Ionization in positron-atom collisions

Depending on the distribution of energy between the scattered positron and the ejected electron in the final channel, two competing processes are involved in positron-impact ionization of atoms, namely (a) direct head-on ionization of an electron and (b) positronium formation to the continuum. Using Faddeev's three-body scattering formalism for positron--hydrogen-atom collisions, we get an amplitude which takes account of both these processes. The doubly differential cross section thus obtained for the forward scattering angle shows a cusp when plotted against the emission energy. The cusp is due to the effect of positronium formation to the continuum and heretofore has not been predicted.