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 ΔVV of the order of 10^-5 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-m{μ} 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, ΔVV10^-6. Heat-treated crystals presented a different behavior of thermal annealing of the F band with several annealing stages between 15 and 80^∘{}K.