Correlation of Annihilation Radiation in Oriented Single Metal Crystals

Precision measurements of the two-photon annihilation in crystals of Al and Cu are presented. These show relatively small variation with crystal orientation. This is interpreted to mean that the strong high-momentum tail in the Cu distribution is due to annihilation with the $3d$ electrons and that the Fermi surface in Al is very nearly spherical but has slight bulges in the [100] and [111] directions. One-electron calculations of the distributions for Cu and Al agree quite well with the experiments and further confirm the importance of core annihilation in Cu. The calculated positron lifetime in Cu does not disagree with previous experimental observations by more than a factor of two. Electron-positron correlation by any factor much larger than this is therefore precluded.