The Co-60 gamma-ray-induced changes in the fracture surface energy φ of KCl at 300° and 115°K, as measured by the Obreimov-Gilman slow cleavage technique, are given by φ=φ0+a1DF12+a2DF23,where φ0 is the fracture surface energy of unirradiated crystal, DF is the F-band optical density per centimeter, and a1 and a2 are constants independent of DF. The square root term represents a plastic-flow contribution which decreases with increasing irradiation and is dominant at room temperature. The two-thirds power term represents an extrinsic, point-defect (EPD) contribution which increases with increasing irradiation and dominates at low temperature. The formation of aggregate centers by room-temperature ``F-light'' bleaching causes an increase in the EPD contribution, which is reversed when the aggregate centers are destroyed by re-irradiation. The external surface energy of Co-60 gamma-ray and 40-kV x-ray irradiated KCl at 300°K, as measured by a pendulum sclerometer, follows a similar relation which is derived under the assumption of a quasistatic friction mechanism. Qualitative observations on spontaneous and forced crack healing in KCl are consistent with the above findings.
Photocurrent oscillations in boat-grown oxygen-doped-GaAs (dark resistivity from 3.0 to 106 ohm cm) are studied at different temperatures under white-light illumination and at 80°K under monochromatic excitation. Higher-resistivity samples show oscillations 20–50°K below room temperature and the oscillation characteristics (period and amplitude) follow closely the variation in the steady-state photocurrent with temperature. Lower-resistivity samples oscillate only at temperatures \lesssim100°K. The oscillation characteristics of all the samples of different dark resistivity follow the spectral response curves at 80°K. From the experimental observations, an energy diagram is presented to explain the phenomenon of photocurrent oscillations in these crystals. It is concluded that two acceptor levels, respectively 0.22 eV and 1.3 eV below the conduction band, are responsible for this phenomenon. The 1.3 eV level is found to be responsible both for the photoconduction and field-dependent trapping of the photoexcited electrons.
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