ESR and PL centers induced by gamma rays in silica

We have studied the point defects created by gamma 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 x 10(15) cm(-3) Mrad(-1) to 6 x 10(17) cm(-3) Mrad(-1). Samples of natural silica are rather susceptible to gamma ray exposure, as E' concentration saturates (typically 5 x 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 gamma-induced absorption band at 5.8 eV. Finally, exposure to gamma 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.