Positron interactions with high-Z atoms at relativistic energies

We consider the interaction of a positron with a bound electron of a high-Z atom. In particular, we present relativistic calculations of the cross sections for knockout and/or positronium formation in a thorium target, as well as an internal process in a hypothetical Z=164 atom. Both plane-wave and distorted-wave Born approximations are discussed. We find peaks in the final sum energy of the electron-positron pairs due to the momentum distribution of the atomic electron. However, these peaks are too low in energy and are too broad compared with peaks claimed in e+e- scattering or with the anomalous positron peaks in heavy-ion experiments.