia can be caused by
exceedingly little radiation. There is little doubt, however, that
mutations can be caused by any small amount of radiation. The less
radiation the less the chance. But the chance will always be there.
A very great increase in the natural rate of mutations could indeed have
terrifying effects. We can be quite certain, however, that radiation
from atomic tests will increase the chance of mutations by only a very
small amount.
The argument is essentially the same as the one concerning the danger to
the individual. The tests are responsible for 0.001 or 0.002 roentgens
per year to the human reproductive cells. This is equivalent to
approximately 0.05 roentgen per generation. Most of this radiation is
due to gamma rays from Cs¹³⁷ which has been deposited on the ground or
absorbed in the body. The number of mutations caused by this radiation
is to be compared with the number of natural mutations.
Some of the natural mutations are caused by heat and chemicals. Some are
due to background radiation, to cosmic rays or to gamma and beta rays
emitted by natural radioactive substances in or near our bodies. Our
best estimate is that 10 per cent of the natural mutations are due to
the background radiation.
Over a period of one generation the background radiation dosage to the
human reproductive cells is approximately five roentgens. Assuming a
simple proportionality between dosage and the number of mutations, it
follows that fifty roentgens would be required to induce a number of
mutations equal to the total number of natural mutations (from
background radiation and all other causes). That is, fifty roentgens is
a “doubling dose.”
The atomic tests are therefore increasing the number of mutations by
about 0.05 ÷ 50, which is 0.1 per cent. This kind of increase in the
rate of mutations would certainly not seem to be a serious reason for
worry.
Actually the number of mutations from the tests is very small even
compared to geographical and altitude variations in the natural
radioactivity. The Inca empire existed for many generations in the high
country of Peru. The people of Tibet have been exposed for generation
after generation to the greater cosmic ray intensity which bombards them
through a thinner layer of atmosphere. These people have been exposed to
much greater additional radiation than anything which is caused by
atomic tests. Yet genetic differences have not been noticed in the human
race or for that matter in any other living species in Peru or Tibet. We
are certainly talking here about questions which may strike hard on some
individuals but which from the point of view of the community or race
are not serious.
It has been often repeated that all mutations due to radiation are
harmful. There is every reason to believe that mutations due to
radiation are not different in kind from other mutations. Should we then
seriously believe that all mutations are harmful? That most of them are
is admitted. If all of them were indeed always harmful, we must deny the
simplest facts of evolution.
There will be some who maintain that the human race is not capable of
improvement. Such an argument is irrefutable. It is also unreasonable.
What cannot be further improved is perfect, and not many people will
maintain that our species can claim perfection.
Another and much more plausible argument has been advanced: In the wild
state living species do perfect themselves by means of natural
selection. Human society by caring for the imperfect and defective
individual has eliminated natural selection. Therefore further mutations
will not improve mankind.
It is very hard to discuss this question for the simple reason that the
argument involves the interaction of two processes extremely different
in magnitude and in fact different in kind. On the one hand it concerns
itself with evolution which proceeds in the slow deliberate way of a
glacier. On the other hand it focuses attention upon the process of
human civilization with its technical and social changes which has
gained momentum like an avalanche. The momentum is still there and it is
still increasing and where we shall land we do not know. To consider the
motion of the glacier while being carried along by the avalanche puts
things completely out of proportion. Long before the present rates of
mutation could have any effect upon the human species we shall live in a
very different world and we shall have started to influence our own
behavior including those of selection, natural or otherwise, in ways
which today we cannot foresee.
If we discuss the question how civilization will influence natural
selection, we shall not do it with the hope of arriving at a firm
answer. We shall do it rather in order to illustrate how doubtful all
the arguments are which concern the interplay of two processes which
cannot be measured in the same scale.
It is true that we can and do preserve the lives of children who,
because of inherited weaknesses, would perish under natural conditions.
It is true too that we do this for reasons and for feelings concerning
the individual and we do it without regard to the consequences to the
race. However, under our present condition of civilization a disease
which can be corrected by administering chemicals or using the surgeon’s
knife is no longer effectively a disease. In our present condition such
a life can be as valuable to society and to the race as a life which
does not have these superficial shortcomings. That we can and do
preserve more life in this manner only emphasizes that under present
conditions biological differences which used to be important no longer
matter.
On the other hand, in social living many properties which used to be
indifferent for a wild being have become of great significance. Ability
to communicate and to get along with our fellows is not the only one,
but is perhaps the most obvious one of such properties. The struggle for
existence has become more gentle, and the chance of any individual to
live on in his children is governed by new ways of behavior.
Nevertheless the difference between the individual adapted to civilized
living and the one who is not adapted is of great importance and will
become of greater importance. It is likely that civilization will not
eliminate evolution of the race. Rather it will direct it into new
paths.
But the greatest change might be expected from an entirely different
direction. We are going to understand in real detail the intricacies of
human inheritance. Then we shall be faced with problems and shall find
possibilities of an entirely new and different kind. The interest of a
person in his children is not a superficial one. It is one of the most
strong and lasting forces in biology, sociology and history. A clear
understanding of the details of inheritance may bring about some grave
difficulties because a new situation is never fitted easily into
existing patterns of living. In the end more understanding may bring
about improvements of a kind beside which all the worthwhile things that
have been so far accomplished, might look unimportant.
The real importance of radioactivity for heredity does not lie in the
fact that we may speed up the glacier by one inch in a millennium. The
real importance of nuclear radiation is rather that it is helping us to
understand the strange processes of life and the curious substances
which connect one generation to the next.
CHAPTER XIV
The Cobalt Bomb
Nuclear explosions seem horrible for many reasons. They were presented
to an unprepared world as a dramatic surprise—as the climax to the
slaughter of the Second World War. Their power of destruction is
fantastic. Before we had adjusted our thinking to atomic bombs, an even
more potent tool of warfare—the hydrogen bomb—was invented. Worst of
all: To the fear of destruction there was added the dread of the
unknown. It is not surprising that discussion of nuclear weapons has not
proceeded on a purely rational level.
To the nightmare of the atomic and hydrogen bombs has been added—not as
a reality but as a further threat—the cobalt bomb. The idea of such a
bomb is to intensify the most terrifying aspect of nuclear explosions:
the radioactivity. This radioactivity could be used to poison the enemy.
It could get out of hand and poison everyone.
Cobalt⁶⁰ is a radioactive isotope of the fairly common metal cobalt. It
can be easily produced by absorbing slow neutrons in the natural and
stable cobalt⁵⁹. It has a half-life of five years and it emits
penetrating gamma rays. These properties make it useful in cancer
therapy.
Many cancerous growths are more sensitive to radiation than healthy
tissue. Therefore radiation can be used to reduce—sometimes even to
destroy—dangerous tumors. The penetrating rays of cobalt⁶⁰ can reach the
cancer even deep inside the human body. The lifetime of cobalt⁶⁰ is long
enough so that this substance is easily installed in hospitals.
But the same properties which make cobalt⁶⁰ useful also make it
potentially dangerous. A nuclear explosion produces many neutrons and
these could be absorbed in ordinary cobalt. The radioactivity produced
in this way lives long enough to become widely distributed. Its ray can
easily penetrate a foot of masonry and several hundred feet of air. A
cobalt bomb would indeed be a most unpleasant object. (See pictures 7
and 8.)
One widely discussed possibility is that future nuclear tests will be
used to develop a cobalt bomb or other bombs for radiological warfare.
Actually tests have little to do with the cobalt bomb. Once one has a
powerful nuclear weapon, such as a hydrogen bomb, it is relatively easy
to make a radiological bomb. Further tests are not necessarily required.
To the extent that any testing need be carried out, it is only necessary
to activate a moderate amount of substance to find out in what way a
certain bomb would function as a tool of radiological warfare. Tests of
this kind would add only a negligible amount of radioactivity to the
atmosphere. Therefore, in connection with the test program we need not
worry about the cobalt bomb or any related experiment. The question of
the cobalt bomb or radiological warfare in general is not whether it is
feasible—it is—but rather whether it serves a useful military purpose.
It is not impossible that situations might arise in which radiological
warfare could be militarily advantageous. Instead of cobalt, other
materials may be placed near the nuclear bombs. In this way other
radioactive substances can be produced. By an appropriate choice of such
a substance one can get a radioactive material which, when deposited
near the point of explosion, will contaminate the site for a time which
can be adjusted to the military requirements. The lifetime of the
radioactive material may be long enough to give an opportunity to the
people to escape from the contaminated area. At the same time, one may
precipitate almost all the activity near the explosion so that distant
localities would not be seriously affected. It is conceivable,
therefore, that radiological warfare could be used in a humane manner.
By exploding a weapon of this kind near an island one might be able to
force evacuation without loss of human life. No instrument, not even a
weapon, is evil in itself. Everything depends on the way in which it is
used.
Public opinion has all but persuaded itself that nuclear weapons will be
used not for a military objective but to terrorize and kill the greatest
number of people. This is technically feasible. In fact, it does not
even require the atomic bomb. For the last hundred years this
possibility has been with us. Bacteriological warfare may cause
widespread destruction. Yet no one has resorted to this horrible way of
making war. We do not believe that anyone will expose his enemy and
ultimately himself to indiscriminate bacteriological or radiological
destruction. Our guarantee against this danger is not that it cannot be
done. Our guarantee is the better and saner part of human nature: the
will to survive and the feeling of common decency.
