Note on the production of secondary rays by α rays from polonium

In the ‘Philosophical Magazine’ for May, 1896, Professor Bragg points out that since theα rays are absorbed by a solid according to the same law as by a gas, it seems reasonable to expect that the atoms of the solid are ionised in the same manner as are those of a gas. Hence we should expect that slowly-moving negative rays would be given off at the surfaces where theα rays enter or leave a solid. Professor J. J. Thomson has shown that when a solid is subjected to bombardment by a stream of canalstrahlen, it emits cathode rays. In the course of some experiments which were undertaken with the idea of determining the velocity of projection and the ratio of the charge to the mass of the slowly-moving negative orδ rays emitted by polonium, the writer has obtained some results which appear to indicate clearly the existence of slowly moving negatively charged secondary rays which are produced when an aluminium or copper plate is bombarded by a stream ofα particles.