Neutron-induced opalescence in silica glass

Distribution of light scattering intensity along the length of silica optical fibers irradiated in nuclear reactors was measured by optical time domain reflectometry (OTDR). Longitudinal variations of the OTDR signal (“OTDR traces”) were found to contain two components associated with radiation-induced attenuation of light (RIA), on the one hand, and with longitudinal variation of the Rayleigh scattering coefficient (RSC), on the other. Based on the shape of the OTDR traces and its variation during irradiation, we concluded that the RSC dependence on the fast-neutron fluence is strongly non-monotonic: it features a ∼10-time increase at a fast-neutron fluence of ∼10 18 –10 19 n f /cm 2 followed by a fall-off during subsequent irradiation. Such RSC variation with fluence was interpreted as being associated with the formation of thermal spikes – peculiar silica regions ∼10 nm in size. Thermal spikes arise as the result of fast-neutron bombardment of silica and possess a higher fictive temperature and, consequently, a higher density. In the first stage of fast-neutron irradiation (10 17 – 10 19 n f /cm 2 ), thermal spikes become a factor of silica inhomogeneity to increase RSC. However, as soon as most silica is occupied by thermal spikes, further irradiation, on the contrary, makes silica more homogeneous to reduce RSC. The RSC increase and subsequent fall-off are argued to be similar to the anomalous RSC behavior in solids and liquids in the vicinity of phase transition temperature. For this reason, the RSC behavior upon fast-neutron irradiation can be named neutron-induced opalescence (NIO).