Theoretical Aspects of Positron Collisions in Gases

Abstract

Publisher Summary This chapter discusses recent progress in the scattering and annihilation of low-energy positrons by simple atoms—in particular, hydrogen and helium. Superficially, positron–atom scattering might seem to be very similar to electron–atom scattering, the only difference being the sign of the charge of the incident particle. However, this change of sign of the charge has several very important consequences. The positron is distinguishable from the electrons in the target atom and therefore exchange effects with the incident particle are absent. In their place, however, are electronpositron correlations that take the form of states of positronium, the bound system of an electron and a positron. The potential energy of the positron in the static approximation is positive, whereas the polarization potential is negative, so that two major components in the positron–atom interaction tend to cancel each other. As a result, the low-energy scattering cross sections are much smaller than the corresponding electron–atom cross sections, and the actual values are quite strongly dependent on the accuracy of the polarization potential. A further feature of positron–atom collisions that has no counterpart at all in electron–atom collisions is electron–positron annihilation into two or three γ rays.