CHAPTER XV
What About Future Tests?
Many people feel that tests should be discontinued. This feeling is
widespread and strong. The question of tests is obviously important. It
may influence our security as individuals. It certainly will influence
our security as a nation. If in a free, democratic country the majority
believes that something should be done—it will be done. The sovereign
power in a democracy is “the people.” It is of the greatest importance
that the people should be honestly and completely informed about all
relevant facts. In no other way can a sound decision be reached. The
basic and relevant facts are simple. The story can be presented without
unnecessary frills or undue emotion. When this has been done, the right
decision will be reached by common sense rather than by exceptional
cleverness.
Unfortunately much of the discussion about continued experimentation
with nuclear explosives has been carried out in a most emotional and
confused manner. One argument concerning tests is so fantastic that it
deserves to be mentioned for that very reason: It has been claimed that
nuclear explosions may change the axis of the earth.
Of course, nuclear explosions do produce such changes. Only the changes
are so small that they are impossible to observe and even difficult to
estimate. Searching for effects connected with past tests that may
displace the axis of the earth, or the position of the North Pole, we
could find no effect that would have caused a change of position even as
great as the size of an atom. One could design tests with the specific
purpose to produce such a change, but these man-made effects could not
be compared even remotely with the forces of nature. The motion of the
Gulf Stream has a small effect on the North Pole; but this effect is
incomparably greater than what any nuclear explosion could accomplish.
It is good to know that the old top on which we live does have some
stability.
The argument about world-wide radioactive fallout is more serious. It is
asserted that fallout is dangerous and that we are ignorant of the
extent of the danger.
In a narrow, literal sense both these statements are correct. But in the
preceding chapters we have seen that the danger is limited. We do not
know precisely how great it is. We do know, however, that the danger is
considerably smaller than the danger from other radiations to which we
continue to expose ourselves without worry. The danger from the tests is
quite small compared with the effects of X-rays used in medical
practice. The fallout produces only a fraction of the increase in cosmic
ray effect to which a person subjects himself when he moves from the
seashore to a place of higher altitude like Colorado. People may or may
not be damaged by the fallout. But it is quite certain that the damage
is far below a level of which we usually take notice.
Fallout in the vicinity of the test sites did cause damage. In the past
this damage was not great although in one Pacific test it was serious.
Precautions have been increased and we may hope that future accidents
will be avoided altogether. The safety record of the Atomic Energy
Commission compares favorably with other enterprises of similar scale.
It seems probable that the root of the opposition to further tests is
not connected with fallout. The root is deeper. The real reason against
further tests is connected with our desire for disarmament and for
peace.
There can be no doubt that the desire for peace is most deep, and this
desire is felt by all thinking and honest people on our earth. All of us
certainly hope that the catastrophe of war can be avoided. This great
and universal wish for peace is the driving force behind the desire for
disarmament. In the minds of most people it would be an important step
toward disarmament if the testing of nuclear weapons were stopped by all
nations. This belief is widely held, but it is not necessarily
well-founded. In fact, there are arguments on the other side which
should be considered carefully.
It is generally believed that the First World War was caused by an arms
race. For some strange reason most people forget that the Second World
War was brought about by a situation which could be called a race in
disarmament. The peace-loving and powerful nations divested themselves
of their military power. When the Nazi regime in Germany adopted a
program of rapid preparation for war, the rest of the world was caught
unawares. At first they did not want to accept the fact of this menace.
When the danger was unmistakable, it was too late to avert a most cruel
war, and almost too late to stop Hitler short of world conquest.
Unfortunately, disarmament is safe only when no one wants to impose his
will by force of arms upon his neighbors.
In the uneasy world in which we live today no reasonable person will
advocate unilateral disarmament. What people hope is that all sides will
agree to reduce their military power and thereby contribute to a more
peaceful atmosphere. The elimination of tests has appeared possible and
proper for two reasons. One is that tests are conspicuous, and therefore
it is believed that we can check whether or not testing has actually
been stopped by everyone. The second reason is that nuclear explosives
already represent such terrifying power that further tests appear
useless and irrational. These arguments are simple and almost
universally accepted. They are based on misconceptions.
A nuclear explosion is a violent event, but in the great expanses of our
globe such tests can be effectively hidden if appropriate care is taken
to hide them. There can be no doubt that this is possible. The question
is only how much it costs to hide a test and how big is the explosion
that can be carried out in secret for a certain amount of expenditure.
If an agreement were made to discontinue the tests, the United States
would surely keep such an agreement. The very social and political
structure of our country excludes the possibility that many people would
collaborate in breaking an international undertaking. Whether Russia
would or would not keep such an agreement would depend on the ingenuity
of the Russians, on their willingness to make economic sacrifices, and
on their honesty. Of these three factors we can have a firm opinion
about the first. The Russians are certainly ingenious enough to devise
secret methods of testing. As to the other questions, whether the
Russians will want to invest the effort and whether they will be bound
by their word, we feel that each man is entitled to his own opinion.
According to past experience, an agreement to stop tests may well be
followed by secret and successful tests behind the iron curtain.
In a more general way we may ask the question: Is it wise to make
agreements which honesty will respect, but dishonesty can circumvent?
Shall we put a free, democratic government at a disadvantage compared to
the absolute power of a dictatorship? Shall we introduce prohibition in
a new form, just to give rise to bootlegging on a much greater scale? It
is almost certain that in the competition between prohibition and
bootlegging, the bootlegger will win.
All of these arguments, however, would become irrelevant if it were true
that further testing would not accomplish any further desirable result.
It has been said and often repeated that we now possess adequate nuclear
explosives to wipe out the cities of any enemy. What more do we need?
Our main purpose in further experimentation with nuclear bombs is not,
of course, to make city-busters more horrible. We would prefer not to
have to use our nuclear weapons at all. We keep them as a counterthreat
against the danger that we ourselves should be subjected to a
devastating attack. To understand what we are actually trying to do in
the tests, we have to take a closer look at some military problems.
In the Second World War strategic bombing was used for the first time on
a really massive scale. It may well be and, in fact, it is probable that
such strategic bombing will not be repeated in the future.
There are two military reasons for the bombing of cities. One is that
factories are located in cities, and these factories support the war
effort. The other reason is that cities are centers of transportation
through which the supplies of war materials pass. By destroying these
centers the flow of the war supplies can be interrupted.
Nuclear warfare is likely to be quite different from past conflicts. The
great concentration of firepower which a nuclear weapon represents makes
it possible to attack on enemy anywhere, at very short notice. This is
true no matter what the particular target is, whether one is trying to
attack the planes, ships, tanks, or troop concentrations of an enemy.
The great mobility of nuclear firepower makes it highly probable that
the nuclear conflict will be short. What the factory produces during
this conflict will not affect the outcome of the fighting. The only
weapons on which anyone can rely are the weapons which are already
stockpiled. Therefore, it will be militarily useless to bomb factories.
The same fact of mobility also implies that no great flow of war
material will need to be maintained. Practically all movement can be
executed by light and fast methods, by planes, submarines, and small
battle groups. Under these conditions the cities will lose their
importance as centers of transportation.
The only purpose in bombing cities will be to spread terror among the
enemy. This was rarely done in past wars. In fact, terror is
self-defeating because it provokes retaliation from the other side.
We believe that the role of nuclear weapons in a future war is by no
means the killing of millions of civilians. It is rather to stop the
armed forces of an aggressor. This is not easy to do because it requires
not only nuclear weapons, but very special kinds of nuclear weapons
which are hard to develop and harder to perfect. But with proper
experimentation and proper planning the defensive use of nuclear weapons
is possible.
The idea of tactical nuclear weapons is not new. The possibility of
using nuclear explosives in small wars has been frequently discussed.
What kind of weapons do we need in order to fight these small wars and
to defend the freedom of people wherever such defense becomes necessary?
It has often been suggested that in small wars, small weapons will be
used, while big weapons are appropriate for big wars. Such a statement
is much too simple and has no relation to reality. In every case the
right kind of weapon is the one which performs the job of stopping the
enemy’s armed forces without inflicting unnecessary loss on the innocent
bystander. For this purpose we need a great number of weapons which are
adaptable to specific purposes, which are easy to transport and easy to
deliver, and give rise to the kind of effect which the situation
requires.
For instance, a nuclear weapon may be carried by a fighter plane and
used to shoot down an attacking bomber. Since the carrying capacity of
the fighter plane is severely limited, the weapon for this purpose must
be small and light. A major objective of the test program is to develop
such purely defensive weapons.
The encounter between the fighter plane and the bomber may well take
place in our own country over populated areas. This possibility would
fill most people with alarm lest the population underneath the explosion
should be hurt. Fortunately, in a recent nuclear test in Nevada, five
well-informed and courageous Air Force officers demonstrated that there
is complete safety to people on the ground. They did this by standing
directly beneath the explosion at ground zero.
This important test took place only a few months ago—on July 19, 1957.
An F-89 jet fighter plane flying at 19,000 feet above sea level
delivered an air-to-air atomic rocket to a preassigned point in the sky.
