Investigation of Low-Temperature Ultrasonic Absorption in Fast-Neutron Irradiated Si${\mathrm{O}}_2$ Glass

Ultrasonic attenuation measurements were made in normal and fast-neutron-irradiated fused silica from 7 to 50 Mc/sec and from 1.5° to 200°K. A broad attenuation curve, attributed to a structural relaxation with a distribution of activation energies, occurs at low temperatures. The shape of the loss curve is dependent upon the distribution of activation energies and the amplitude is proportional to the number of structural units which contribute to the relaxation process. Heavy fast-neutron irradiation produced no change in the shape of the curve while the amplitude decreased markedly. A loss associated with a specific defect, an elongated Si—O—Si bond with two equilibrium positions for the bridging oxygen atom, is consistent with the results of this study. The presence of a large number of these defects suggests a new concept of the structure of glass. Evidence is presented to show that thermal spikes, rather than displacement collisions alone, are responsible for the fast-neutron damage in Si${\mathrm{O}}_2$.