Effect of Fast-Neutron Irradiation on the Pressure and Temperature Dependence of the Elastic Moduli of SiO₂ Glass

The low-temperature ultransonic attenuation, the elastic moduli, and the temperature and pressure dependence of the moduli of SiO₂ glass are modified by heavy fast-neutron irradiation. A sample of Amersil optical-grade fused silica was irradiated to an integrated flux density in excess of 5×10¹⁹ neutronscm². In this sample the ultrasonic loss is decreased by about 85, the shear modulus increased less than 2, and the compressibility decreased by about 20. The anomalous temperature dependence of the compressibility changed from -3.3×10^-2 per°K before irradiation to -9.5×10^-3 per°K after irradiation over a temperature range of from 100°K to 300°K. The anomalous pressure dependence of the compressibility changed from 7.35×10^-2 per atm before irradiation to 2.86×10^-2 per atm after irradiation over a pressure range from atmospheric pressure to 5×10⁴ psi. Similar effects were found in the pressure and temperature dependence of the shear modulus. The effect of irradiation on the low-temperature ultrasonic loss and on the temperature dependence of the sound velocity in this glass was also studied. No change in the ultrasonic attenuation was detected at an irradiation level several orders of magnitude larger than that required to eliminate effectively the comparable 50°K loss peak in synthetic quartz.