The ground zero men were 15,000 feet immediately below. They wore no
helmets, no sun-glasses, and no protective clothing.
At the instant of the explosion the men looked up, saw the fireball and
felt the heat. There was no discomfort, only a gentle warmth. Then they
waited for the shock wave to arrive—approximately ten seconds. When the
shock came, it was actually just a loud noise. However, one of the men
ducked his head instinctively. (See pictures 9 and 10.)
The blast and the thermal pulse were over. But the Air Force men stood
their ground. One question still remained: Would there be any fallout?
They checked their radiation instruments and waited while the cloud
drifted slowly away. There was no significant rise in the radiation
level. The test had been a complete success. The effects of the
explosion were utterly insignificant on the ground. But high in the air
an enemy plane could have been demolished even if the nuclear explosion
had missed it by a considerable distance.
In order that nuclear weapons should be effective against armed
invaders, it is clear that great numbers of these weapons are needed.
Such great numbers of weapons, some of which must be ground-burst, will
produce a considerable amount of radioactive contamination, and this
contamination will endanger friend and foe alike. In particular, the
radioactivity is likely to kill people in the very country whose liberty
we are trying to defend. For this reason it is most important that we
should be able to use nuclear weapons which cause the least possible
contamination. In recent nuclear tests more and more attention has been
paid to the development of such clean weapons, and most fortunately
these efforts are well on the way toward success.
The radioactive fallout from nuclear testing gives rise to a possible
danger which is quite limited in size. The danger from the fallout in a
nuclear war, however, would be real and great. If we stop testing now,
and if we should fail to develop to the fullest possible extent these
clean weapons, we should unnecessarily kill a great number of
noncombatants. Not to develop the explosives with the smallest
radioactive fallout would, indeed, be completely inexcusable.
The only alternative is that nuclear weapons should not be used at all.
Since these weapons have been presented as purely evil instruments, most
people hope that they will never be used, and indeed one should hope
that wars, and therefore the use of these weapons, can be avoided.
But in our conflict with the powerful communistic countries which strive
for world domination, it may be too much to hope for uninterrupted
peace. If we abandon our light and mobile weapons, we shall enable the
Red bloc to take over one country after another, close to their borders,
as opportunities arise. The free nations cannot maintain the massive
armies throughout the world which would be required to resist such
piecemeal aggression. On the other hand, the flexible power of clean
nuclear explosives would put us in a position where we could resist
aggression in any part of the world, practically at a moment’s notice.
The announced policy of our country is to maintain peace and stability
in the world. By being patient and prepared we are trying to arrive at a
world order based on law and justice for all peoples. There is no doubt
that this policy is supported by the overwhelming majority of Americans.
Our armed forces need the greatest possible flexibility in order to give
strength to this policy. Such flexibility we can possess only if we have
in our possession the strongest, best developed weapons which are also
the cleanest, so that they may be used for defense rather than for
random destruction.
If we renounce nuclear weapons, we open the door to aggression. If we
fail to develop clean explosives, we expose people to disaster from
radioactive fallout in any serious military conflict. To our way of
thinking these are weighty arguments in favor of continued
experimentation and development of nuclear weapons. But still another,
more general, point of view should be considered.
The spectacular developments of the last centuries, in science, in
technology, and in our everyday life, have been based on one important
premise: to explore fearlessly any consequences to which greater
knowledge and improved skills can lead us. When we talk about nuclear
tests, we have in mind not only military preparedness but also the
execution of experiments which will give us more insight and more
ability to control the forces of nature. There are many specific
political and military reasons why such experiments should not be
abandoned. There also exists this very general reason—the tradition of
exploring the unknown. We can follow this tradition, and we can at the
same time be increasingly careful that radioactivity, carelessly
dispersed, should not interfere with human life.
CHAPTER XVI
Has Something Happened to the Weather?
The weather is no longer quite as unpredictable as it used to be. Yet we
are hardly ever sure of it even a few hours in advance. One week is
about the limit of the period of any prediction. Where the best men lack
knowledge untrammeled fantasy has a field day. Weather has so far
remained a safe topic of conversation and of speculation.
Nuclear explosions have, of course, been made responsible for the
weather—for any kind of unusual weather. Be it rain or drought or a hard
season of hurricanes—the nuclear tests are dragged in. The weather
bureau says: no. But then—the weather bureau has not always been
correct. Indeed it would be a miracle if the popular talk and the
popular press would not have seen some connection between atomic
explosions and the wayward behavior of the seasons.
In one case—and to our knowledge only in one case—there has occurred a
chain of events starting with a nuclear test and ending in a copious and
unusual downpour. In the spring of 1955 a test shot of moderate size was
fired in Nevada. At the same time the last storm of the season was
blowing itself out in California. According to the usual rules of
meteorology the radioactive cloud should have been carried east by the
steady westerly winds which blow over the temperate zone. But this time
the cloud was caught up by the swirl of the dying California storm and
some of the radioactivity was carried to the west coast.
Hours after the explosion radioactive rain began to fall in California.
The activity was weak enough and did not give rise to any worry. But a
remarkable thing happened. As the active cloud arrived over California
the storm revived. It developed into an abundant rain which is not usual
at that place and time. Did we—quite unintentionally—do something about
the weather?
The weather bureau said: no. One must certainly admit that this single
case proves nothing. Only greatly improved methods of weather
observation and weather prediction would make it possible to decide if
such a chain of events consists of the strong links of cause and effect
or else of a simple sequence of haphazard occurrences.
Even though our knowledge is incomplete there is at least one simple
fact which should be borne in mind. All the energy in that Nevada
explosion was not quite sufficient to evaporate the water droplets in a
cloud one mile broad, one mile wide, and one mile deep. This is not a
very big rain cloud. Such a cloud would give about one third of an inch
of rain water over one square mile—not an impressive amount. Even the
biggest hydrogen bomb would give only energy enough to evaporate a cloud
ten miles by ten miles and towering to the top of the “boiling” portion
of our air, which we call the troposphere. This would give roughly three
inches of rain over a hundred square miles—a more impressive amount but
vanishing in the vastness of the Pacific Ocean.
Nuclear explosions are violent enough. But compared to the forces of
nature—compared even with the daily release of energy from not
particularly stormy weather—all our bombs are puny. Offhand one might
guess that our nuclear fireworks could not swing the scales in the
massive energy changes that we see around us in the common occurrences
of wind and rain.
But the interplay of clouds and sunshine, of water evaporating,
freezing, dropping and thawing—in short the vagaries of weather—are both
involved and tricky. Small causes can give rise to big effects. Some
processes of air masses sweeping over oceans and continents are
irresistible and predictable. Others, like the first upsurge of hot air
from the overheated ground, may be a question of close competition and
trigger action. This is what makes it so difficult to predict the
weather.
One of the most delicate processes we must think about is the formation
of water droplets. When some water molecules are mixed with air
molecules, we have moist air. If such air rises, expands and cools, the
water molecules lose some of their agitated motion and have a greater
tendency to stick together to form droplets. But it is not easy to get
them started on this joint enterprise.
If two or three molecules stick together, they soon are shaken apart.
If, however, two or three dozen are collected, this is enough to start a
growth which ends in a droplet of water. If moist air is cooled,
droplets will form, provided there is a meeting place from which the
growth can start. If there is no such meeting place, there are no
droplets and we get no cloud. If there are few meeting places, each will
collect a rather great amount of water, we will get big drops, and we
may get rain. If there is an abundance of meeting places many tiny
droplets are formed which will remain suspended as a cloud. The present
attempts at rain-making are connected with a birth-control of droplets.
We have seen earlier that in each radioactive decay charged particles
are emitted. As these move along their paths, they tear up more atoms
and leave in their wake an assembly of charged particles. These charged
particles strongly attract the molecules of water. They attract the
molecules of air much less. The reason is that in a water molecule
positive and negative charges are separated to a considerable extent
whereas in the nitrogen and oxygen molecules of air the charges are
distributed more evenly. As a result the track of each particle emitted
in a radioactive decay provides many meeting places for the formation of
water droplets.
Actually, cooled moist air has been used for many decades to make the
tracks of fast charged particles visible. In one of the photographs you
can see a picture of such “vapor trails.” It is a photograph through an
apparatus called the Wilson Cloud Chamber. The myriads of radioactive
disintegrations in the debris of a nuclear explosion can give vapor
trails which coalesce into a real cloud. In this way weather might be
influenced. (See pictures 11 and 12.)
In spite of all this it remains highly probable that testing of nuclear
explosions, as practiced at present, does not influence the weather.
Radioactivity does furnish an opportunity for droplets to form. But
other abundant sources are also available for droplet formation. Dust,
smoke and many forms of air pollution will do the trick. Foam scattered
from ocean waves evaporates and leaves a speck of salt behind. This
particle of salt may be carried by the winds for many miles and may
eventually become the germ around which a new drop will condense. The
cosmic rays by which we are bombarded give rise to vapor trails similar
to those produced by the radioactive decay products. Among the many
processes of nature and the usual by-products of civilization the few
atomic tests do not play an important role. This statement can stand,
not as a certainty, but as a very good guess.
