Positronium Formation in Helium

The cross section for formation of positronium on impact of a positron with a normal helium atom is calculated by Born's approximation for positron energies up to 125 ev. As judged by the size of the maximum cross sections obtained at positron energy of 27 ev, the coupling between elastic scattering and positronium formation is far from weak. This suggests that virtual formation of positronium may play an important part in the elastic scattering of slow positrons by helium atoms. The coupled equations for dealing with this effect are derived.