Annihilation of Positrons in Flight

The cross section for annihilation of positrons in flight has been measured for positron energies of 0.765, 1.02, 2.2, and 3.33 Mev. Positron energies were selected by means of a magnetic lens monochromator. A differential energy interval of about 0.1 Mev was defined by the pulse height from an anthracene crystal in which annihilation occurred. The annihilation radiation was detected in a scintillation counter biased to record only quanta above 0.51 Mev. The angular distribution of the annihilation radiation and the total annihilation cross section, at all energies, agrees with the theoretical values within the experimental accuracy, estimated to be about ±{}5%.