THEORETICAL STUDIES OF LOW ENERGY POSITRON SCATTERING BY AN EQUIVALENT ONE-ELECTRON MODEL OF HELIUM J.T. Dunn, P. Van Reeth and J.W. Humberston Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK Theoretical investigations using a very elaborate scattering wavefunction and an accurate one-electron model of helium are being made to study the elastic scattering, positronium formation and annihilation cross sections.