Radiation-Induced Dilatations in Vitreous Silica

Abstract

It is shown that when vitreous silica is irradiated in a nuclear reactor, three effects cause dimensional changes: a homogenization effect assigned to fast neutrons (period $\sim 3\times {10}^{20} \mathrm{nvt}$), a compaction (about 3%) assigned mainly to fast neutrons (period ∼3×${10}^{19}$), and a dilatation caused by ionization. The ionization-induced dilatation is negative at the beginning of the irradiation, but becomes positive after a brief irradiation. It probably is responsible for the positive dilatation which occurs after the maximum compaction. Most specimens show internal stresses which give evidence of nonuniform effects. The cause is a radiation gradient due to absorption and/or scattering of the radiation in the specimen. In the period of rapid compaction the strain shows one sign, reaches a maximum, then goes to zero near the maximum in the compaction, and changes sign in the period of expansion. The strain does not seem to saturate when the dilatation saturates. The deformation of the specimens is of the order of magnitude expected from the internal stresses. A thermal effect like a radiation annealing was found in the region between room temperature and 90°C. Results obtained from irradiations by x rays, electrons, as well as by mixed radiation in a nuclear reactor are reported. Because of the concomitant ionization effect it was not found possible to determine the displacement threshold energy by electron bombardment. The ionization effects are taken as evidence of electrostatic effects in the radiation-induced dilatations of silica. However, the electrostatic effects cannot account for all of the volume changes observed in the irradiation of vitreous silica.