Positron trapping defects in neutron-irradiated vitreous and crystalline SiO2

Two types of positron-trapping defects have been found to form in silica glass (v-SiO2) by fast-neutron irradiation: type-I and type-II defects. The type-I defects give positron lifetime of about 0.3ns, while the type-II defects provide 0.5ns. Very similar lifetime components also appear in crystalline alpha-quartz (c-SiO2) after irradiation, which suggests that the two types of positron-trapping defects are common in v-SiO2 and c-SiO2. The detailed annealing and photo-quenching studies of positron annihilation and ESR for these two types of defects suggest that the type-I defects are NBOHC (non-bridging oxygen hole centers), while the type-II defects are Si vacancies which can not be detected by ESR Higher dose irradiation than 1.0x10(20) n/cm(2) causes c-SiO2 to change to metamict (vitreous) phase (m-SiO2). Positronium atoms are found to form in microvoids with average radii of about 0.3nm in v-SiO2 and m-SiO2. This suggests that the microvoids are intrinsic structural open spaces and reflect the topologically disordered structure of these phases in subnanometer scale.