A magnetic focusing arrangement is combined with coincidence counting of the annihilation radiation to permit detection of very low positron intensities in the presence of other radiations. Sources of ${\mathrm{P}}^{32}$ yielded (7.5±2)×${10}^{-10\mathrm{}}$ positrons per beta decay. An experiment with a gold-covered source showed that secondary effects in the source material may contribute an important fraction of these positrons. One may conclude that the number of positrons per beta decay of ${\mathrm{P}}^{32}$ does not exceed by more than a factor two the value 5.5×${10}^{-10\mathrm{}}$ expected theoretically from electromagnetic effects and is probably close to the theoretical value.

Sources of ${\mathrm{Y}}^{90}$ emit (3.6±0.9)×${10}^{-5\mathrm{}}$ positrons per beta decay, due primarily to the 1.75-Mev electric monopole transition in ${\mathrm{Zr}}^{90}$.

• Received 19 December 1955

DOI:https://doi.org/10.1103/PhysRev.102.415