To examine the possibility that some radio-isotopes which have been considered to decay by $K$-capture do in part decay with the emission of very slow positrons careful measurements of the $\gamma$-rays emitted by two of them have been made. Preliminary measurements were made of the $F$ and $I$ lines of ThB as a check on the spectrometer used. Also the annihilation radiation line of ${\mathrm{Cu}}^{64}$ was measured to establish its shape and position. In the course of this, good evidence was found for radiation resulting from annihilation while the positron is in flight. The weak high energy gamma-ray previously reported was found and its energy determined to be 1.34±0.01 Mev. The $\gamma$-radiation of both ${\mathrm{Be}}^7$ and ${\mathrm{Cr}}^{51}$ consists of a single line at 485±5 kev and 320±5 kev, respectively. No evidence of annihilation radiation was found in either case.

• Received 2 June 1948

DOI:https://doi.org/10.1103/PhysRev.74.677