Angular Correlation of Annihilation Photons in Ice Single Crystals

Abstract

Linear-slit angular-correlation curves were obtained at - 148 °C for the [0001], [10¯10], and [11¯20] directions in single crystals of ice. Besides the narrow central peak, pronounced narrow side peaks were also observed. They occurred at angles $\theta =\frac{2\pi \hslash g_z}{\mathrm{mc}}$, where $g_z$ is the projection of reciprocal-lattice vectors $\overrightarrow{\mathrm{g}}$ on the direction perpendicular to the slits and the sample surface. The relative area of the central plus the side peaks was (15.2 ± 0.4)% for all curves. All the peaks are interpreted as due to parapositronium annihilation. The side peaks are explained as evidence for the positronium center-of-mass wave function being a Bloch function, i.e., that positronium is delocalized in ice.