Mechanism of hydration-induced gamma-ray degradation in ion-beam-figured fused silica

Ion beam figuring (IBF) improves laser damage resistance in fused silica, yet its radiation reliability after surface hydration remains unclear. We show that surface hydration significantly reduces the damage threshold of irradiated IBF samples, whereas non-hydrated counterparts remain stable. Degradation follows a coupled pathway where IBF-induced deoxygenation creates reactive Si-rich surfaces that hydrate into Si-OH groups. Gamma irradiation subsequently ruptures O-H bonds, generating non-bridging oxygen hole centers (NBOHCs) and thereby enhancing ultraviolet (UV) absorption. ReaxFF simulations support this microscopic transition from Si-rich defects to hydrated structures. Our results reveal a mechanism involving deoxygenation, hydration, and defect formation, highlighting the critical role of surface hydration in the radiation stability of optics.