INFLUENCE OF HEAT-TREATMENT ON THE TUNNELING STATES IN NEUTRON-IRRADIATED QUARTZ

We report the results of a systematic study of the influence of heat t reatment on the tunneling states (TS) in neutron-irradiated quartz. Me asurements of the ultrasonic attenuation and velocity change are carri ed out before and after heat treatment, as a function of temperature ( 0.3-300 K) and in the ultrasonic frequency range (50-650 MHz), on well -characterized specimens. Two doses have been studied, irradiated, and heat treated under similar conditions. One dose is lying well below t he so-called threshold dose, the other is situated near the threshold. From numerical fits of the data with the tunneling model the density of states (P) over bar and the coupling parameter gamma(l) were determ ined independently. This allows us to investigate the influence of the heat treatments on the number and nature of the TS. Attention is paid to both the influence of the annealing temperature and the length of the heat treatment. All samples still show the typical TS behavior aft er heat treatment, but the density of states is drastically reduced. R egarding the coupling, the two doses show a different behavior: while there is no influence on the coupling for the higher dose, a significa nt decrease in gamma(l) is observed for the lowest dose. This decrease can be interpreted in terms of changes in the tunneling entity itself and is in agreement with the changes in the distribution of the TS.