Positron Annihilation in Helium

Abstract

The modified adiabatic scattering function previously used to compute phase shifts for positron-helium scattering below the positronium threshold is applied to the problem of annihilation in helium. A large enhancement factor relative to the Dirac rate is found, varying with energy from 2.30 to 3.16. The probability of finding the spectator electron in states of the ${\mathrm{He}}^{+}$ ion other than the ground state is computed and found to be small but probably observable.