Electron-Positron Annihilation in Flight

Abstract

The positron-electron annihilation cross section has been measured for 50-, 100-, and 200-Mev incident positron energies. Three small proportional counters in a magnetic field determined the incident electron or positron momentum, and a large scintillation counter immediately behind the absorber indicated the disappearance of a particle. The positron annihilation cross section was determined by subtracting the electron loss rate (due mainly to bremsstrahlung energy losses) from the positron loss rate. The cross sections obtained at 50, 100, and 200 Mev were, respectively, 11.0\ifmmode\pm\else\textpm\fi{}2.5, 6.3\ifmmode\pm\else\textpm\fi{}1.2, and 3.7\ifmmode\pm\else\textpm\fi{}0.6 millibarns per electron in a beryllium absorber, in good agreement with Dirac's two-quantum annihilation cross section. The presence of annihilation radiation was detected in coincidence with the disappearance of a positron within a small cone in the forward direction.