Among the many surprises that the future holds one may be closely
connected with the weather. In the age of the airplane we are getting
more and more information about the air masses around us. Air travel
demands this information and also furnishes it. New techniques, such as
radar, can detect the formation of a cloud and can measure the size of
droplets at a great distance. In fact the information received is so
plentiful that one may doubt whether we can properly understand it and
utilize it.
Fortunately we no longer need to rely exclusively on our own brains.
Human thought is a remarkable thing but it is slow. The modern computing
machines, the “electronic brains,” are simpletons as compared to the
apparatus which each of us wears in his skull. But the electronic
computers have one advantage: they are fast. Soon they will be a million
times as fast as our mental processes. The expression “fast as thought”
is dated—it is a contemporary of the horse-and-buggy.
The electronic machines can digest weather information as fast as it is
received. Some progress has already been made. In a few years all
weather predictions may be machine-made.
This need not mean that weather can be predicted with certainty or for a
long time ahead. The trigger processes which, starting from an
insignificant and unnoticed spot of turbulence, can grow into the
dimensions of a cyclone will set a limit to any art of prediction.
But to the extent that weather cannot be predicted it may be influenced.
If small causes may have big effects then even the puny means available
to man may change the weather—provided we know how and where to apply
the lever.
First we shall have to acquire a better understanding of the
weather-science of meteorology. Then we shall have to look for the
appropriate trigger mechanism. This may be a cloud of dust of the right
kind—or else a chemical—or perhaps a great number of radioactive
particles. In one way or another atomic explosions may be used as the
trigger but the trigger will not be effective until and unless the rest
of the machinery is understood.
Of course atomic explosions cannot be used in really significant numbers
unless we learn how to avoid those radioactive by-products which are
really dangerous. Fortunately the use of nuclear fusion, best known from
the hydrogen bomb, makes it possible to regulate the kind of
radioactivity one obtains. We may make only such kinds of activity which
decay before they have a chance to get into the human body.
Experience has proved that to talk about weather is not dangerous. To do
something about the weather will be more risky. Shall the weather become
a ward of the government? Shall we have Republican Rainstorms and
Democratic Droughts? In this way we shall certainly lose the last safe
topic of conversation.
In the narrower confines of Europe where sovereign nation is a few hours
from sovereign nation (as the wind blows) the situation will be much
more serious. But even the whole planet may prove too small for fiercely
conflicting interests when more knowing fingers are placed on more
sensitive triggers.
To govern the weather can be most useful. It could give ample livelihood
to all the people of the earth and to many more billions. Such endeavor
is surely good and it would appear peaceful. But in this case as in many
other cases knowledge will lead to power and power will lead to disaster
if it is not tempered by wisdom.
Yet this knowledge or some similarly dangerous knowledge will come to us
in our lifetimes. Nuclear explosions do not stand alone as a potential
source of mischief.
CHAPTER XVII
Safety of Nuclear Reactors
At the beginning of the scientific and industrial revolution two old
ambitions were found to be impossible dreams. One was the transmutation
of elements, the other the machine of perpetual motion.
Modern nuclear physicists had to retract one of these statements:
elements can be transmuted. But the product is expensive, for the time
being much more expensive than gold.
The perpetual motion machine remains impossible in principle but the
problem may be considered solved in practice. It can be proved, of
course, that a machine can do useful work only if it burns up some fuel.
But the price of fuel is quite often less than the cost to operate and
maintain the machine.
Nuclear fuel even today is no more expensive than conventional fuel in
many parts of the United States. Nuclear fuel is neither heavy nor bulky
and can be therefore transported easily. In those parts of the world
where ordinary fuel is expensive, nuclear energy will soon become of
great importance. Furthermore, we shall learn to use most of the energy
in uranium rather than just the part contained in its rare and valuable
isotope, U²³⁵.
One only has to add a neutron to common U²³⁸ to get radioactive U²³⁹. In
the course of time this decays into plutonium. This element can be used
like U²³⁵: It produces fission, a great amount of energy and enough
neutrons to keep the process going. We shall also learn to extract
energy from other nuclear fuels. Thorium acts like uranium, while
deuterium can give energy by building up bigger nuclei rather than
breaking them into smaller pieces. Therefore the source of energy will
be universally available and quite inexpensive. This really means that
we are as well off as though we had a machine of perpetual motion.
But, of course, all this does not mean that the machine will do its job
free of charge. Even a perpetual motion machine would need servicing and
maintenance. Unfortunately our nuclear machines need a lot of such
servicing and therefore for the time being, nuclear energy is not the
cheapest.
The main reason why a nuclear energy source, or a nuclear reactor is
difficult and expensive to run is that the reactor after a short time of
operation becomes strongly radioactive. Therefore it cannot be
approached and it has to be handled by remote control. We can hardly
expect that energy will be free like air or water. But when we learn how
to handle inexpensively our nuclear machines, we shall be able to obtain
energy for a reasonable price at any place on the earth. Sooner or later
conventional fuel will become scarce. But nuclear energy will allow the
industrial revolution to continue and to expand into every corner of the
earth.
There can be little doubt that during the next decades nuclear reactors
will greatly multiply and by the beginning of the next century they will
be found everywhere. It is therefore of the greatest importance that
these reactors should be operated safely. On the face of it, a nuclear
reactor is a sluggish instrument which can be made to run itself. But
the ease of operation is deceptive. (See picture 13.)
One need not fear that a nuclear reactor might explode like an atomic
bomb. Nuclear explosives are very carefully constructed so that they can
release a lot of energy in a short time. Nuclear reactors on the other
hand are put together so as to make it possible that energy will be
released only at a moderate rate. Some reactors if improperly handled
may explode, but the violence of the explosion cannot greatly exceed
that of a similar weight of high explosive.
Nevertheless a reactor accident could become exceedingly dangerous. The
reactor is charged with radioactive fission products and some other
radioactive substances produced by neutron absorption. Any accident
which will allow even a portion of these products to escape into the air
will endanger people at a considerable distance in the downwind
direction. One reason why reactors can be dangerous is that in
protracted operation of the reactor, fission products which have longer
lives accumulate. It is precisely these longer-lived products which are
more dangerous because they have a better chance to find their way into
the human body.
Reactors are now planned which will produce 300,000 kilowatts of
electricity. If such a reactor operates for half a year and then
explodes and releases its radioactive content into the atmosphere, its
radioactivity will be comparable to that of a hydrogen bomb. In one
important respect such an accident would be worse than a hydrogen
explosion. The nuclear explosive lifts most of its radioactive products
to a high altitude and the poisonous activity gets dispersed and diluted
before it descends. The activity from a reactor on the other hand will
remain close to the ground and might endanger the lives of the people in
an area of hundreds of square miles. It will contaminate an even greater
territory.
In the extensive operation of many reactors in the United States no one
has yet been killed by the radioactivity. This has been due to extremely
careful operation and also to good luck. We must be prepared that sooner
or later accidents will occur. On the other hand we must try to take
sufficient precautions to avoid the kind of catastrophic accident which
we have mentioned above. With great care such accidents can indeed be
avoided.
In thinking of all kinds of man-made machines we find some which move
fast and seem dangerous like, for instance, airplanes; others which are
stationary and apparently harmless, like the bath tub. Yet more
accidents happen in bath tubs than in air travel. The most dangerous
element in all operations is the human element. We ourselves constitute
the greatest safety hazard. This is a situation no different in nuclear
technology than in any other kind of technology. What is new in nuclear
technology is that a reactor is usually very safe but may become
extremely dangerous when something unexpected happens to it. Also we
dare not use the method of trial and error. An error in the reactor
business could exact a far heavier toll of lives than an error in the
testing of H-bombs. We cannot wait to learn by experience; we must
forestall accidents.
An especially difficult safety problem is connected with the use of
reactors in small countries. A serious accident could endanger the lives
of people in adjacent countries. Thus modern technology may force
cooperation across national boundaries.
There is only one way to avoid traffic accidents and that is care
exercised by everyone, particularly the drivers. Similarly reactor
safety will depend on the people who operate the reactors. At the same
time a lot of help can be obtained by careful construction and scrutiny
of each new reactor.
One of the first acts of the Atomic Energy Commission was to establish a
Committee for Reactor Safeguards. With the passing of years this
committee had to take on more heavy responsibilities. At first it had to
operate under secrecy. With the wider and more public use of reactors
the safety considerations are becoming more available to the public. The
question of safe operation of a machine cannot be separated from a
thorough understanding of the working of the machine. We cannot attempt
to give an adequate description of a reactor or of the safety rules. A
few general statements have to suffice.
A working reactor is full of neutrons. In a small fraction of a second
these neutrons produce fission and a new generation of neutrons comes
into being. In slow reactors which contain lots of light elements like
hydrogen or carbon, the neutrons move with speeds little greater than
that of sound and a generation may last as long as a millisecond (one
thousandth of a second). In fast reactors which contain almost
exclusively heavier elements like uranium or iron, neutrons move with a
great speed which is about three per cent of the speed of light. In this
case one generation replaces another in less than a microsecond (one
millionth of a second).
