Recent Advances in Nuclear Physics

Abstract

I WOULD like to relate some of the results that have been obtained by physicists in the general field of nuclear physics within the past five or six years. For the benefit of those who have not had time to follow these developments and who may be somewhat curious about them, I shall discuss the subject in quite an elementary way, in spite of the fact that I shall necessarily thereby repeat things which are quite well known to many persons who have been particularly interested in such developments. The subject of nuclear physics is one which deals with man's attempts to learn about the innermost parts of the atom. If I may review a little of the history of the development of our knowledge of atomic structure, I will say that modern views date from about 1912, when the Bohr theory was developed. At that time it became abundantly evident that all matter consists of atoms which are built on a particular plan, often described as analogous with the solar system. That is, the central part of the atom is a nucleus where most of the mass of the atom resides, and surrounding that are electrons which are bound there and which are responsible for the chemical properties. This view, put forth by Bohr, in 1912, has been the subject of most of the attention of physicists and theoretical chemists since that date until the present, but especially there was a rather “closing-up” development about 1928 or 1930, and physicists now feel that all the chemical properties of all the elements can be explained in terms of this picture because of the numerous successes which it has had. All of the facts of spectroscopy, both atomic and molecular, and a great many of the facts of chemical binding, in simpler cases in which mathematical difficulties are not too great, have been very successfully related to this theory of the structure of the atom and that part of the development only involves a consideration of the outer structure; that is, of these electrons which are outside of the nucleus. It was not until about 1928 that attention began to be devoted to the problem of the structure of the nucleus, itself. I think by following the analogy with the solar system it can be understood why there might be two rather different scientific problems presented there. In the solar system, one has the problem of explaining the motion of the planets, the motions of the various planets around the sun and around their parent bodies, and in that problem one really does not need to consider what the sun is made of or why it is hot as it is, or anything at all about its internal structure. It functions, for that purpose, only as a huge central mass which is able to attract the other planets. So, likewise, in the atom. The nucleus is a large central mass which, for purposes of studying the main facts of spectroscopy and of chemistry, needs only to be considered as a thing which exerts electric forces on the electrons and keeps them moving around the central nucleus.