Fast-Neutron-Induced Changes in Quartz and Vitreous Silica

The course of the changes produced by exposure in nuclear reactors and the subsequent alteration of the radiation-induced property changes caused by heating were followed by measuring the dilatation, refractive index, rotatory power, and birefringence. The fast-neutron dosages determined by a comparison of radiation damage in several substances are used to give a consistent scale for presenting the effects seen from the initial changes to nearly the saturation state, despite the many irradiation facilities used for the work. The property changes resulting from heating specimens irradiated to various extents are given. The damaging phenomena are explained in terms of the interaction of the scattered atoms with the silica structure, and the annealing phenomena in terms of the behavior of the anticipated structural changes. On the basis of the effect of displaced atoms and thermal spikes on solid structures there are delineated a number of classes of radiation damage among insulators. For one of these, silica is taken to be the prototype. Thermal spikes cause the displaced atoms to be accommodated by a local deformation, which is depicted as analogous to the rapid inversions in which a minor reorientation of the Si-O tetrahedra occurs without extensive disruption of the structure. The resulting gradually accumulating disorder is considered the basic formulation of a metamictization for which the damaging of quartz thus becomes the simplest example. The same basic processes are considered to take place in vitreous silica; but since the substance is already disordered, the changes which take place are minor, the slight compacting of the structure evidently representing mainly the partial freezing-in of a high-temperature state produced by the thermal spike.