γ-ray-induced bleaching in silica:: Conversion from optical to paramagnetic defects

We report experimental results on optical and ESR measurements performed in gamma-irradiated natural silica samples having different content of OH groups. A partial bleaching of the optical absorption band B-2 beta at 5.15 eV and the related photoluminescence emissions at 3.1 eV and 4.2 eV is observed together with the growth of an ESR doublet split by 11.8 mT. The kinetics of the two processes as a function of the gamma dose are correlated and depend on the OH content. Our experiments indicate the occurrence of a gamma-ray-induced conversion, from optically active centers to paramagnetic ones and vice versa, changing the relative concentration of the two defects until a steady state is reached. A conversion model is proposed in which gamma-ray-activated H atoms either are trapped in optically active defects, giving rise to paramagnetic ones, or interact with the last ones, restoring the original optical centers. Our results are consistent with a structural model in which the optical active center is a twofold coordinated Ge and the 11.8 mT doublet originates from a O=Ge-H center.