STRUCTURES IN DIFFERENTIAL CROSS-SECTIONS FOR POSITRON IMPACT IONIZATION OF HYDROGEN

A singular structure in the differential cross section for electron ejection by positron impact occurs when the positron and the ionized electron have low relative momentum in the final channel. This is the analogue of the process of 'electron capture to the continuum', well known in positive-ion impact ionization. Near the singularity a dip, due to interference between two double-binary scattering processes, is predicted if the scattering angle of positron and electron equals the critical angle of 45-degrees. Dip and singularity coincide in the high energy limit.