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.

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.

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 N₀ 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 2T, one quarter remain; and so forth.

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.

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.

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:

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:

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:

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

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,

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.

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):

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.

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.