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 8x10^ n/cm^2 below 150℃ or with 3 MeV electrons up to 1x10^e/cm^2 below 50℃. 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 τ_i (i=1, 2, 3) and the relative intensities I_i (I_1+I_2+I_3=1). The τ_3 lifetime is about 1.7 ns, being nearly independent of the samples, while τ_2 is strongly dependent on the samples, ranging from 0.7 to 1.2 ns. Narrow peaks on the ACAR curves and the τ_3 lifetime give evidence of Ps formation in structural open-spaces (voids) with radius about 0.3nm. By neutron-irradiation the τ_2 lifetime has reduced to about 0.47 ns irrespective to the samples but its intensity (I_2) 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