Angular distribution of beta radiation from thick sources

The specific activity of materials may be measured by determining the yield of BETA -particles at the surface of a source with a thickness larger than the mean free path of the particles. This value is proportional to the specific activity and does not change with further increase of the thickness. Thls requires knowledge of the angular distribution of the particles. A method was developed involving the measurement of the absorption and scattering of the BETA -radiation from a point source placed between a thick support and foils of various thickness made of Al, Cu, Sn, and Pb. The measurements were carried out by means of a vacuum chamber and an end-window chamber, obtaining the angular distribution of the BETA -radiation of W/sup 185/, Tl/sup 204/, P/sup 32/, and Sr/sup 90/+ Y/sup 90/. Results showed that the angular distribution does not depend on the energy of the radiation and the atomic number of the source. The flux from the thick-layer source in the direction of the window may be computed by Lambert's formula, derived in 1892. (TTT)