Positron Annihilation in Diatomic Crystals

Abstract

The angular correlation between the two photons created by annihilating positron-electron pairs in lattices with a basis is derived in various approximations. The effects of temperature as introduced through the momentum distribution of the positrons and through lattice vibrations are examined. Self-consistent electron wave functions with exchange are calculated for the LiH crystal in the cell approximation. The resulting x-ray structure factors are in close agreement with the recent extensive experimental data of Calder et al. The positron wave function is obtained in the same approximation and the momentum distribution of annihilating positron-electron pairs is calculated. The apparent discrepancy noted by Stewart and March between their angular-correlation data and the electron density distribution in the LiH crystal consistent with the x-ray structure factors can be traced to the phase relations between the wave functions within the unit cell of a lattice with a basis.