The electrical resistance of Harshaw sodium chloride crystals exposed at room temperature to x-ray doses sufficient to produce about 6×${10}^{16}$ $F$ centers/${\mathrm{cm}}^3$ has been measured as a function of time at constant temperatures between 150 and 200°C. The effects of previous heat treatment of the crystals, x-ray dose, temperature of measurement, and preliminary optical bleaching have been observed. Initially the resistance of irradiated crystals is much greater than that of unirradiated ones. The resistance of the colored crystals at first decreases rapidly (for about 10 to 100 minutes, depending on the temperature). In the case of crystals used as received from Harshaw, the resistance even falls slightly below that of the normal uncolored crystals. Subsequently the resistance again increases, reaching a constant value (after about 50 to 1000 minutes) higher than for the normal crystal. If the crystals are optically bleached before the measurement, the resistance increase recovers very much more slowly. The behavior can be restored to normal by annealing above 250°C. The slow resistance increase is seen as evidence for the production by the x-rays of excess positive-ion vacancies.

• Received 16 September 1957

DOI:https://doi.org/10.1103/PhysRev.109.710