Fortunately not all the neutrons get reproduced so rapidly. Some
fissions produce delayed neutrons which are emitted usually with a delay
of several seconds. In a steadily working reactor each generation should
have the same number of neutrons as the previous one. If each succeeding
generation has even a slight surplus, the reactor will become hot and
may explode in a small fraction of a second. The main reason why safe
operation is possible is the fact that fast multiplication can occur
only if each generation becomes more populous _even when one does not
count the delayed neutrons_. A slightly overactive reactor is easily
governed, but there comes a point when the dormant dragon begins to
stir. This happens when there are enough neutrons produced so that
multiplication can occur without waiting for the delayed neutrons. At
that point a well behaved dragon will perform a harmless action. For
instance it may blow a fuse. But a vicious dragon will spit radioactive
fire.
It is not easy to predict whether the dragon will be always well
behaved. But with careful analysis one can make such a prediction. For
instance one must look into the question of whether the reactor is
stable. If it gets hotter, does this make the reactor proceed even
faster so that the rate of heating increases and the reactor runs away?
In a stable reactor excess heat should tend to stop the energy
production and thus the reactor cools and returns to its normal
operating temperature.
But too great a stability may also be dangerous. Heating may be
overcompensated by the cooling mechanism; after the reactor has become
too cold it may then heat up too fast and overshoot again. We must guard
not only against a simple run-away, but also against increasing
oscillations.
In many reactors unusual chemical compounds are used. A reactor accident
may start with nothing worse than an ordinary chemical reaction between
strange compounds under strange conditions. But if this chemical
reaction destroys the reactor sufficiently to allow some fission
products to escape, then such a chemical accident can be as bad as one
of nuclear origin.
In the interior of the reactor materials are exposed to unusually strong
radiation. Under this effect some materials can change their chemical
properties so that what has been inert as a construction material may
become dangerous during the operation of the reactor.
Perhaps the most important single item is the arrangement of mechanical
controls. The reactor is adjusted by a system of sheets or rods made of
a material which absorbs neutrons. This arrangement must be so
constructed that the control rods can be withdrawn only at a very slow
rate. But it must be possible to put them back quite fast. Any danger
signal should shove the absorbers in at maximum speed. The technical
expression is “scram.”
The main point, however, is that all the dangers and safety devices can
be studied and after careful study a nuclear accident can be avoided.
Some reactors are now so thoroughly understood that they can be safely
used for training of future nuclear engineers. Other reactors which are
more powerful or less well studied have to be used more carefully. Some
reactors should be, and are being, enclosed in gas-tight containers. If
an explosion occurs the fission products will be harmlessly confined
inside the container. Of course, one must be quite sure that the reactor
is of such a type that it cannot produce an explosion great enough to
burst the container and what is even more important one should be quite
sure that the container is closed except when the reactor is shut down
and completely safe. Often it may be best to build the reactor
underground.
The safety of a reactor, of course, depends to a great extent on the use
to which the reactor is put. In general a power station is less likely
to give trouble than a moving power source. It is not probable that
nuclear locomotives will ever be safe. In nuclear ships more room is
available and more room permits more safety measures. But even so the
safety of nuclear motors in ships will have to be considered
particularly carefully because ships will have accidents in harbors.
Between the urgent need for progress and the absolute necessity of
safety it is difficult to keep a sense of balance and one can easily
make the mistake of being unnecessarily cautious. Such unnecessary
caution was probably exercised when the Committee on Reactor Safeguards
considered the earthquake hazard of the Brookhaven reactor on Long
Island. A seismologist, who is a Jesuit Father, was asked to tell the
committee[15] of the possibilities and probabilities of an earthquake on
Long Island. The chairman[16] of the committee subjected the expert to a
long and detailed questioning. After half an hour the Committee on
Reactor Safeguards ran out of questions. But the Jesuit Father had not
given any signs of running out of answers. The session being at an end
the expert, looking the chairman of the committee firmly in the eye and
in a more authoritative voice than he had yet used, said, “Mr. Chairman,
I can assure you on the highest authority that there will be no major
earthquakes on Long Island in the next fifty years.”
CHAPTER XVIII
By-products of Nuclear Reactors
Nuclear reactors generate energy with the help of nuclear fission. Every
time a fission occurs we are left with radioactive by-products. It is
most important to prevent the uncontrolled escape of these fission
products from the reactor. Fortunately the dangerous products can be
retained in the reactor—if the machine has been constructed and operated
with reasonable care.
In the end, however, the burnt or partly burnt uranium charge will have
to be removed from the reactor and fresh charge, fresh fuel will have to
be added. What will become of the fission products at this time?
During protracted operation of a reactor most of the short-lived fission
products decay. Those with longer lives accumulate. The discharge of the
reactor is strongly radioactive, and it will remain radioactive for many
years. One certainly must not dispose of this radioactive waste in a
careless manner. There are, however, many ways in which one can store
such waste with reasonable safety.
One can deposit the radioactive material in well-built underground
tanks. One can concentrate the activity, imprison it in concrete blocks,
and deposit it at the bottom of the ocean. If one is very much worried
he might even put the radioactivity in rockets and let it decay
harmlessly in outer space. These procedures will cost money and will add
to the expense of nuclear energy.
It would be far better if we could find a way in which the radioactive
by-products could be made to serve a useful and safe purpose. Some of
the by-products can be used and have been used. These uses are connected
with some hazards. Furthermore, only a small fraction of the fission
products have found good employment up to the present. But the
importance of fission products is growing.
We are using them in research. A radioactive isotope imitates the
behavior of its non-active brother in all chemical reactions and in all
the intricate processes in which matter changes its form inside a living
body. Furthermore a radioactive substance can be detected with the
greatest ease. It can be found in a concentration which is less than a
millionth of a safe radiation dose. What the microscope has been in the
exploration of the structure of organisms, the radioactive elements may
become in the understanding of the chemical functioning of living
matter.
With better understanding there comes the possibility of using
radioactive by-products for diagnosis. As with the medical use of X-rays
the possible small damage due to radiation exposure should be regarded
as the price for the help we can get from early and correct recognition
of diseases.
In the treatment of patients, particularly in the case of persons
stricken by cancer, radioactive destruction of the diseased tissue is
often preferable to the use of the surgeon’s knife. Such radioactive
treatment is new. There is much room for improvement. Appropriate use of
radioactive substances for this purpose may become a far more powerful
tool and much more widespread than it is at present.
But all these applications will use up only a vanishing fraction of the
fission products. Moreover, most of the biologically important elements
are not produced in the fission of uranium. Many useful activities can
be produced by neutron absorption in reactors. But among the fragments
of uranium perhaps only radio-iodine has been put so far to direct
physiological use.
Industry deals with less sensitive objects than living tissue. Therefore
greater amounts of radioactive materials can be used here. And indeed
radioactivity has done a great variety of jobs. The penetrating power of
X-rays has been used to control the thickness of sheets in an easy and
automatic manner. Radioactivity has been incorporated into surfaces
which are exposed to mechanical wear or corrosion, to check the rate at
which the surface is worn away by the appearance of activity in the
lubricant or other fluids which have been in contact with the surface.
By such methods industry has accumulated savings which are rapidly
approaching the billion dollar mark. These savings will increase as
people learn how to use the new materials. But in all these cases it is
important to make sure that the activity will not hurt anyone while it
is used and after it has served its purpose.
Possibly the greatest amount of radioactivity will be needed in food
sterilization and preservation. One may incorporate the activities into
rods which will safely retain the materials but which will allow a
considerable fraction of the penetrating gamma rays to escape.
To sterilize food means to destroy all microorganisms. Many of these are
radiation-resistant and may have to be exposed to 50,000 or more
roentgens—that is one hundred times as much as would kill a mammal.[17]
Such massive irradiation begins to affect the foodstuff itself. In some
cases sterilization by irradiation changes the food more than would be
the case by boiling it or freezing it. In other cases irradiation
produces less undesirable side effects than any other methods.
Another way to use radiation is the preservation of agricultural
products. This need not be done by the difficult procedure of
sterilization. It is enough to control pests and to prevent germination
of the seeds which one is trying to preserve. Thus we need here
approximately one per cent of the radiation that would be required for
sterilization. By so little radiation the food is not altered to a
noticeable extent. It is precisely in such processes, where great
amounts of materials will have to be irradiated, that a substantial
fraction of the fission products might find employment.
In all applications care has to be exercised lest radioactive materials
should inadvertently be scattered around. Where great amounts are needed
as in food sterilization and preservation, caution has to be redoubled.
That trouble may arise has been illustrated by an occurrence in Houston,
Texas.
Radioactive iridium¹⁹², which is a beta and a gamma emitter, was being
used by a certain industrial concern to take X-ray pictures of metal
parts. A shipment of this radioactive material in the form of powder
pellets was being opened by remote control when compressed gas in the
container exploded and scattered some radioactivity around. The area was
shielded but some of the radioactive dust escaped to the rest of the
building. The two men who were operating the remote control apparatus
became contaminated. They washed themselves and cleaned up the area but
did not report the incident.
A few weeks later a standard radiation check showed that the plant was
still radioactive. Company officials became worried and called in
experts. At this late stage the plant was thoroughly decontaminated. The
homes of the two men were also examined and were found to be slightly
radioactive. The men and their families were temporarily moved out while
their homes were being cleaned up. When they returned, neighbors and
friends shunned them. The four year old son of one of the men lost his
playmates. People were afraid to enter the houses. One of the houses was
put up for sale but no one wanted to buy it.
