Positron and positronium studies of irradiation-induced defects and microvoids in vitreous metamict silica

To study irradiation-induced defects and structural microvoids in vitreous silica (v-SiO(2)), positron lifetime, angular correlation of positron annihilation radiation (ACAR), and electron spin resonance (ESR) were measured on v-SiO(2) and quartz (c-SiO(2)) samples irradiated with fast neutrons up to a dose of 4.1 x 10(20) n/cm(2). Two kinds of positron-trapping defects have been found to form in v-SiO(2) by fast neutron irradiation: type-I and type-II defects. Similar defects also appear in the irradiated c-SiO(2), indicating that both the defects are common in v-SiO(2) and c-SiO(2), The detailed annealing and photo-illumination studies of positron annihilation and ESR for these two defects suggest that the type-I defects are non-bridging oxygen hole centers (NBOHC), while the type-II defects are oxygen molecules which cannot he detected by ESR, Higher dose irradiation than 1.0 x 10(20) n/cm(2) causes c-SiO(2) to change to metamict (amorphous) phase (m-SiO(2)). Positronium (Ps) atoms are found to form in microvoids with an average radius of about 0.3 nm in the v-SiO(2) and m-SiO(2). This suggests that microvoids proved by Ps are structurally intrinsic open spaces and reflect the topologically disordered structure of these phases in the subnanometer scale. (C) 2000 Published by Elsevier Science B.V. All rights reserved.