Volume Expansion and Color Center Behavior of NaCl X-Ray Irradiated at Low Temperature

Volume expansion of x-ray irradiated NaCl was measured in the range 15-90°K and its optical behavior was analyzed from 15 to 300°K. An improved photoelastic method showed that strain profiles agree with the shape predicted by the linear theory and that slow cooling (10°C per h) is essential to minimize background strain. Twenty-four hours of irradiation by 45 kV, 30 mA x rays at 15°K produces $\frac{\Delta V}{V}$ of the order of ${10}^{-5\mathrm{}}$ which does not anneal below 80°K while, in contrast, the $F$ band shows in this range one-stage annealing. Growth curves of the $F$ band and of the 343-mμ band were made at 15°K. Annealing at 80°K followed by reirradiation at 15°K indicates that these bands recover quickly and a comparison with results on KCl and KBr suggests that the $F$ centers are annihilated by hole capture. If follows also that in the range 15 to 80°K electronic changes in these centers do not lead to volume changes greater than the experimental error, $\frac{\Delta V}{V}\simeq {10}^{-6\mathrm{}}$. Heat-treated crystals presented a different behavior of thermal annealing of the $F$ band with several annealing stages between 15 and 80°K.