The fact that the houses had been checked by radiation meters and found
to be clean, and the fact that the half-life of iridium¹⁹² is only 75
days so that any trace of activity would disappear in a reasonably short
time, did not dispel people’s fears.
It is fortunate that no one was seriously hurt in this incident. But
there is an important lesson we can learn from it: Ignorance may hurt
more than radioactivity. That a house should lose its value in spite of
the fact that its radioactive contamination has been removed, that a
little boy should be shunned as though radioactivity were infectious
like the plague—these are examples of suffering caused by one of the
greatest sources of human misery: unreasoning fear.
The greatest potentialities of fission products for the future might lie
in still a different direction. Radioactivity can induce mutations. To
what extent this is a danger we have discussed in an earlier chapter. In
the hands of a breeder who tries to bring about changes in animals or
plants radioactivity could become exceedingly useful.
Of course it is true that most mutations are harmful. It is also true
that artificial mutations have been produced for many decades. But now
it is possible to place simple and cheap tools in the hands of many more
people. Therefore the chances will increase to find among the many wrong
mutations the few and decisive changes which lead to improvement.
Do we dare to place dangerous materials in so many hands? We should not
do so without making certain that only competent and responsible
individuals will get radioactive materials. This can be done. Druggists
have dispensed poison; doctors and biologists have bred in their
laboratories the multiplying menace of germs. All this was done and is
being done with safety and to the great benefit of all people.
The use of radioactivity should be even more safe because this material
is easy to detect. If poisons or germs become lost, they may be hard to
find. Radioactive materials, however, give unmistakable evidence of
their presence. It is, of course, never easy to find a needle in a
haystack. But the chance to find it is much better if it is a
radioactive needle.
Radioactive by-products need not remain what they seem to be today: dirt
and danger to be disposed of and hidden. But in the immediate future we
shall incur some expense to keep radioactivity in a safe place.
Some gaseous by-products like the long-lived krypton⁸⁵ (half-life: 10.4
years) might continue to give rise to real difficulties and to
considerable expense. The trouble is, of course, that a noble gas like
krypton will not be bound to any material by strong bonds. It may be
inadvisable to let long-lived gases escape. On the other hand, their
adsorption or their storage at low temperature or high pressure may
prove to cost a considerable amount of money.
We have been talking about the problem of handling the by-products of
nuclear power. This problem will not appear in proper proportion unless
we also give some thought to the by-products of the kind of power we are
using at present.
That we do not like smoke and smog is obvious. To what extent these
residues of incomplete burning can cause cancer or other damage we do
not know. Chemistry is more tricky than radiation. Our lack of knowledge
about the slow biological effects of chemicals is much greater than our
remaining uncertainties about radiation.
In addition to the obvious annoyance and worry caused by the products of
incomplete combustion there exists an interesting question connected
with the result of complete combustion. The carbon that has been
deposited through the geologic ages as coal and as oil is being used up
gradually and converted to a colorless, odorless, harmless gas—carbon
dioxide. There is always some carbon dioxide in our atmosphere. The
amount is approximately 300 parts per million of common air. All the
carbon that has been burned since the beginning of the industrial
revolution could have increased the carbon dioxide in the atmosphere by
ten per cent to the value of 330 parts per million.
This increase could be significant. Carbon dioxide acts like a blanket
or a valve for some kinds of radiation. In the daytime we receive energy
in the form of visible light from the sun. This form of radiation has no
difficulty in penetrating the carbon dioxide gas. However, the incoming
radiation is balanced by invisible heat radiation, which flows out from
the earth into space day and night. This infrared radiation is quite
similar in nature to light, only our eyes are not sensitive to it. Now
the carbon dioxide gas acts like a barrier, though only a partially
effective barrier, to this outgoing heat radiation. If the carbon
dioxide content of our atmosphere were to increase too greatly, it would
act like the glass in a greenhouse and our climate would grow warmer.
A ten per cent increase in the carbon dioxide content of the atmosphere
should have produced an observable rise in temperature. Such a
temperature rise has not, in fact, been observed. The reason is that not
all the carbon dioxide which has been generated in the processes of
combustion has actually remained in the atmosphere. Most of it has found
its way into the great reservoir of our oceans. Some of it is deposited
as lime at the bottom of the oceans. However, some time is required for
the carbon dioxide to be removed from the atmosphere and to reach the
oceans. One would expect, therefore, that there would have been at least
a slight increase in the carbon dioxide content of the atmosphere.
Measurements show that this is the case and that the increase is about
two per cent—which is too small to have changed our climate.
If we continue to consume fuel at an increasing rate, however, it
appears probable that the carbon dioxide content of the atmosphere will
become high enough to raise the average temperature of the earth by a
few degrees. If this were to happen, the ice caps would melt and the
general level of the oceans would rise. Coastal cities like New York and
Seattle might be inundated.
Thus the industrial revolution using ordinary chemical fuel could be
forced to end before the advantages of civilization have spread all over
the earth. However, it might still be possible to use nuclear fuel. With
nuclear fuel the industrial revolution and its countless benefits for
man could continue to every part of the globe. The by-products of the
nuclear age are less bulky and therefore are more easily handled than
the by-products of our coal- and oil-economy. The main advantage of
nuclear energy may yet turn out to be this: With proper care nuclear
energy may turn out to be the cleanest among the available sources of
power.
CHAPTER XIX
The Nuclear Age
The future depends on people. People are unpredictable. Therefore, the
future is unpredictable. However, some general conditions of mankind
depend on things like the development of technology, the control won by
man over nature and the limitations of natural resources. These can be
predicted with a little greater confidence. The future is unknown but in
some respects its general outline can be guessed.
Such guesses are important. They influence our present outlook and our
present actions.
The nuclear age has not yet started. Our sources of energy are not yet
nuclear sources. Even in the military field, where development has been
most rapid, the structure of the armed forces has not yet adjusted
itself to the facts of the nuclear age in a realistic manner. In
politics the atomic nucleus has entered as a promise and as a menace—not
as a fact on which we can build and with which we can reckon.
Some technical predictions seem safe:
Nuclear energy will not render our older power plants obsolete in the
near future. But nuclear energy will make it possible to maintain the
pace—even the acceleration—of the industrial revolution. It will be
possible to produce all the energy we need at a moderate cost.
Furthermore—and this is the important point—this energy will be
available at any place on the globe at a cost which is fairly uniform.
The greater the need for power, the sooner will it be feasible to
satisfy the need with the help of nuclear reactors.
Nuclear energy can be made available at the most outlandish places. It
can be used on the Antarctic continent. It can be made to work on the
bottom of the ocean.
The expanding front of industrialization has been called the “revolution
of rising expectations.” That nuclear energy should be involved in the
current and in the turbulence of this expanding front, is inevitable.
One can say a little more about the effects of scientific and
technological discoveries on the relations among the people of the
globe. With added discoveries raw materials will no longer be needed
with the old urgency. For most substances substitutes are being found.
This may make for greater economic independence.
On the other hand, new possibilities will present themselves. We shall
learn how to control the air and how to cultivate the oceans. This will
call for cooperation and more interdependence.
The dangers from radioactive by-products will act in a similar
direction. The radioactive cloud released from a reactor accident may be
more dangerous than a nuclear explosion. Such a cloud will not stop at
national boundaries. Some proper form of international responsibility
will have to be developed.
What effect the existence of nuclear weapons will have upon the
coexistence of nations is a question less understood and less explored
than any other affecting our future. Most people turn away from it with
a feeling of terror. It is not easy to look at the question with calm
reason and with little emotion.
A few predictions seem disturbing but are highly probable:
Nuclear secrets will not keep. Knowledge of nuclear weapons will spread
among nations—at least as long as independent nations exist.
Prohibition will not work. Laws or agreements which start with the word
“don’t” can be broken and will always be broken. If there is hope, it
must lie in the direction of agreements which start with the word “do.”
The idea of “Atoms for Peace” succeeded because it resulted in concrete
action.
An all-out nuclear war between the major powers could occur but we may
have good hope that it will not occur if we remain prepared to strike
back. No one will want to provoke the devastation of his own country.
Atomic bombs may be used against cities. But there will be no military
advantage in destroying cities. In a short and highly mobile war neither
centers of supply and communication nor massive means of production will
count. If cities are bombed, this will be done primarily for reasons of
psychological warfare. We must be and we are prepared for this kind of
war but only as a measure of retaliation. There is good reason to
believe that as long as we are prepared for all-out war, our civilian
population will not suffer from a nuclear attack.
The certainty of a counterblow gives real protection against all-out
war. No such protection exists against wars limited in territory and in
aims. In the history of mankind such wars have been most frequent. There
is no indication that these limited wars have ended. We must be prepared
for these conflicts with effective and mobile units, and this requires
the use of nuclear firepower.
Nuclear weapons will certainly have a profound effect upon such limited
warfare. Not all of this effect need be and indeed it must not be in the
direction of greater devastation.
