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William H. Bragg's Corpuscular Theory of X-Rays and γ-Rays

Published online by Cambridge University Press:  05 January 2009

Roger H. Stuewer
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Extract

The modern corpuscular theory of radiation was born in 1905 when Einstein advanced his light quantum hypothesis; and the steps by which Einstein's hypothesis, after years of profound scepticism, was finally and fully vindicated by Arthur Compton's 1922 scattering experiments constitutes one of the most stimulating chapters in the history of recent physics. To begin to appreciate the complexity of this chapter, however, it is only necessary to emphasize an elementary but very significant point, namely, that while Einstein based his arguments for quanta largely on the behaviour of high-frequency black body radiation or ultra-violet light, Compton experimented with X-rays. A modern physicist accustomed to picturing ultra-violet light and X-radiation as simply two adjacent regions in the electromagnetic spectrum might regard this distinction as hair-splitting. But who in 1905 was sure that X-rays and γ-rays are far more closely related to ultra-violet light than to α-particles, for example ? This only became evident after years of painstaking research, so that moving without elaboration from Einstein's hypothesis to Compton's experiments automatically eliminates from consideration an important segment of history—a segment in which a major role was played by William Henry Bragg.

Type
Research Article
Information
The British Journal for the History of Science , Volume 5 , Issue 3 , June 1971 , pp. 258 - 281
Copyright
Copyright © British Society for the History of Science 1971

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References

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57 The three letters from W. H. Bragg to A. Sommerfeld are deposited in the Archive for History of Quantum Physics (AHQP) at the American Philosophical Society Library (Philadelphia), University of California Library (Berkeley), and the Universitets Institut for Teoretisk Fysik (Copenhagen); the letter from Sommerfeld to Bragg was found by Sir Lawrence Bragg in his father's papers, and a copy of it was kindly sent to me by Sir Lawrence. I should like to express my gratitude to Sir Lawrence and to Dr.-Ing. Ernst Sommerfeld for permission to reprint them all here. The translation of Sommerfeld's letter is my own. For W. Friedrich's Munich Dissertation see “Räumliche Intensitätsverteilung der X-Strahlen, die von einer Platinaantikathode ausgehen”, Annalen der Physik, xxxix (1912), 377–430Google Scholar; E. Bassler's 1909 work on the polarization of X-rays is discussed and referenced on p. 378.

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81 Ibid., 192.

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84 Ibid., 188.

85 Ibid., 193.

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97 On deposit in the AHQP, reference (69).

98 See identical statement by Bragg in “X-rays and Crystals”, Nature, xc (1912), 360.Google Scholar

99 Ibid.

100 For Nobel Lecture see “The Diffraction of X-Rays by Crystals”, Nobel Lectures: Physics (Amsterdam, 1967), i, 370–382.Google Scholar

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103 On deposit in the O. W. Richardson Collection in the Miriam Lutcher Stark Library at the University of Texas. I am indebted to Sir Lawrence Bragg and to the Stark Library for permission to quote from this letter.

104 The exact relationships between Thomson and Compton scattering may be seen from the plots presented by Ann T. Nelms in “Graphs of the Compton Energy-Angle Relationship and the Klein-Nishina Formula”, National Bureau of Standards Circular Number 542 (1953).Google Scholar

105 There is of course a longer wavelength Compton component in scattered X-rays as well as in scattered γ-rays. Since, however, the change in wavelength for X-rays is only a few per cent, while for γ-rays it is of the order of 100 per cent, the longer wavelength γ-ray component is much easier to observe than the longer wavelength X-ray component.

106 For a discussion of Kossel's work see Heilbron, John L., “The Kossel-Sommerfeld Theory and the Ring Atom”, Isis, lviii (1967), 451–485, especially 462–466.Google Scholar

107 Curiously, as I shall show in a future publication, Arthur Compton, while in no way building on Einstein's work, may have received this crucial insight (that a single quantum must interact with a single electron) from Bragg's work. I should also mention that my reason for terming Einstein's hypothesis “long-neglected” in spite of Millikan's 1915 photoelectric effect experiments is that Millikan (in common with virtually every other contemporary physicist except Einstein) did not accept these experiments as proof of Einstein's hypothesis. See my “Non-Einsteinian Interpretations of the Photoelectric Effect” in Stuewer, Roger H., ed., Historical and Philosophical Perspectives of Science (Minneapolis: University of Minnesota Press, 1970).Google Scholar

108 Eddington, Arthur, The Nature of the Physical World; paperback reprint (Ann Arbor, 1958), 194.Google Scholar