Nuclear Physics Read online

  Nuclear Physics

  W. Heisenberg

  PHILOSOPHICAL LIBRARY

  New York

  PREFACE

  This book was based originally on a series of lectures, and is intended for readers who, while interested in natural sciences, have no previous training in theoretical physics and yet are familiar to a certain extent with physical ideas. In conformity with the express wish of the Verband deutscher Elektrotechniker,a under whose auspices the lectures were given, a short history of atomic physics, as well as a general review of contemporary knowledge of atomic and nuclear structure, are included here as an introduction. Obviously, a thorough understanding of nuclear physics cannot be gained from a short survey of this nature, but it may at least succeed in providing a basis for an understanding of the lectures on nuclear physics which follow. In my treatment of nuclear physics, I have departed somewhat from the method followed by other popular books on the subject, inasmuch as I have attempted to begin my discourse with the theory of the processes and reactions within the atom, and to discuss practical applications in conclusion only. At the same time, it was essential to make the theory intelligible without resort to mathematics, with the aid of illustrative models and by citing as analogies certain more widely known related phenomena. Nuclear physics lends itself to such a treatment more than many other branches of physical science. However, this method obviously has its natural limitations, and for a more profound understanding of the entire complex of relationships, a mathematical presentation of the subject is, of course, essential. For a thorough study of nuclear physics in this sense, there are many excellent books available. In the present volume, the technical apparatus of nuclear physics is discussed in the seventh chapter only; the eighth, and last, chapter presents a survey of the practical applications achieved up to the present time.

  Since the publication, during the war, of the first edition of this book, reports have been published on the great progress in the field of nuclear physics, and especially on those technical developments relating to the atomic nucleus which had till then been restricted to the secret laboratories of the belligerent nations. These new developments are described, in general outline, in the last chapter of this book, where the practical applications of nuclear physics are discussed. Furthermore, those discoveries which were made or published after the war only, are dealt with in the text elsewhere.

  The present English edition, appearing some time after the German one, may be of interest in connection with the history and the principles of nuclear physics rather than with respect to its recent development. Since the writing of the book and even since its last revision in 1948 an enormous development of nuclear physics has taken place, in its principles as well as in technical applications. Therefore some of the content of the book may now be commonplace to many readers, some parts are definitely out of date, since new discoveries have changed the picture. In a new edition the shell structure of the nucleus should play a central rôle, since it has simplified our knowledge of the nucleus considerably through the work of Mayer-Göppert, Haxel, Jensen and Suess. In dealing with the nuclear forces one should mention all the new types of mesons that have been found in recent years and their modes of interaction. But it would probably be entirely impossible to give an account of the present state of nuclear physics in a short work. Therefore this book may still serve as an introduction to a field, the knowledge of which would require much more extended studies.

  W. HEISENBERG

  Table of Contents

  PREFACE

  ACKNOWLEDGMENT

  1. ATOMIC THEORY, FROM ANTIQUITY TO THE END OF THE NINETEENTH CENTURY

  2. MOLECULES AND ATOMS

  3. RADIOACTIVITY AND THE BUILDING BLOCKS OF THE NUCLEUS

  4. THE NORMAL STATES OF ATOMIC NUCLEI

  5. THE NUCLEAR FORCES

  6. THE NUCLEAR REACTIONS

  7. THE TOOLS OF NUCLEAR PHYSICS

  8. THE PRACTICAL APPLICATIONS OF NUCLEAR PHYSICS

  APPENDIX—RESEARCH IN GERMANY ON THE TECHNICAL APPLICATION OF ATOMIC ENERGY

  TABLES

  LITERATURE ON NUCLEAR PHYSICS

  ACKNOWLEDGMENTS FOR ILLUSTRATIONS

  Notes

  ACKNOWLEDGMENT

  Thanks are due to the editors of Nature for permission to reprint the article on page 189.

