Orientation of nuclei excited on annihilation of a positron in a heavy atom shell

The orientation cross sections and spin-tenzors for a nucleus excited on nonradiative annihilation of a positron by an electron of the nlj shell of the atom are calculated. The relativistic electron and positron wave functions obtained by numerical integration of the Dirac equation with a single mean Hartree-Fock-Slater atomic potential are employed. The results of a calculation of the factors determining the cross sections and orientation of the excited nucleus as a function of the positron energy Esub(+) in the 0.55 to 6.55 MeV range for the E0, E1, M1 multipole transition series on annihilation at the K shell of the atom are presented graphically. The calculations are performed for atoms with Z=41, 49, 60, 70, 82 and 92. Estimates are also given for positron annihilation at the L shell of lead (Z=82). The values cited can be used to estimate the cross sections for nuclear excitation on positron annihilation as well as the angular distributions of the decay products of the nucleus thus excited (quanta, fission fragments etc.). Some possible applications of a low energy positron beam in nuclear spectroscopy are briefly discussed