ESR and PL centers induced by gamma rays in silica

We have studied the point defects created by γ irradiation in various types of commercial silica glasses, including both natural and synthetic samples, with different OH content, in the low dose regime (0.05-100 Mrad). We found that the growth rate of E' centers depends strongly on the silica type, ranging from 2 × 10 ¹⁵ cm ^-3 Mrad ^-1 to 6 × 10 ¹⁷ cm ^-3 Mrad ^-1. Samples of natural silica are rather susceptible to γ ray exposure, as E' concentration saturates (typically 5 × 10 ¹⁷ cm ^-3) for doses as low as a few Mrads. For both synthetic and natural samples, the radiation hardness is higher in wet than in dry silica. Moreover, we found a strict correlation between the concentration of E' centers and the γ-induced absorption band at 5.8 eV. Finally, exposure to γ rays generates in all the samples a photoluminescence band at 4.4 eV, whose excitation spectrum has a maximum at 4.95 eV. This band exhibits a sublinear growth kinetics in all the investigated samples.