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THE ATOMIC NUCLEUS1 In his recent address to the Fourth International Thomistic Congress, His Holiness Pope Pius XII stressed the need of philosophers acquainting themselves with the recent advances in nuclear science. This presents something of a problem for the professional philosopher. For at no time in the long history of science, have we ever witnessed anything comparable to the vast and intensified research, in great part Government subsidized, which is being brought to bear on the study of the atomic nucleus. The primary literature is vast and not always readily available in summary accounts. This survey of nuclear physics then may be of help to the philosopher and of interest to scientists in fields other than physics. The term nucleus was first introduced by Ernest Rutherford, the "father of nuclear physics", some forty years ago in connection with his most significant discovery, the conception of the atom as comprising a tiny but massive positively charged core surrounded by a very tenuous but relatively extensive electron structure. The development of Rutherford 's concept of the atom and its integration with Planck's quantum theory of radiation by men like Bohr, Heisenberg, Dirac and Schroedinger was the principal concern of atomic physics during the first three decades of this century. With its climax in the achievement of a unified theory of quantum mechanics, accounting for the physical and chemical behavior of the atom in both elementary and compound form, the center of the theoretical physicist's interest shifted from the extranuclear electron structure, the domain of atomic physics proper, to the nucleus and the score of "elementary" particles that are born of the heart of the atom. The last twenty years consequently, has witnessed the creation and development of a distinct branch of speculative science known as nuclear physics. Its present status is akin to that of atomic physics before the discovery of quantum mechanics. A great many nuclear phenomena have been studied and classified, but to date no satisfactory theory has been devised for integrating them. Such a theory, to be adequate, should account for what we know of elementary nuclear 1 Address given at the Fourth National Meeting of Franciscan Sisterhoods, Marian College, Indianapolis, Indiana, November 25—26, 1955. 350 The Atomic Nucleus351 particles, nuclear reactions and nuclear forces. And until such a theory appear, the material we intend to cover in this paper can be treated under these three topics even though the first and third overlap somewhat. i. Elementary Nuclear Particles It might be more exact to speak of the elementary particles of nuclear physics rather than elementary nuclear particles, since the first description implies nothing more than that such particles are objects of study in nuclear physics, whereas the latter designation intimates that the nucleus may be compounded of these elementary entities as a wall is of bricks or cinder-blocks. The latter is only partially correct, if true at all, in the sense that the physicist is wont to distinguish between the nucléons proper (the proton and neutron) which are assumed to retain their general identity within the nucleus as contrasted with neutrinos, electrons, gamma photons and mesons which have only a potential existence there and are created or annihilated in the course of changes of energy states of the nucléons in much the same way as photons are created or annihilated in connection with the energy changes of the electrons in the extranuclear structure of the atom. Even with this qualification the title is still misleading. For the term "elementary" is used in a purely relative sense. It does not imply invariable stability, for the elementary particles Usted by the nuclear scientist are almost all unstable and quickly decay into another elementary form. They are "elementary" only in as much as they do not seem to be composed in the ordinary physical or chemical sense of the word, as we would speak of an atom being composed of a nucleus and electron cloud, or a deutron being composed of a proton and a neutron, or an alpha particle of two neutrons and two protons, and so on. The term "particle" too may be misleading unless we keep in mind that all of...


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