In a nuclear war it will not make sense to use massed manpower. Any such
concentration will provide too good a target for atomic weapons. To use
big, costly and conspicuous machines of war will be unwise. Such
machines will be defeated by nuclear explosions in the same way as the
mailed knight went down before firearms.
Any fighting unit in a nuclear war will have to be small, mobile,
inconspicuous and capable of independent action. Such units whether on
sea, land or in the air cannot rely and will not rely on fixed lines of
supply. There will be no possibility and no need to occupy territory and
to fight at fixed and definite fronts. If a war should be fought for
military reasons and for military advantage, it will consist of short
and sharp local engagements involving skill and advanced techniques and
not involving masses that slaughter and are being slaughtered.
If an invader adopts extreme dispersion, it will become impossible to
defeat him with atomic weapons. But a very highly dispersed army can be
defeated by a determined local population. Therefore the main role of
nuclear weapons might well be to disperse any striking force so that the
resistance of people defending their homes can become decisive. Nuclear
weapons may well become the answer to massed armies and may put back the
power into the hands where we believe it belongs: the hands of the
people.
At this point we are brought back to the main topic of this book:
radioactivity. In a limited nuclear war the radioactive fallout will
probably kill many of the innocent bystanders. We have seen that the
testing program gives rise to a danger which is much smaller than many
risks which we take in our stride without any worry. In a nuclear war,
even in a limited one, the situation will probably be quite different.
That noncombatants suffer in wars is not new. In a nuclear war, this
suffering may well be increased further due to the radioactive poisons
which kill friend and foe, soldier and civilian alike.
Fortunately there exists a way out. Our early nuclear explosives have
used fission. In the fission process a great array of radioactive
products are formed, some of them intensely poisonous. More recently we
have learned how to produce energy by fusion. Fusion produces fewer and
very much less dangerous radioactivities. Actually the neutrons which
are a by-product of the fusion reaction may be absorbed in almost any
material and may again produce an assortment of radioactive nuclei.
However, by placing only certain materials near the thermonuclear
explosion, one may obtain a weapon in which the radioactivity is
harmless. Thus the possibility of clean nuclear explosions lies before
us.
Clean, flexible and easily delivered weapons of all sizes would make it
possible to use these bombs as we want to use them: as tools of defense.
When stopping an aggressor we would not let loose great quantities of
radioactive atoms which would spread death where we wanted to defend
freedom. Clean nuclear weapons would be the same as conveniently
packaged high explosives. They would be nothing more.
The possibility of clean explosions opens up another development: the
use of nuclear explosives for the purposes of peace. Conventional high
explosives have been used in peace fully as much as in war. From mining
to the building of dams there is a great variety of important jobs that
dynamite has performed. Nuclear explosives have not been used in a
similar way. The reason is the danger from radioactivity. Once we fully
master the art of clean explosions peaceful applications will follow and
another step will be made in controlling the forces of nature.
All this is of course only a small part in the process of the increasing
power of man and the increasing responsibility of man. As the impossible
of yesterday becomes the accomplished fact of today we have to be more
and more aware of our neighbors on this shrinking planet. The arts of
peace may lead to conflicting interest as easily as they may lead to
fruitful cooperation. If we ever learn to control the climate of the
world, a nation may find itself in the same relation to another nation
as two farmers who have to use the waters of the same river.
Rivals are men who fight over the control of a river. When the same word
“rivals” comes to mean cooperation for the best common use of the river
or any other resource—that will be the time of law and of peace. Surely
this sounds like Utopia and no one sees the way. But the general
direction in which we should go is not to consider atomic explosives and
radioactivity as the inventions of the devil. On the contrary, we must
more fully explore all the consequences and possibilities that lie in
nature, even when these possibilities seem frightening at first. In the
end this is the way toward a better life. It may sound unusually
optimistic in the atomic age, but we believe that the human race is
tough and in the long run the human race is reasonable.
GLOSSARY
Activity: Short for radioactivity. Also the strength of a radioactive
source measured in disintegrations per second.
Air burst: A nuclear explosion at such an altitude that the fireball
does not touch the earth’s surface. An air burst produces very
little local fallout.
Alpha ray (particle): Energetic but non-penetrating radiation emitted by
heavy radioactive nuclei. An alpha particle consists of two
neutrons and two protons, and is identical with the nucleus of the
ordinary helium atom.
Atom: A positively charged nucleus surrounded by negatively charged
electrons.
Atomic bomb: A fission bomb.
Atomic cloud: The cloud remaining after the energy of the explosion has
been carried off by the shock wave and the thermal radiation. It
consists of condensed water vapor, ground material, and bomb
debris including the radioactivity.
Atomic energy: Energy released in nuclear reactions, for example in
fission. Atomic energy and nuclear energy mean the same thing, but
the latter name is more appropriate.
Atomic reactor: Same as nuclear reactor.
Background radiation: Natural radiation due to cosmic rays, and due to
radioactive substances in the earth, in the atmosphere, and in our
own bodies.
Beta ray (particle): An energetic electron or positron emitted by some
radioactive nuclei. Practically all of the fission products are
beta (electron) emitters.
Blast wave: Same as shock wave.
Cesium¹³⁷: A radioactive fission product. It emits a 0.5 million volt
beta ray and a 0.7 million volt gamma ray with a half-life of 30
years. The daughter nucleus is stable barium¹³⁷.
Chain reaction: Self-maintained sequence of fissions. Neutrons released
by the fission of one nucleus are used to induce fission in
another nucleus.
Chromosome: A small irregularly shaped body found in cells. Chromosomes
carry the genes, which are responsible for heredity.
Clean bomb: A nuclear bomb which produces heat and blast, but only a
negligible amount of radioactivity. The energy of such a bomb is
derived almost entirely from the fusion process.
Cobalt⁶⁰: Radioisotope—decays into nickel⁶⁰ with the emission of a weak
beta ray. The half-life for this decay is 5.3 years. The nickel⁶⁰
immediately ejects two gamma rays with a total energy of 2.5
million electron-volts.
Cobalt bomb: A radiological bomb which produces a large quantity of
cobalt⁶⁰.
Control rod: A rod of neutron-absorbing material used to control the
power level of a nuclear reactor.
Cosmic rays: Energetic particles from outer space. They induce nuclear
reactions in the earth’s atmosphere and thus contribute to the
background radiation. This cosmic radiation is more intense at
high altitudes than at sea level.
Counter: A device which detects nuclear radiation.
Critical mass: The amount of fissionable material required to sustain a
steady chain reaction. With less than the critical amount, the
reaction stops because too many neutrons are lost.
Cyclotron: A machine that accelerates charged particles to high energy.
Energetic charged particles can be used to induce nuclear
reactions.
Daughter: The nucleus which remains after decay of a radioisotope.
Decay: Spontaneous process in which a radioactive nucleus emits an
alpha, beta, or gamma ray.
Delayed neutrons: Those released after a fraction of a second to a
half-minute or so by the fission products. They comprise less than
one per cent of the total number of neutrons released in the
fission process but are useful for the purpose of control in
reactors.
Deuterium: Stable hydrogen isotope. Its nucleus (called a deuteron)
consists of one proton and one neutron.
Disintegration: Same as decay.
Dose: A quantity of radiation—usually measured in roentgens.
E = mc²: Einstein’s equation relating mass (m) and energy (E). The speed
of light (c) enters as a proportionality constant. The equation
asserts that one pound of mass is equivalent to ten megatons of
energy. In the fission process only one-tenth of one per cent of
the mass is converted. Therefore, to produce ten megatons of
energy by fission 1000 pounds of uranium would be required.
Electromagnetic radiation: Includes radio waves, visible, infrared, and
ultraviolet waves; also X-rays and gamma rays. The latter two are
energetic, penetrating forms of radiation.
Electron: A particle having a unit negative charge and a weight equal to
1/1840 of the weight of the lightest atom (hydrogen).
Electron capture: process in which an atomic electron unites with a
proton in the nucleus producing a neutron and a neutrino.
Electron-volt: The amount of energy acquired by an electron which is
accelerated through an electric potential of one volt. Typically,
the energy required to “knock” an electron out of an atom is a few
electron-volts or so; particles ejected from radioactive nuclei
have energies between a few hundred thousand and a few million
electron-volts.
Element: A collection of atoms whose nuclei all have the same charge. An
element may consist of many isotopes.
Enriched material: Uranium which contains a greater proportion of the
235-isotope than is found in the natural ore.
Excited state: A state of an atom, molecule, or nucleus having excess
energy. As soon as possible this excess energy is released and the
system goes to the ground state.
Fallout: Radioactive particles from an atomic explosion. They may be
carried in the atomic cloud to large distances from ground zero
and then “rained down” to the earth’s surface.
Fireball: The luminous ball of hot air and bomb material which expands
and cools as the shock wave races out.
Fission: The breaking-up of a heavy nucleus into two or more fragments.
A large amount of energy and some free neutrons are released in
the process.
Fissionable material: Isotopes which undergo fission when bombarded by
_slow_ neutrons: uranium²³⁵ and plutonium²³⁹.
Fission products: Fission fragments and their daughters, including
hundreds of different radioactive species, among them strontium⁹⁰
and cesium¹³⁷.
