Thermal bleaching of x-ray-induced defect centers in high purity fused silica by diffusion of radiolytic molecular hydrogen

Abstract

High purity Type-III fused silicas (1200 ppm OH) were subjected to 100 keV X-irradiation at 77 K and investigated by electron spin resonance (ESR) before and after heat treatments to higher temperatures. Theoretical fits to the isochronal and isothermal anneal curves of the nonbridging oxygen hole center (OHC) have been accomplished using Waite's equation for diffusion-limited bimolecular recombination in solids under the working hypothesis that the diffusing species is radiolytic molecular hydrogen. The analysis supports the notion that there exists a statistical distribution in activation energies for H 2 diffusion in amorphous SiO 2, and percolation theory is invoked to explain the generally lower activation energies measured in the present experiment vis-à-vis the results of more classical gas evolution studies. The occurrence of molecular hydrogen as a product of radiolysis in high-OH Type-III silicas appears to provide a natural radiation-protection mechanism (room temperature fading) which might be inadvertently thwarted by heat treatments which drive out hydrogen prior to irradiation. The significance of these results in the fiber optic context is emphasized.