  We have investigated the universal scaling behavior of differential cross
sections for the single K-shell ionization by electron or positron impact. The
study is performed within the framework of non-relativistic perturbation
theory, taking into account the one-photon exchange diagrams. In the case of
low-energy positron scattering, the doubly differential cross section exhibits
prominent interference oscillations. The results obtained are valid for
arbitrary atomic targets with moderate values of nuclear charge number Z.
