Vacancies in as-grown and electron irradiated α-SiO2

Positron annihilation is a technique uniquely suited to detect vacancies independent of whether they are EPR active or not. In α-SiO2 it has long been suspected that EPR inactive precursors should exist for the E’ centre. Here we show that vacancies are indeed present in as-grown α-SiO2 at a concentration of ~1017/cm3. About 1000 °C these vacancies migrate whereup they form vacancy clusters. 2 MeV electron irradiation introduces additional vacancies but at a highly nonlinear rate. The introduction rate decreases abruptly by a factor of 5 around a dose of 1017e−/cm2. The irradiation-produced vacancies are annealed out at 500 °C well below that for the vacancies in the as-grown α-SiO2. We ascribe the 500 °C annealing stage to recombination with interstitials closely associated with the irradiation-produced vacancies.