Production or annihilation of positrons with bound electrons

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Abstract
We have investigated the recently reported discrepancy between theory and a new experiment [Palathingal et al., Phys. Rev. A 51, 2122 (1995)] for single-quantum annihilation of positrons with bound electrons. Fully relativistic calculations of total cross sections for the production and the annihilation of positrons involving an electron bound in the K or L shells of atoms have been performed in screened atomic potentials, for a number of elements ranging from germanium (Z=32) to thorium (Z=90) and for positron kinetic energies up to 4 MeV above threshold. Very good agreement with earlier calculations, that had been performed over a more restricted range of elements and energies, is obtained, also confirming that the Z and shell dependence previously seen continues at higher energies. The results presented here extend theory to energies where recent experiments have been performed, at forward angle, which were assumed to characterize the Z and shell dependence of the corresponding total cross sections. Our new theoretical predictions are not consistent with the strong dependence of the K- and L-shell bound-pair-annihilation cross sections on the nuclear charge of the target reported in those experiments. An explanation of these discrepancies is proposed, namely, that it is not correct to assume the same ratios for total cross sections as for forward distributions. This is demonstrated using existing data for the related photoeffect process, and it is seen that such forward data are indeed consistent with the pair-annihilation experiments.

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