Partitioning of the positron-helium total scattering cross section

Abstract

From measurements of total, ionisation and positronium formation cross sections the authors have determined the 'total cross section without ionising channels' and compared this with calculations of the angle-integrated elastic and excitation cross sections. The comparison shows that the theoretical results for elastic scattering and excitation are in satisfactory agreement with experimental data. The measurements and calculations of positron ionisation indicate that the positron cross section is smaller than the electron cross section between threshold and about 35 eV, a behaviour which is in accordance with threshold laws derived from the Wannier theory. The partitioning of the total cross section also elucidates the 'convergence problem' of electron and positron scattering on helium, that is, the fact that measured electron and positron total cross sections merge at 200 eV, whereas for the elastic cross sections theory predicts convergence at a much higher energy.