Creation of paramagnetic defects by gamma irradiation in amorphous silica

An electron spin resonance (ESR) study of the defects induced by gamma -rays in various types of natural and synthetic silica is reported. Three main structures were identified: the E' center and two doublers with field splitting of 7.4 and 11.8 mT, respectively, both centered around the E' center signal. Another structure partially overlapping the E' center line was also detected, consisting in three peaks with a maximum field splitting of 1.36 mT. We have investigated the growth kinetics of these centers on increasing the gamma -ray accumulated dose. In all investigated materials the growth of E' centers can be interpreted as caused by gamma -activated conversion of one or more precursors. The 1.36 mT structure can be considered as a hyperfine structure of the E' centers generated by the same precursors but interacting with a nuclear spin. The 7.4 mT doubler features a sublinear growth with the gamma dose in the whole investigated range (from 0.01 to 1000 Mrad) and a strict correlation with the gamma -induced photoluminescence band at 4.4 eV. Finally the 11.8 mT doubler is generated only in natural silica samples and can be related to the gamma -induced bleaching of the emission hands at 3.1 and 4.2 eV.