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

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 (σ/sub Ps/) in the present method differs from the corresponding cross sections for capture by protons (σ/sub H/) 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 σ/sub Ps//σ/sub H/ 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