Basic Bethe : seminal articles on nuclear physics, 1936-1937 9780883184950, 0883184958

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The History of Modern Physics 1800-1950

Volume 6

The History of Modem Physics, 1800-1950

EDITORIAL BOARD Gerald Holton, Coeditor Harvard University Katherine R. Sopka; Coeditor Four Corners Analytical Sciences Stephen G. Brush University of Maryland Roger H. Stuewer University of Minnesota Spencer R. Weart Center for History of Physics, American Institute of Physics John A. Wheeler University of Texas

The History of Modern Physics, 1800-1950

TITLES IN SERIES

Volume 1 A Isos by Samuel A. Goudsmit

Volume 2 Project Y: The Los Alamos Story Part I: Toward Trinity by David Hawkins

Part II: Beyond Trinity by Edith C. Truslow and Ralph Carlisle Smith

Volume 3 American Physics in Transition: A History of Conceptual Change in the Late Nineteenth Century by Albert E. Moyer

Volume 4 The Question of the Atom: From the Karlsruhe Congress to the First Solvay Conference, 1860-1911 by Mary Jo Nye

Volume 5 Physics For a New Century: Papers Presented at the 1904 St. Louis Congress

Volume 6 Basic Bethe: Seminal Articles on Nuclear Physics, 1936-1937 by Hans A. Bethe, Robert F. Bacher, and M. Stanley Livingston

INTRODUCTORY NOTE

The Tomash/American Institute of Physics series in the History of Modern Physics offers the opportunity to follow the evolution of physics from its classical period in the nineteenth century when it emerged as a distinct discipline, through the early decades of the twentieth century when its modern roots were estab¬ lished, into the middle years of this century when physicists continued to develop extraordinary theories and techniques. The one hundred and fifty years covered by the series, 1800 to 1950, were crucial to all mankind not only because pro¬ found evolutionary advances occurred but also because some of these led to such applications as the release of nuclear energy. Our primary intent has been to choose a collection of historically important literature which would make this most significant period readily accessible. We believe that the history of physics is more than just the narrative of the development of theoretical concepts and experimental results: it is also about the physicists individually and as a group—how they pursued their separate tasks, their means of support and avenues of communication, and how they interacted with other elements of their contemporary society. To express these interwoven themes we have identified and selected four types of works: reprints of “classics” no longer readily available; original monographs and works of primary scholar¬ ship, some previously only privately circulated, which warrant wider distribu¬ tion; anthologies of important articles here collected in one place; and disserta¬ tions, recently written, revised, and enhanced. Each book is prefaced by an introductory essay written by an acknowledged scholar, which, by placing the material in its historical context, makes the volume more valuable as a reference work. The books in the series are all noteworthy additions to the literature of the history of physics. They have been selected for their merit, distinction, and uniqueness. We believe that they will be of interest not only to the advanced scholar in the history of physics, but to a much broader, less specialized group of readers who may wish to understand a science that has become a central force in society and an integral part of our twentieth-century culture. Taken in its entire¬ ty, the series will bring to the reader a comprehensive picture of this major discipline not readily achieved in any one work. Taken individually, the works selected will surely be enjoyed and valued in themselves.

IV

The History of Modern Physics

Volume

1800 1950 -

'

Basic Bethe Seminal Articles on Nuclear Physics, 1936-1937 HANS A. BETHE ROBERT F. BACHER M. STANLEY LIVINGSTON

Preface by HANS A. BETHE Introduction by ROGER H. STUEWER

Tomash Publishers

American Institute of Physics

Q &k( 1-2/3),

or, both conditions in one: Fig. 3. Schematic graph representing the energy of isobars of even atomic weight as a function of nuclear charge Z. The upper parabola contains the nuclei of odd charge, the lower one those of even charge. The arrows represent possible /3-disintegrations. The two “even” nuclei marked 5 are stable, all the others unstable.

weight, we shall get a smooth curve. The nucleus nearest to the minimum of that curve will be the stable nucleus for the given atomic weight. Chances are equal that this stable nucleus has even or odd charge. For even atomic weight, on the other hand, we obtain two separate curves for nuclei with odd and such with even charge. We may assume that each of these curves is smooth. On the lower curve (even charge) we find always one, but in many cases several, stable nuclei, i.e., nuclei which have less energy than both their neighbors (Fig. 3). We may try to determine the magnitude of the forces between like particles from the statistics of isobars. It is reasonable to assume that the two curves representing the energies of even charge and odd charge isobars, are parabolas just shifted vertically by an amount 8. The minimum of the parabolas will in general lie at a fractional value of the nuclear charge, ZA. Let us denote by $ the difference between ZA and the nearest odd number; /3 obviously may be any number be¬ tween — 1 and +1. The curvature of the parabola may be determined from our general formula (18) for nuclear energies: K=-^d2E/dZ2=(iae+yA2'3)/A.

(21)

5>k(1 + 2|/3|).

(21d)

To find out the meaning of 8, we remember that we have proved that the energy of the “odd” nucleus must be midway between the two even ones if there are no forces between like particles. In this case, therefore, we would have E' 0dd — E (ZA) + k (1 + /32), so that, in the absence of forces between like particles, 8' = k. Therefore the effect of the forces between like particles is given by 8" = 8-8’ = 8-k

(21e)

and, according to (2Id), we have two stable isobars if

6">2k|0|

(21 f)

and only one stable element of atomic weight A if 8" /80 only one nucleus of weight A is stable while for | /S |