Positron annihilation in neutron- and electron-irradiated silica glass

Positron lifetime and angular correlation of annihilation radiation (ACAR) have been measured on fused and synthetic silica glass before and after irradiation with fast-neutrons up to 8x1018 n/cm2 below 150degC or with 3 MeV electrons up to 1x1018 e/cm2 below 50degC. Before irradiation positron lifetime spectra show characteristic long-lifetimes due to ortho-positronium (o-Ps) formation and are well decomposed into 3 components with the time constants τi (i=1,2,3) and the relative intensities Ii(I1+I2+I3=1). The τ3 lifetime is about 1.7 ns, being nearly independent of the samples, while τ2 is strongly dependent on the samples, ranging from 0.7 to 1.2 ns. Narrow peaks on the ACAR curves and the τ3 lifetime give evidence of Ps formation in structural open-spaces (voids) with radius about 0.3nm. By neutron-irradiation the τ2 lifetime has reduced to about 0.47 ns irrespective to the samples but its intensity (I2) has increased with the dose. This component suggests positron trapping without Ps formation at irradiation-induced defects such as peroxy radicals. Annealing behavior of lifetime parameters is also presented and compared with previous ESR work. (author)