Analysis of the long range α- Particles from radium C

In a previous paper, an account has been given of a new method of detection of groups of α-rays expelled from a radioactive substance, even when these are present in small quantity compared with the main group or groups. In this way we were able to show that the α-rays from actinium C were complex, and to prove the presence of a group of α-rays arising from the dual disintegration of radium C, although present in numbers about 1 in 3000 of the main group of α-rays of longer range. The method depends on the linear magnification of the ionisation current, due to a few millimetres of the track of an α-particle, by means of valves and the recording of the momentary current on a photographic film by a special oscillograph. By the use of a differential method, it was found possible to record only those α-particles which were stopped in the shallow ionisation chamber. In this way a type of straggling curve for the α-rays was obtained, the maximum of which corresponded to the mean range of the group of α-particles in the air or other gas employed. By comparison with a standard range, this mean range could be fixed with precision when isolated homogeneous groups of α-rays were present.