  1. ATOMIC THEORY, FROM ANTIQUITY TO THE END OF THE NINETEENTH CENTURY

  I. MATTER AND ATOMS IN THE PHILOSOPHY OF ANTIQUITY

  Nuclear physics is one of the most recently developed branches of physics. The term nucleus was first introduced by Rutherford about forty years ago, and the more detailed knowledge of the nuclei of atoms is only about fifteen years old. But the concept of the atomic structure of matter—the view that there exist certain smallest, ultimate, indivisible units, which are the basic building blocks of all matter—dates back to the philosophy of Antiquity, and was suggested by Greek philosophers as a daring hypothesis 2,500 years ago. Anybody who desires to understand something of modern atomic theory, will do well to study the history of the concept of the atom in order to become acquainted with the origins of those ideas which now have come to full fruition in modern physics. For this reason, the following lectures, the object of which is a description of the physics of the atomic nucleus, are prefaced by a short survey of the history of atomic theory.

  The idea of the smallest, indivisible ultimate building blocks of all matter first came up in connection with the elaboration of the concepts of Matter, Being and Becoming, which characterized the first epoch of Greek philosophy. At the very dawn of ancient philosophy we find a remarkable statement by Thales, who lived in Miletus in the sixth century B.C.: He said that water was the source of all things. As Friedrich Nietzsche expounded, this sentence expresses three of the most essential and fundamental ideas of philosophy. Firstly, the question as to the source of all things; secondly, the demand that this question be answered in conformity with reason, without resort to myths or mysticism—in those times, no idea was regarded as more evident than that the source of all things must be sought in something material, such as water, and not in life—thirdly, the postulate that ultimately, it must be found possible to reduce everything to one principle. Thales’ statement was the first expression of the idea of a fundamental substance, from which the whole universe had arisen, although in that age the word substance was certainly not interpreted in the purely material sense which we ascribe to it to-day.

  In the philosophy of Anaximander, a pupil of Thales, who also lived and taught in Miletus, the idea of a fundamental polarity—the antithesis of Being and Becoming—was substituted for the concept of a single fundamental substance. Anaximander argued that if only one fundamental substance were to exist, this infinite, homogeneous substance would completely fill the universe, and therefore, the great many varieties of phenomena would remain unexplained, and for this reason, Change and Becoming must have arisen from that indeterminate prime basis of all things. Anaximander seems to have regarded the process of Becoming as some sort of degeneration or debasement of this undifferentiated Being—as an escape, as it were, ultimately expiated by a return into that which is without shape or character.

  In the philosophy of Heraclitus, the concept of Becoming occupies the foremost place. He regarded that which moves—fire—as the basic element. In the teachings of Parmenides, a fundamental polarity—that of Being and Not-Being—is the central concept. Parmenides, too, regarded the wide variety of phenomena as resulting from the combined action and reaction of two opposed principles.

  Anaxagoras, who followed Thales by about a century (he probably lived about 500 B.C.), was responsible for a definite transition to a more materialistic view of the world of phenomena. He assumed that there existed an i
nfinite number of basic substances, the mutual interactions of which produced the variety of world processes. In his view these basic substances possessed the character of purely material elements in a much greater degree; he conceived of them as being eternal and indestructible in themselves, and he considered that the change and sequence of phenomena were produced solely by their sharing in the movement which threw them together at random.

  Empedocles, about ten years later, saw the existence of four ‘elements’—earth, air, fire and water—as the ‘prime root’ of all things. He regarded the primordial state of all things as consisting in an undifferentiated, homogeneous mixture of the elements, bound by Love in a state of eternal bliss, whereas Hate tended to separate these elements and to shape out of them the variegated drama of Life.

