ESR AND PL CENTERS INDUCED BY GAMMA-RAYS IN SILICA

We have studied the point defects created by gamma irradiation in vari ous types of commercial silica glasses, including both natural and syn thetic samples, with different OH content, in the low dose regime (0.0 5-100 Mrad). We found that the growth rate of E' centers depends stron gly 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 suscepti ble 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 a nd natural samples, the radiation hardness is higher in wet than in dr y silica. Moreover, we found a strict correlation between the concentr ation of E' centers and the gamma-induced absorption band at 5.8 eV. F inally, exposure to gamma rays generates in all the samples a photolum inescence 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 inve stigated samples.