Fusion: The combining of light nuclei into heavier ones with a release
of energy. For example, deuteron + triton → alpha + neutron. About
18 million electron-volts are released in this process.
Gamma ray: Energetic, penetrating electro-magnetic radiation emitted by
certain radioactive nuclei, frequently after a beta emission.
Genes: Parts of the chromosomes. They are big molecules that determine
heredity.
Ground state: The state of least energy and greatest stability of atoms,
molecules, and nuclei.
Ground zero: The point on the surface of the earth directly above or
below a nuclear explosion.
Half-life: The time required for one half of a large number of identical
radioactive nuclei to disintegrate.
H-bomb: Same as hydrogen bomb.
Heavy hydrogen: Same as deuterium.
Heavy water: Water with heavy hydrogen substituted for ordinary
hydrogen.
Hydrogen bomb: A high-yield thermonuclear bomb.
Iodine¹³¹: A radioactive fission product with a half-life of 8 days. It
emits an electron of average energy 0.2 million electron-volts and
a gamma ray of energy 0.4 million electron-volts.
Ion: A charged atom or molecule. Ions are produced in abundance when
energetic charged particles pass through matter.
Ionization: The process of removing electrons from neutral atoms or
molecules. Neutrons and gamma rays as well as energetic charged
particles are very effective in producing ionization.
Iridium¹⁹²: 75 day radioisotope. It emits an electron of average energy
0.2 million volts and a 0.3 million volt gamma ray.
Isotopes: Atoms whose nuclei have the same number of protons but a
different number of neutrons. Such atoms have the same chemical
behavior.
Kiloton: The amount of energy released by a thousand tons of TNT.
Krypton⁸⁵: A radioactive fission product. It has a ten year half-life
and emits an electron of average energy 0.2 million volts and a
0.5 million volt gamma ray.
Leukemia: A usually fatal disease in which white blood cells are
overproduced.
Local fallout: Radioactive fallout in the neighborhood of a nuclear
explosion.
Megaton: The amount of energy released by a million tons of TNT.
Meson: A particle intermediate in weight between the electron and the
proton. Actually, there are two kinds of mesons, called pi and mu.
The pi meson weighs 276 times as much as the electron and is
connected with the forces that hold the nucleus together. The mu
meson weighs 212 times as much as the electron and contributes
appreciably to the cosmic radiation.
Microsecond: One millionth of a second. It takes light 5 microseconds to
go a mile.
Million volt particle: Short for million electron-volt particle.
Moderator: A material used in nuclear reactors to reduce the speed of
neutrons.
Molecule: A combination of atoms held together chemically.
Mutation: A genetic change, which is transmitted to offspring and
affects hereditary characteristics. Such changes in genes may be
caused by radiation as well as chemical and thermal agents.
Neutrino: A weightless, uncharged particle which carries off energy in
the process of beta decay.
Neutron: A neutral particle, one of the basic constituents of the
nucleus. A neutron weighs slightly more than a proton, and when
free, decays into a proton plus an electron and a neutrino.
Noble gases: Helium, neon, argon, krypton, and xenon. They do not
combine chemically with any elements including themselves.
Nuclear bomb: A bomb which derives its energy from nuclear fission or
fusion.
Nuclear reactor: A machine for maintaining a controlled chain reaction.
Nucleus: The core of an atom, consisting of neutrons and protons. Its
charge is equal to the number of protons. Its weight is equal to
the number of protons plus the number of neutrons.
Periodic system: The chemical elements arranged in order of increasing
atomic charge. Elements with similar chemical properties occur
periodically.
Plutonium: Element with charge 94, produced by capturing a neutron in
uranium²³⁸ followed by two beta emissions. Like uranium²³⁵,
plutonium is valuable as an atomic fuel.
Positron: The positive counterpart of the electron.
Potassium⁴⁰: A natural radioactive isotope. It has a half-life of one
billion years and emits beta and gamma rays.
Proton: A constituent of the nucleus. It has one unit of positive charge
and weighs slightly less than a neutron.
Radiation: Energetic charged particles, neutrons and gamma rays which
cause ionization in matter. Radiation is produced in nuclear
explosions but also occurs naturally from cosmic rays and from the
decay of radioactive substances in our surroundings.
Radioactivity: Spontaneous nuclear decay, releasing an alpha, beta, or
gamma ray.
Radioisotope: Short for radioactive isotope.
Radiological bomb: A bomb designed to create radioactive contamination.
Radium: Element with charge 88. The principal isotope has a weight of
226 and emits an alpha particle with a half-life of 1620 years.
Range: Distance traveled by an energetic charged particle in matter
before it stops. Heavy charged particles move in a straight line
inside matter, but electrons frequently change their course. For
this reason the range of electrons is only about one-half the
total distance traveled.
Reactor: Same as nuclear reactor.
Roentgen: A measure of radiation dose—defined in terms of the amount of
energy deposited per unit weight of irradiated material. A dose of
400,000 roentgens in living tissue deposits enough energy to raise
the temperature by 1°C. A dose of only 400 roentgens in a human
being will cause death fifty per cent of the time.
Shock wave: Expanding front of high pressure and strong winds produced
by an explosion.
Spontaneous fission: Natural fission, not induced by a neutron. The
half-life for this process in uranium²³⁸ is 8 × 10¹⁵ years.
Stratosphere: The atmosphere above the weather zone. The altitude of the
stratosphere varies from thirty to fifty thousand feet depending
on latitude and season.
Stratospheric fallout: World-wide fallout from big bombs whose clouds
rise into the stratosphere. On the average the radioactivity
remains in the stratosphere for about ten years and is then
deposited more or less uniformly over the surface of the earth.
Strontium⁹⁰: A radioactive fission product. It has a half-life of 28
years and emits two electrons of average total energy 1.2 million
electron-volts. Strontium is chemically similar to calcium and
gets deposited in bones.
Thermal radiation: Electromagnetic radiation, mainly visible, but also
ultraviolet and infrared, emitted from the fireball of a nuclear
explosion and transmitted long distances in the surrounding cold
air.
Thermonuclear bomb: A bomb which derives a significant fraction of its
energy from the fusion of hydrogen isotopes.
Thermonuclear reaction: A fusion reaction induced by high temperature.
Thorium: Element with charge 90. The principal isotope has a weight of
232 and emits an alpha particle with a half-life of 14 billion
years.
Trigger process: A small cause which leads to a big effect.
Tritium: An isotope of hydrogen. Its nucleus (called a triton) consists
of one proton and two neutrons. Tritons are radioactive beta
emitters having a half-life of 12.25 years.
Troposphere: The weather portion of the atmosphere, from sea level to
about forty thousand feet.
Tropospheric fallout: World-wide fallout, mainly from small bombs (less
than a megaton) whose clouds remain in the troposphere. This
fallout occurs on the average two weeks to a month after the
explosion and stays in a latitude close to the latitude of the
explosion.
Uranium: Element with charge 92. Natural uranium contains 1 part of U²³⁵
to 139 parts of U²³⁸. U²³⁵ is a fissionable material and U²³⁸ can
be converted to plutonium, which is fissionable.
X-ray: Penetrating electromagnetic radiation, usually made by bombarding
a metal target with energetic electrons. X-rays and gamma rays are
really the same thing.
FOOTNOTES
[1]The word “noble” is perhaps a misnomer—these atoms do not even seek
the company of each other.
[2]Quotes are put around the word atom because, having lost one of its
electrons, it is no longer an ordinary neutral atom in its ground
state.
[3]Yet.
[4]Actually the same state may be occupied by two neutrons and two
protons. The reason is that neutrons and protons are magnetic
particles with a north and a south pole. Consequently the demand for
a difference can be satisfied by having one neutron (or proton) with
its north pole pointing up and another with its north pole pointing
down.
[5]It seems that neutrinos emitted in the company of electrons have the
symmetry of a right screw; those emitted together with a positron
have the symmetry of a left screw.
[6]Actually the weights rarely add up to the original 238 because, as a
rule, one or more neutrons are emitted which carry off some of the
original mass.
[7]Only a very few unlucky ones are overtaken by beta decay first.
[8]She and her husband were the discoverers of two elements, rhenium and
masurium. One of these exists.
[9]A great portion of the energy might be lost if the neutron is quite
fast. In this case the neutron can cause internal excitation of the
nucleus.
[10]There seems to be a good possibility that he died from a hepatitis
entirely unrelated to the initial radiation exposure.
[11]Half-lives of radioactive nuclei are uninfluenced by the extreme
temperatures or pressures of the explosion, or by the state of
motion of the particles or where they happen to be.
[12]A small amount may drift down to the ground in the winds. This may
get deposited on leaves and grass.
[13]The last line of the table is based on our own estimates.
[14]Recent evidence suggests this number is sometimes twenty-three.
[15]Dubbed by its friends “Committee for Reactor Prevention.”
[16]One of the authors.
[17]This difference is not surprising. When we sterilize, we have to
kill _all_ germs, even those which are most resistant to radiation.
Furthermore small organisms may escape the radiation effects by mere
chance. On the other hand a big and complicated organism will cease
to function when the most sensitive among its essential tissues have
been destroyed.
Transcriber’s Notes
—Silently corrected a few typos.
—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by
_underscores_.
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