Optical Absorption of Fused Silica and Sapphire Exposed to Neutron and Gamma Radiation with Simultaneous Thermal Annealing

The simultaneous effects of radiation and thermal annealing on optical absorption in fused silica and sapphire subjected to gamma irradiation up to doses of 10 Mrad, and mixed neutron/gamma irradiation up to total neutron fluence of 1.7 × 10¹⁷ n · cm⁻² (211 Mrad gamma) are reported and discussed. Thermal annealing was performed post-irradiation from 200 ^∘C to 800 ^∘C, and concurrent-irradiation thermal annealing was performed at 800 ^∘C. Optical absorption was measured over the range of 220 nm to 1100 nm. Gamma-radiation-induced attenuation peaks in sapphire at 260 nm, 300 nm, and 355 nm are observed under concurrent-irradiation thermal annealing through the transfer of electrons between precursor oxygen vacancy and divacancy sites and the increased mobility of vacancy sites. It is shown that concurrent-irradiation thermal annealing does not restore the materials to their unirradiated state. Under concurrent-irradiation thermal annealing, high-OH content fused silica retained a radiation induced absorption band in the ultraviolet due to a 213 nm absorption band from unannealed E’ centers. Low-OH content fused silica retained an absorption band from 220 nm to 900 nm with multiple peaks generated from E’ centers. Sapphire retained an absorption band from 220 nm to 350 nm consistent with the transfer of electrons between precursor oxygen vacancy and divacancy sites, the additional production of oxygen vacancy sites from neutron interactions, and increased mobility of vacancy sites. These results help to inform optical material selection and placement to support the development of optical instrumentation in applications such as fission or fusion reactors.