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

Abstract

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.

Disputed discovery: the beginnings of X-ray diffraction in crystals in 1912 and its repercussionsThis Laue centennial article has also been published inZeitschrift für Kristallographie[Eckert (2012).Z. Kristallogr.227, 27–35].

“…William Henry Bragg, in a response to the Bremsstrahlen theory, presented Sommerfeld with an alternative view which became known as the neutral-pair hypothesis (Stuewer, 1971;Wheaton, 1983, pp. 87-90).…”

“…However, the Braggs soon became aware that the phenomenon is indeed a diffraction phenomenon of the primary X-rays. While W. H. Bragg hoped for an explanation that would finally account both for the wave and particle aspect of X-ray phenomena (Stuewer, 1971;Wheaton, 1983, p. 208), his son began to adopt the idea of a wave interference -even though entirely different from Laue. On 11 November 1912, Bragg junior reported at Cambridge about the Munich experiment.…”

The Nobel Science: One Hundred Years of Crystallography

“…In 1906 Charles Glover Barkla had shown that X-rays could be polarized and this became key evidence that they were wave-like in nature. However due to Sir William Henry Bragg's work on radioactivity at the University of Adelaide, the very same evidence that was seen to support the wave theory of light could also be interpreted by a corpuscular description of X-rays (Stuewer 1971). It would take intuition from Max von Laue to add indisputable evidence for the wave-like nature of X-rays.…”

“…At this point however, it was not clear whether X-rays were even related to visible light (Stuewer 1971) let alone whether they were waves or particles (Holton et al 2014). In 1906 Charles Glover Barkla had shown that X-rays could be polarized and this became key evidence that they were wave-like in nature.…”

Disputed discovery: The beginnings of X-ray diffraction in crystals in 1912 and its repercussions

“…However, there were serious doubts about whether X-rays are waves. William Henry Bragg, in a response to the Bremsstrahlen theory, presented Sommerfeld with an alternative view which became known as the neutral-pair hypothesis (Stuewer, 1971;Wheaton, 1983, pp. 87-90).…”