  This pronounced tendency to materialism reached its highest development with the philosophers Leucippus, a contemporary of Empedocles, and Democritus who was Leucippus’ pupil. The antithesis of Being and Not-Being became crystallized in the doctrines of Leucippus as the antithesis of ‘Full’ and ‘Empty’. The concept represented by ‘Full’ was regarded as manifesting itself in the ultimate, indivisible particles, the atoms, between which there was nothing but emptiness. The atom was pure Being, eternal and indestructible, but inasmuch as there existed an infinite number of atoms, pure Being could, within certain limits, be repeated an indefinite number of times. Thus, for the first time in history, there was voiced the idea of the existence of smallest ultimate, indivisible particles—the atoms—as the fundamental building blocks of all matter. In this manner, the concept of matter became analysed, in fact, into two sub-concepts: atoms and the void in which the atoms move. Formerly, space had seemed to be filled by matter; it was, as it were, stretched or expanded by material substance, and absolutely empty space had been inconceivable. But now, empty space was allotted a very important function: it became the vehicle for geometry and kinematics, by making possible the various arrangements and movements of atoms.

  Although the atom was regarded as having a special position in space, also a shape, and as executing certain movements, it was not allotted any attribute other than these purely geometrical properties. The atom had neither colour nor smell nor taste, and the properties perceptible by human senses, together with their changes and mutations, were supposed to be produced by the movement and displacement of atoms in space. Just as both tragedy and comedy could be written with the same latters of the alphabet, the vast variety of events in the universe were regarded as the products of the selfsame atoms, of their different positions and different motions. Democritus said: ‘A thing merely appears to have colour; it merely appears to be sweet or bitter. Only atoms and empty space have a real existence.’

  The basic ideas of atomic theory were taken over and modified, in part, by the later Greek philosophers. Plato, in his dialogue Timaeus, co-ordinates these ideas with Pythagoras’ theory of the harmony of numbers, and identifies the atoms of the elements—earth, air, fire and water—with the symmetrical bodies, cube, octahedron, tetrahedron and ikosahedron. The Epicureans, too, adopted the essential concepts of the atomic theory, and appended to it an idea which was to play an important part in natural science at a later date: the idea of natural necessity. According to this theory, the atoms are not thrown together arbitrarily, at random, like dice, nor set in motion by forces such as Love or Hate, but their paths are determined by natural laws, or by the working of blind necessity.

  After this point, there was no further development in atomic theory, either in the philosophy or in the science of Antiquity.

  II. MODERN ATOMIC THEORY, UP TO THE END OF THE NINETEENTH CENTURY

  All the progress which we have mentioned, occurred in the course of a few centuries. Two thousand years elapsed then before they were recalled, and before another thinker took up these ideas and transformed them into something fruitful. During the latter part of Antiquity, and during the Middle Ages in particular, the philosophy of Aristotle was accepted as an incontestable foundation, and for the Christian outlook reality had changed to such an extent that the attention of mankind was not attracted by material Nature for a long time.

  The first philosopher to revive these neglected trends of thought was the Frenchman Gassendi. A theologian and philosopher, he was born in Provence in 1592 and died in Paris in 1655. He was a contemporary of both Galileo and Kepler, and as such he witnessed the first achievements of a revived natural science. It was about this time, after a barren interval of nearly 2,000 years, that the soil once again became fertile for the progress of scientific knowledge.

  The first representatives of this new natural science, including Gassendi, revolted against the authority of Aristotle and turned to other philosophers of the classical era. Thus, Gassendi embraced the teachings of Democritus, which he at once invested with a completely materialistic form. He, too, held that the world was built of ultimate, indivisible units, or atoms, so small as to be invisible. And like Democritus, he regarded the wide variety of phenomena as the product of the variety in the arrangement and movements of atoms. The idea had already suggested itself that physical phenomena could be made intelligible in a much more concrete—one might even say, banal—way with the aid of the atomic theory. Thus, a mixture of water and wine might be compared to a mixture of two different types of sand which has been stirred so thoroughly that the two kinds of grains are completely intermingled, and distributed statistically, by pure chance. The atoms of water and wine would correspond to the grains of sand in their random and indissoluble union. Furthermore, the idea suggested itself that the states of aggregation of matter could likewise be explained by the atomic theory, even though not in the clear and intelligible manner to which we are accustomed in modern times. To-day we know that in ‘solid’ water—ice—the atoms are packed tightly in ranks and files, as it were. In ‘liquid’ water, they are also tightly packed, but are in a state of disorder, and move about in this disorderly state. Finally, in water vapour, or steam, the atoms (or more correctly, certain groups of atoms, which we call molecules) move in a way which may be likened to a swarm of fruit-flies, at considerable distances from each other.

