EPR reversible signature of self-trapped holes in fictive temperature-treated silica glass

Post-mortem electron paramagnetic resonance spectroscopy experiments have been carried out between room temperature and 20 K to examine the radiation-induced defects in fictive temperature (T_f) treated Heraeus F300 silica (0.1 ppm OH, 1500 ppm Cl₂). In particular, we focus our attention on Self-Trapped Hole (STH) centers detected in 1000 °C, 1100 °C, and 1200 °C T_f treated samples irradiated at room temperature by gamma rays at 6 kGy. By repeating annealing cycles between 77 and 300 K on the same samples, we observed that the EPR signal attributed to STH decreases as the temperature increases but in a reversible manner. We evidenced a deviation from the Curie law for T > 70 K and suggested an interpretation based on the decrease in the "strain-assisted TH" population by reversible excitation of the trapped hole to a delocalized state with an activation energy of 7.8 meV. This also means that the precursors of hole trapping sites (a local strain atomic configuration) remain stable until 300 K at least.