EPR imaging of irradiated silicon dioxide: increased concentrations of E′ defects near the surface

Although 60Co γ-rays are sufficiently energetic to pass through 0.5 mm of amorphous silicon dioxide with negligible attenuation, higher concentrations of 60Co γ-ray-induced E′ defects have been detected by EPR imaging in surface layers than in the bulk material for some samples of SiO2. The layers with enhanced defect concentrations are about 100 μm thick. Total spin concentrations and local spin concentrations were examined for these samples, but do not correlate with the enhanced surface sensitivity toward defect formation for particular samples.