Positron annihilation in oriented single crystals of calcite: two photon angular correlation

Precision measurements of the angular correlation of two photon annihilation in oriented single crystals of calcite are reported. 10 millicurie Na22was used as the positron source and the resolution of the angular correlation set-up was 0.7 milliradian. Measurements were made parallel and perpendicular to optic axis and also for the (100) plane. Using the Goldanskii and Prokopev model for polaron states for positrons in ionic crystals the FWHM is used to calculate the effective masses of positrons and anion effective charges for different crystallographic directions. The observed variations of effective charge must be taken as the direct experimental evidence for charge distortions