Positronium formation from He, Be, C, O, and Ne by the impact of high-energy positrons

Abstract
Positronium formation from He, Be, C, O, and Ne by energetic positrons has been studied using an approximation which is complete through all second-order terms in collisional potentials. We have calculated differential and total positronium-formation cross sections for a wide range of energies. We obtain clear evidence of residual structure near the Thomas angle in the keV range of energies due to the destructive interference of second-order amplitudes. At a fixed energy this structure gradually disappears as the target charge increases and for a fixed target this structure becomes more prominent with the increase of energy. The total cross section for capture by positrons (σPs) in the present method differs from the corresponding cross sections for capture by protons (σH) at least by an order of magnitude at the same velocity. Our cross sections are qualitatively different from the first-order Brinkman-Kramer results. For example, the ratio σPs/σH decreases with the increase of energy in the present method whereas in the Brinkman-Kramer approximation this ratio increases slightly. A number of possible experimental tests are discussed.