Effect of lattice structure on the positron annihilation with inner shell electrons

The effects of lattice structure on the high momentum part of the Doppler broadening of the positron-electron annihilation radiation was studied for a number of materials. The momentum distribution of the annihilation radiation was measured using two Ge detectors to record "in coincidence" the energies of both the upshifted and downshifted gamma rays. First-principles calculations were performed to complement the experimental data. We present results for diamond and graphite, alpha and beta Sn, amorphous and crystalline Si, well-annealed and deformed Cu, and Fe, with and without vacancy clusters. The bulk systems provide a good test case for the present approach, since the delocalized positron has no effect on the surrounding lattice. These results show that the two-detector technique, which exhibits enhanced elemental sensitivity, can also be used as a probe of the geometry of the annihilation environment, and hence can be used for identifying defect structures. (C) 1997 Elsevier Science Ltd. All rights reserved.