Effects of anharmonicity and of thermally activated structural relaxation in irradiated quartz

The acoustic properties of neutron-irradiated quartz have been investigated at ultrasonic frequencies by the pulse-echo method, and at hypersonic frequencies by Brillouin scattering. Four samples irradiated at different doses were studied in the range 10 to 300 K. The contribution to the acoustic attenuation due to the thermally activated relaxational process can be extracted from ultrasonic data. This relaxational part may be fitted by using the asymmetric double-well potential model. A comparison of these results to the hypersonic frequencies allows the determination of the relative parts of the attenuation, due to structural relaxation and to anharmonicity. It is shown that the attenuation due to structural relaxation increases with the neutron dose, while the attenuation due to anharmonic effects does not disappear. The evolution of the number of defects and of the activation energy of the relaxation process as a function of the irradiation dose is shown.