Angular Correlation of Annihilation Radiation in Alkali Halide Crystals

A new type of instrument has been used for measurements on Mg metal and single crystals of LiF, NaCl, KCl, KBr, and KI. The data, after correction for instrument resolution, yield the momentum distribution of either the photons or the center of mass of the positron and electron just prior to annihilation. The momentum distribution obtained for NaCl is accounted for by assuming (a) that the positron is bound to the chloride ion in an $s$ state, (b) that annihilation takes place with one of the electrons of the closed shell, and (c) that the overlap wave function is of the form ${\mathrm{}(r-b)\mathrm{}}^2{e}^{-mr}$. Best agreement with experiment was obtained for 30% of the annihilations taking place with $s$ electrons for which $b=0.75\mathrm{}$ A, and $m=3.64\mathrm{}$ ${\mathrm{A}}^{-1\mathrm{}}$ and the other 70% with $p$ electrons for which $b=0.70\mathrm{}$ A and $m=4.00\mathrm{}$ ${\mathrm{A}}^{-1\mathrm{}}$.