| Pulse-echo ultrahigh frequency ultrasonic measurements in gamma irradiated natural brazilian X-cut quartz crystals reveal the existence of Debye relaxation peaks at low (18 K) and high (155 and 255 K) temperatures which can be attributed to transitions of the hole trapped at the oxygen ion nearest to the substitutional aluminum impurities. The observed activation energies are in agreement with previous anelastic and dielectric loss measurements but the relaxation times are found to be respectively one to two, and five orders of magnitude smaller for the high and low temperatures peaks. The low-temperature peak shows an apparent radiation annealing for gamma doses in excess of one megarad attributed to the closing of tunnelling transitions of the hole from low-lying intermediate energy levels due to electrostatic shifts of the asymmetry of the double-well potential by ionized defects. Uncoupling of the sound waves to the split ground state levels of the hole may be due to the anisotropy of the deformation potential constant. |
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