Positronium Formation by Positron Impact on Atoms at Intermediate Energies

Abstract

Publisher Summary This chapter discusses positronium (Ps) formation at intermediate energies from the threshold for ionization up to energies of a few hundred electron volts, where the effective coupling between the rearrangement and the direct channels becomes very small. In the interval above the ionization threshold, and extending to at least 100 eV, the Ps formation cross section is comparable to, or in some cases larger than, that for excitation; however, at high energies, the ratio of the formation cross section to the total inelastic cross section decreases rapidly. In the intermediate energy range up to energies of several hundred electron volts, the use of coupled channel models is feasible in principle and can be expected to provide accurate cross sections, including the formation cross section. At higher energies, although most approximations lead to similar values for the ground-state capture cross section the differential cross section is very sensitive to the details of the approximation employed and in contrast to the comparatively well-understood situation in charge exchange, there are still large gaps in the understanding of high-energy Ps formation.