Deformed sodium chloride crystals were irradiated by γ rays from spent fuel elements while their ultrasonic properties (resonant frequency and decrement) were being measured. It was found that the damping coefficient associated with the viscous drag on a moving dislocation varied with time during irradiation, and was also a function of the state of deformation of the crystal. The variations in the damping coefficient were interpreted in terms of the model developed by Robinson and Birnbaum for dislocation damping in alkali halides.

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Room-temperature radiation experiments have been carried out on deformed single crystals of ’’ultrapure’’ and nominally pure NaCl and KCl where it has been observed that the decrement may initially increase to some maximum value and then decreases with radiation time. In contrast, the compliance defect was always found to decrease as soon as the crystal was irradiated. If the radiation was stopped during an experiment the decrement kept on decreasing whereas the compliance defect remained essentially constant. Simpson and Sosin have observed a similar decrement maximum with copper foils vibrated at 500 Hz and exposed to 1.0-MeV electrons. Their defect-dragging model is examined in relation to the present work and it is concluded that the model cannot explain the results. The results are also compared with those of Anderson and Pollard who have carried out fast neutron irradiation experiments on NaCl and KCl crystals. It was found that the behavior of B/B0, the fractional change in the damping coefficient, is similar for both neutron and gamma ray experiments at least in the early stage of irradiation.

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