X-Ray Expansion and Coloration of Undoped and Impurity-Doped NaCl Crystals

Expansion and coloration measurements of undoped and impurity-doped single crystals of NaCl have been made under x-ray irradiation (43 kvp and 20 ma) for intervals up to three hours. Expansion was measured in a direction normal to the x-ray beam with a sensitive capacitive dilatometer that was designed to compensate automatically for thermal fluctuations. It was found that the initial rate of creation of $F$ centers exceeds the rate of creation of vacancy pairs as computed from the expansion data; however, after continued irradiation the rates of $F$-center and vacancy-pair production are approximately evenly matched. This suggests that the initial production of $F$ centers results from both vacancies originally contained in the crystal and vacancies created during irradiation, while the later coloration is due essentially to vacancies that are created in the irradiation process. Within the framework of this interpretation it is possible to estimate the initial negative-ion vacancy concentration of a number of crystals. This concentration ranges from 1×${10}^{16}$/${\mathrm{cm}}^3$ for a Harshaw crystal to a value larger than 6×${10}^{17}$/${\mathrm{cm}}^3$ for a calcium-doped crystal grown at NRL. Although the presence of a large initial concentration of negative-ion vacancies in calcium-doped NaCl explains its enhanced color sensitivity to x-rays without a corresponding enhancement of x-ray-induced expansion, this explanation is not in accord with simple expectations of charge compensation and the law of mass action.