Positron collisions with helium at intermediate energies

Calculations are reported of total and differential cross sections for elastic scattering and excitation of the 1 1 S to 2 1 S and 1 1 S to 2 1 P transitions in helium by positrons with energies between 60 and 400 eV. Results are given in three-state close-coupling, Born, Glauber, eikonal Born series, optical model and a distorted-wave Born model. They are compared with electron impact results. Excitation cross sections are summed and an estimate of the ionisation cross section derived from the total cross section measurements of Twomey et al. Cross sections for 2 1 S to 2 1 P and coincidence parameters for 1 1 S to 2 1 P transitions at 80 eV are briefly discussed.