Abstract
An improvement of the Coulomb-Born approximation is made including the effect of exchange in positronium (Ps) and atom scattering. The exchange amplitude for the simplest Ps-H system is a nine-dimensional integral. We developed a methodology of evaluating the Coulomb-Born-Oppenheimer amplitude for Ps-atom scattering and apply it to calculate the Ps-ionization cross-sections in Ps-H and Ps-He collisions. We present the first and preliminary results using the present methodology.