Partitioning of the positron-helium total scattering cross section

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 present measurements and calculations of positron ionisations 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 comparison of experimental data on total scattering, positronium formation and ionisation has produced evidence for a cusp in the angle-integrated elastic scattering cross section at the positronium formation threshold. The authors' partitioning of the total cross section also elucidates the 'convergence problem' of electron and positron scattering on helium, that is, the measured electron and positron total cross sections merge at 200 eV, whereas the elastic cross sections theory predicts convergence at a much higher energy. (author)