Determination of Anisotropic Momentum Distributions in Positron Annihilation

A theoretical method is presented which permits the experimental determination of the momentum distribution of the photon pairs originating in positron annihilation in crystalline solids, as a function of both the magnitude and the direction of that momentum. The distribution is expanded in a series of lattice harmonics. It is shown how linear combinations of the two-quantum angular correlations, obtained from measurements on a number of suitably oriented single crystals, may be unfolded to yield the momentum-dependent coefficients in that expansion. The method is illustrated by a model computation.