Effect of neutron irradiation on the structure and optical properties of fused SiO2

Abstract

The effect of fast-neutron irradiation on the structure of fused silica has been studied by IR reflection spectroscopy in a wide range of neutron fluences, from 1017 to 1021 n/cm2. The spectral characteristics of the bending and stretching modes of the bridge bonds in silica have been shown to be nonlinear functions of neutron fluence. The kinetics of radiation-induced changes in the optical properties of silica in the UV through visible spectral region have been analyzed in relation to those in its structure and microscopic characteristics. A strong correlation has been found between the dose dependences of the optical and structural properties of silica, such as the 465-nm luminescence intensity, the intensity and position of the amorphous halo, the reflectivity and frequency of the IR bands at 1125 and 480 cm−1, the density of the material, and its ionic polarizability. We assume that there is a threshold dose in the range 1019 to 1020 n/cm2 which produces sharp changes in the optical and structural properties of SiO2 due to a transition to a metamict-like state. An analytical expression has been derived for the radiation-induced changes in the intensity of the first halo. The driving force and mechanism of the radiation-induced structural changes in fused SiO2 have been tentatively identified.