Influence of defects and temperature on the annihilation of positrons in neutron-irradiated silicon

Positron-annihilation lifetime spectra have been obtained from neutron-irradiated silicon. Isochronal annealing shows that positrons trapped in divacancies have a lifetime of 325 ±{} 20 psec and in quadrivacancies, a lifetime of 435 ±{} 30 psec. From this a positron lifetime of 270 psec is predicted for monovacancies. Divacancies are found to anneal out according to a second-order process with an activation energy of 0.8 ±{} 0.1 eV. Breakup of divacancies is found to follow a first-order process with an activation energy of approximately 1.7 eV. The positron trapping cross section for divacancies varied with temperature approximately as T^-2.5, which is in good agreement with theoretical calculations based on the cascade capture model of the electron capture cross section. At 296^∘{}K the positron capture cross section was estimated to be about 4 ×{} 10^-16{cm}²$.