Room Temperature Instability of E′γ Centers Induced by γ Irradiation in Amorphous SiO2

We study by optical absorption measurements the stability of E′γ centers induced in amorphous silica at room temperature by γ irradiation up to 79 kGy. A significant portion of the defects spontaneously decay after the end of irradiation, thus allowing the partial recovery of the transparency loss initially induced by irradiation. The decay kinetics observed after γ irradiation with a 0.6 kGy dose closely resembles that measured after exposure to 2000 pulses of pulsed ultraviolet (4.7 eV) laser light of 40 mJ/cm2 energy density per pulse. In this regime, annealing is ascribed to the reaction of the induced E′γ centers with diffusing H2 of radiolytic origin. At higher γ doses, the decay kinetics becomes unexpectedly slower notwithstanding the progressive growth of the concentration of induced defects. In particular, the annealing kinetics of E′γ centers after 79 kGy irradiation is inconsistent with the reaction parameters between the defect and H2. To explain this result, on the basis of the quantitative analysis of the kinetics, we propose water-related species to be responsible for the slow room temperature annealing of E′γ after irradiation with such a dose. This model is qualitatively supported by results obtained by IR absorption measurements, which show an increase of the absorption in the spectral region of Si-OH groups.