On the scattering of the α-particles by matter

Abstract

In the course of the experiments undertaken by Professor Rutherford and myself to determine accurately the number of α-particles expelled from 1 gramme of radium, our attention was directed to a notable scattering of the α-particles in passing through matter. The effect of scattering is well known in the case of β -particles. A narrow pencil of β -rays emerges after passing through a metal plate as an ill-defined beam. A similar effect, but to a much smaller extent, was known to exist also for the α-particles. Professor Rutherford* showed that the image of a narrow slit produced by the α-rays on a photographic plate broadens out when the slit was covered with a thin sheet of mica, while a well-defined image was obtained in vacuum with the uncovered slit. The question of the actual existence of the scattering effect of the α-particles has been discussed further by Kucera and Masek,† by W. H. Bragg,‡ L. Meitner,§ and E. Meyer.|| Some experiments have been made, using the scintillation method to determine the magnitude of the scattering of the α-particles in passing through matter. The apparatus used is shown in fig. 1. The main part consists of a glass tube nearly 2 metres in length and of about 4 cm. diameter. The α-particles from a strong but small source placed at R passed through a narrow slit S and produced an image of this slit on a phosphorescent screen Z, which was cemented to the end of the glass tube. The breadth of the slit was 0.9 mm., and the breadth of the geometrical image on the screen was about 2 mm., depending upon the dimensions and the distance of the source. The numbers of scintillations at different points of the screen were counted directly by means of a suitable microscope M, of 50 times magnifi­cation. The area of the screen which could be seen through the microscope was about 1 mm. 2 The number of scintillations counted varied between two or three a minute and about 80 per minute. As regards the microscope and the most convenient method to count thescintillations, the hints given by E. Regener* in his recent paper proved very useful. The microscope was mounted on a slide PP so that the scintillations produced at varying distances from the centre of the beam could be observed. The actual position of the microscope was read on a millimetre scale fixed to the slide.