The decrease in velocity of α-particles in passing through matter

Abstract

It is becoming recognised that one of the most fruitful sources of information as to the internal structure of atoms is provided by the phenomena attending the passage of swift electrified particles through them. In particular from a consideration of the scattering α-particles it has been found that the atom consists essentially of a very concentrated charge at the centre of the atom surrounded by eletricity of the opposite sign, probably electrons, distributed throughout the remainder of the atom. Further, Darwin and Bohr have attempted to obtain information as to the number and distribution of electrons in the atom by a consideration of the absorption or loss of velocity of the α-particles in passing through matter. The only data for this purpose so far obtainable are provided by the velocity curves, or relations between velocity and thickness of matter traversed, in aluminium as determined by Rutherford, and in mica as determined by Geiger. It seemed, therefore, of interest to make a more complete investigation of the velocity curves in various substances, more particularly as the earlier observations are subject to slight errors due to the assumption that equal thickness of matter have the same air equivalent at different parts of the range of α particles. In the present experiments the velocity curves in gold, copper, aluminium, mica and air have been determined, using as source the α-particles of radium C as in the experiments of Rutherford and of Geiger. The velocities of the α-particles before and after passing through sheets of matter of various thickness were measured by the deflection of the α-particles in a known magnetic field. The apparatus used is shown in fig. 1. It consists essentially of a source of radiation D, a series of absorption foils T, a slit L, to form a pencil of the α-particles, and a zinc sulphide screen M on which the position of the pencil could be observed, the whole being enclosed in an evacuated chamber C between the poles of a large electromagnet.