Calculation of the cross sections for positron- and proton-impact ionization of helium

Cross sections for positron- and proton-impact ionization of atomic helium have been calculated using a coupled-state method. The single-electron Hamiltonian of the helium atom was diagonalized in a finite Hilbert space. We included nine radial pseudostates for each s, p, d, and f angular momentum. The time-dependent Schroedinger equation was solved by using a unitary matrix, U matrix, and a hyperbolic trajectory for both projectiles. Our positron results, covering the energy range 60--1000 eV, are compared with available measurements and theoretical calculations. Agreement with the most recent measurement is reasonably good over the entire positron energy range. The proton-impact cross sections are obtained for the proton energy range 10--1000 keV and also show good agreement with the measurements over the entire energy range. We conclude that the lower ionization cross sections for positron impact in comparison with those for proton impact are due to the bigger deflection suffered by the positron in the collision