POSITRON-IMPACT AND ELECTRON-IMPACT DOUBLE-IONIZATION OF HELIUM AT LOW AND INTERMEDIATE ENERGIES

Using a correlated four-body final-state wavefunction the angular dist ribution of fragments following double ionisation of helium by positro n and electron impact is studied. Distortion of angular patterns due t o projectile-electron and projectile-nucleus interactions is investiga ted. Further, the influence of two-body Coulomb-density of states in t he four-body continuum is envisaged. It is shown that these interactio ns, previously neglected in Born-type treatments, play a major role in determining the cross section at low and intermediate energies.