Annealing of radiation induced oxygen deficient point defects in amorphous silicon dioxide: evidence for a distribution of the reaction activation energies

The selective annealing of point defects with different activation energies is studied, by performing sequences of thermal treatments on gamma irradiated silica samples in the temperature range 300-450 degrees C. Our experiments show that the dependence on time of the concentration of two irradiation induced point defects in silica, named ODC(II) (standing for oxygen deficient centre II) and the E-gamma' centre, at a given temperature depends on the thermal history of the sample for both of the centres studied; moreover in the long time limit this concentration reaches an asymptotic value that depends on the treatment temperature alone. These results suggest the existence of a distribution of the activation energies of the reaction process responsible for the annealing of the defects investigated, intimately related to the intrinsic disorder of the amorphous lattice. Furthermore, our data show that the thermal treatment can modify this distribution of activation energies and as a consequence the thermal properties of the centre itself.