Effect of moisture on the self-healing of vitreous silica under irradiation

Although it is widely understood that water interacts extensively with vitreous silicates, atomistic simulations of the response of these materials to ballistic radiation, such as neutron or ion radiation, have excluded moisture. In this study, molecular dynamics simulations were used to simulate the collision cascades and defect formation that would result from such irradiation of silica in the presence of moisture. Using an interatomic potential that allows for the dissociation of water, it was found that the reaction between molecular water or pre-dissociated water (as OH^- and H⁺) and the ruptured Si-O-Si bonds that result from the collision cascade inhibits a significant amount of the structural recovery that was previously observed in atomistic simulations of irradiation in perfectly dry silica. The presence of moisture not only resulted in a greater accumulation of non-bridging oxygen defects, but reduced the local density of the silica and altered the distribution of ring sizes. The results imply that an initial presence of moisture in the silica during irradiation could increase the propensity for further ingress of moisture via the low density pathways and increased defect concentration.