The Three-dimensional Reproduction of Tracks of β-particles Ejected by X-rays

Abstract

THE use of a stereoscopic camera by C. T. R. Wilson in the photography of tracks of β-particles ejected by pencils of X-rays led to the revelation of two types of asymmetry of initial direction of ejection (Proc. Roy. Soc., A, 104, 1923). One is characterised by an excess of tracks having their initial portions in or near the plane containing the X-ray pencil and its electric vector; the other by an excess of tracks having a forward component in their initial velocities. They have been termed ‘lateral’ and ‘longitudinal’ asymmetry respectively. The former distribution reveals the partial polarisation of the primary beam, the preponderance of primary X-rays the electric vector of which is in the plane containing the cathode stream. The latter distribution has been accounted for qualitatively on the basis of Richardson's hypothesis that the absorption of a quantum hv of energy by the electron is accompanied by the transference of the momentum of the absorbed radiation causing a resultant motion in the forward direction. A third feature of interest and importance revealed by Wilson's photographs was the existence of short, unidirectional, fish-like tracks with tails towards the X-ray source and produced only by X-rays of wavelength not exceeding about 0.5 Å.U. The phenomena of fish-tracks are most completely and satisfactorily explained by the application of the Compton quantum theory of scattering of X-rays.