The ionisation of various gases by the β-rays of actinium

Experiments on the absorption of theβ -rays from radio-active substances have shown that the rays from actinium are less penetrating than those from uranium or radium. This is shown by Table I, which contains the coefficients of absorption of different materials obtained with theβ -rays of actinium and uranium by Godlewski and by Rutherford and Crowther respectively. It will be seen that the coefficient of absorption obtained in the case of aluminium with theβ -rays of actinium is approximately double that obtained with theβ -rays of uranium. This indicates that the velocity of the actinium rays is on the whole less than that of the uranium rays. It also follows from the table that the coefficients of absorption obtained with theβ -rays of actinium, do not increase so rapidly with the atomic weight of the absorbing material as the coefficients obtained with theβ -rays of uranium. We would expect this on comparing the absorption of the cathode rays produced in a discharge tube with the absorption of theβ -rays of uranium. Lenard found that with the cathode rays the ratio of the coefficient of absorption to density is practically the same for all substances, while from the experiments of Rutherford, Crowther, and others it follows that with theβ -rays of uranium this ratio increases rapidly with the atomic weight of the absorbing material. Therefore, with rays whose velocity lies between that of the cathode rays and theβ -rays of uranium, the increase will be less rapid than that obtained with uranium rays.