Gamma Ray Absorption Measurement Using Positron Annihilation Coincidence Detection

A new principle of density and thickness measurement by gamma ray absorption is developed. The standard deviation of the density and thickness readings caused by the natural randomness of a gamma emitter can be effectively reduced by applying detection of two identical photons produced by positron annihilation. It can be shown that the relation of the original annihilation rate to that absorbed in the material is a function of thickness and density, and the deviation of the readings depends on the efficiency of the detectors. Twice the accuracy of the conventional single gamma counting method can be achieved with a detector efficiency of 62%. A better efficiency gives higher accuracy.