POSITRON-HYDROGEN AND POSITRONIUM-PROTON SCATTERING AT INTERMEDIATE AND HIGH-ENERGIES

Positron scattering from atomic hydrogen is studied at energies rangin g from 4 to 100 Ryd using the close-coupling approach and cross sectio ns from both the e(+)-H and Ps-p entrance channels are reported in a b asis comprising 1s, 2s and 2p states of the hydrogen and positronium a toms. Comparative studies with widely used approximation methods such as the first Born, unitarized Born and second Born approximations are presented. With the exception of elastic scattering for the Ps(1s)-p e ntrance channel, it was found that on-shell approximations such as the first Born and the unitarized Born approximations are reasonably accu rate for collisions not involving particle transfer provided the energ y is sufficiently high. Rearrangement collisions on the other hand are dominated by off-the-energy-shell scattering and the accurate approxi mation at high energies is the second Born approximation. Further, the effects of positronium channels on the direct reactions are investiga ted and we conclude that the positronium formation channels can be omi tted for energies greater than 20 Ryd. Elastic scattering in the Ps(1s )-p entrance channel is seen to be unique, the high-energy behaviour o f the phase shifts is different from any other quantum system.