Positron Annihilation in Sintered Boron Nitride

Abstract

Positron annihilation spectroscopy is a very sensitive technique to investigate the electronic structure and defects in solids ¹. The most common mode of positron annihilation with the electron in a solid is two gamma rays to be emitted in coincidence in approximately opposite directions. If the center of mass of the annihilating electronpositron pair is at rest, all of the rest mass energy of the pair (2mc²) would transform to two gamma rays of 0.511 Mev (mc²) emitted at 180° to each other. However, since the actual annihilating pair has some momentum, the two gamma rays are emitted in directions which deviate from 180° by an angle $\Xi$ given by

$$ \Xi = {p \over {mc}} $$

(1)

where p is the linear momentum of the annihilating pair and c is the speed of light. The two-gamma-ray angular correlation technique (ACAR) measures the angular deviation of Eqn.(l) and provides information on the electron momentum distribution.