All charged particles emit radiation when accelerated. Indeed on the Maxwell view, that radiation (which takes energy from the particle) is the “wake” left by that acceleration in an æther of crossed electric and magnetic fields, and the concept underlies Hertz' corroboration of Maxwell in 1888. So when ► cathode-rays were about 1900 identified by most physicists as streams of ► electrons, their impact on the anti-cathode in Rontgen's vacuum tube should produce an irregular sequence of dislocated electromagnetic impulses due to the electrons' deceleration. This is “braking,” hence the term Arnold Sommerfeld (1868–1951) coined in 1909 of Bremsstrahlung.
Wilhelm Conrad Röntgen (1845–1923) had thought in 1895 he had found the elusive longitudinal e-m wave in his discovery of ► x-rays. But Sommerfeld in 1899 found two species in the new radiation: at the low-energy end periodic waves like ultra-violet light, at the high end a broad spectrum to be expected from discontinuous impulses dissected by Fourier frequency expansion. This distinction was reinforced by Charles Barkla (1877–1944) in 1907: superimposed on the spectrally-spread out x-radiation from electron impacts (Bremsstrahlung) was a series of sharp strong peaks characteristic of the anti-cathode metal (fluorescent x-rays) that Barkla showed were polarized. Sommerfeld returned to the issue in 1911 with a non-relativistic analysis of γ-rays (Bremsstrahlung from exiting β-electrons) to show their energy is emitted markedly in the forward direction like “directed radiation,” or “needle radiation,” see Fig. 1. Niels Bohr had to contend with Bremsstrahlung as fundamental evidence for his atom in 1913, although Joseph Larmor (1857–1942)  and J. J. Thomson (1856–1940)  had defused the notion of the ► Bohr atom necessarily destroying itself by radiation from orbiting electrons.
KeywordsBrownian Motion Compton Scattering Comptes Rendus Secondary Literature Cloud Chamber
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