  This idea was taken up by other investigators, and its application to the material world progressed by leaps and bounds. For the Greeks, the conception of atoms was still the means which enabled the world, as a whole, to be understood and which accounted for observable reality. Now it became the means for the understanding of the behaviour of crude, inanimate matter.

  The next scientific investigator whom we must mention was an Englishman, Robert Boyle (1627–1691). He was a chemist and physicist rather than a philosopher. His most important work concerned the theory of gases, and he discovered the law that the product of the pressure and volume of a gas at a given temperature is always constant. Chemistry is indebted to Boyle for other important discoveries, too, more especially for the introduction of the concept of the chemical elements in the modern sense. The Greeks had already associated the notion of elements with fundamental natural phenomena—rest and motion, earth and fire—but Boyle associated this notion with chemical processes in a thoroughly materialistic way. Chemistry was able to convert different substances into each other. Boyle’s query was: From what substances can the infinite variety of homogeneous substances existing in nature be built up? And furthermore: What are the elements that cannot be resolved any further, and of which all substances are composed, in one way or another? This problem arose out of the originally quite different question raised by the alchemists in the centuries before Boyle. Alchemy had developed out of the fundamental idea that every substance could ultimately be reduced to one basic substance, and that it must be possible, in principle at least, to convert any substance, any type of matter, into any other—mercury into gold, for instance. But all efforts in this direction had always remained futile; such transmutation could never be effected by chemical means. It appeared obvious that matter was not homogeneous in this sense—when treated by
chemical means—but there had to exist basic substances which no chemical process could change into another. Since Boyle’s time it has become a matter of common knowledge that there exists a whole series of these basic substances, or chemical elements, as against the approximately half a million uniform chemical compounds known to-day. The number of chemical compounds exceeds by far that of the basic elements. Nevertheless, the number of the elements is still large enough to make it difficult for us to regard them as the ultimate, indivisible building blocks of matter. Of course, Boyle knew relatively only few of the ninety-six elements known to us to-day, but nevertheless he succeeded in formulating quite clearly the aims and tasks of chemistry. He said: ‘What we have to do is to determine into what basic substances matter can be analysed by chemical means, and what these basic substances are.’ Thus we see that his chemical elements had nothing more in common with earth, air, fire and water, the elements of Democritus.

  A century later came Lavoisier, the real father of modern chemistry. He was born in 1743 and died, a victim of the French Revolution, in 1794. His permanent contribution to science was the founding of quantitative chemistry. He was first to interpret rightly the process of combustion. Up to his time, it had been believed that in the combustion of any body a substance called phlogiston was released, and therefore, bodies would necessarily become lighter after combustion. Lavoisier adopted the opposite view, that combustion consisted in the combination of an element with oxygen, and as a result, the body must become heavier. His theory was proved correct by experiment. At the same time, he accomplished something of vast importance, in that he stimulated chemists to investigate changes in mass due to chemical changes.

  Now we come to a law which was formulated by Lavoisier in 1774, but became the common property of chemists only several years later: the law of the conservation of mass. Lavoisier already claimed that in every chemical change the total mass of the substances involved remains constant—meaning that the total quantity of converted matter weighs exactly as much after the conversion as it did before it. The discovery and formulation of this law marks the actual beginning of modern chemistry, and in a very few years it became the connecting link between the chemistry of Boyle and the atomic theory of Gassendi.