Measurement of the Annihilation-in-Flight Cross Section at 0° for 8.5-Mev Positrons

Abstract

The differential cross section at 0\ifmmode^\circ\else\textdegree\fi{} for the annihilation in flight of 8.5-Mev positrons has been measured. The positrons were created in a thick Ta target which was bombarded by 20-Mev electrons from a linear accelerator. They were directed onto a Be target where annihilation occurred, and the annihilation photons were measured by use of a thick-crystal spectrometer. The measured value for the cross section is 1.3\ifmmode\pm\else\textpm\fi{}0.2 barns/steradian per electron, which is in agreement with theory.