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.
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