Annihilation probability density in positron scattering by He

Abstract

We have calculated annihilation probability densities in positron collisions against the He atom. Our scattering wave functions were obtained with the Schwinger multichannel method [J. S. E. Germano and M. A. P. Lima, Phys. Rev. A 47 , 3976 (1993)]. It has been found that direct annihilation, in which electronic cloud deformation shields the nuclear repulsive potential effectively attracting the positron to a binary encounter, dominates the annihilation process at low impact energies. Closer to the real positronium formation threshold, the signature of virtual positronium has been noticed. At room temperature, significant annihilation probability has been observed over a somewhat extended region.