Annihilation in low-energy positron - helium scattering

Abstract

Elaborate wavefunctions representing low-energy positron - helium elastic scattering, which were obtained in the course of calculating accurate values of the scattering phase shifts, are used to determine the electron - positron annihilation rate and the Doppler-broadened annihilation -ray spectrum. This spectrum is also measured using room-temperature positrons in a Penning trap and a non-Gaussian lineshape is observed for the first time. Excellent agreement is obtained between the theoretical spectrum and the present results.