An experiment has been performed to investigate single-quantum annihilation of positrons in the field of high-$Z$ nuclei. This mode of annihilation has been observed and the absolute magnitude, energy dependence, and $Z$ dependence of the cross section measured. Backgrounds which might contaminate the derived signals have been semiempirically investigated and found to be an order of magnitude or more below the rate identified as single-quantum annihilation. The data on absolute magnitude of the counting rate and its dependence on positron energy agree with the calculations of Jaeger and Hulme. The $Z$ dependence agrees with a cross section proportional to $Z^5$.

• Received 18 May 1960

DOI:https://doi.org/10.1103/PhysRev.124.1851