-Ray Spectrum of Radio-Helium

Abstract

IN order to investigate the energy distribution of the -rays from radio-helium¹, we have employed an expansion chamber constructed by Dr. J. C. Jacobsen and kindly lent to us. In the middle of the top plate, a thin-walled brass cylinder (0.05 gm. per cm.², 1.5 cm. diameter) was inserted, into which an activated beryllium cylinder could be dipped. The latter was made of beryllium powder (0.12 gm. per cm.²) stuck on to a brass tube (0.1 gm. per cm.²) and could be moved automatically from a position around a neutron source (beryllium-radon) to the position in the Wilson chamber in 0.4 sec., this movement also starting the timing arrangement for expansion and light. A suitable lead shielding was arranged which cut down the electrons due to the -rays to a number small compared with the number of -rays. The radioactivity induced in the brass tube carrying the beryllium is negligible as the latter is exposed to the neutrons only for a few seconds at most every minute.