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.

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.

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.