Measurement of the Stored Energy in X-Rayed Sodium Chloride

The stored energy in sodium chloride crystals irradiated with x rays at room temperature was measured by dissolving the crystals in water and measuring the temperature changes. In the range of $F$-center densities between 2.5×${10}^{17}$ and 6.5×${10}^{17}$ per ${\mathrm{cm}}^3$, the curve of stored energy versus $F$-center density was a straight line, whose extrapolation did not intersect the origin. Quantitative interpretation of the slope of this line is difficult because of lack of knowledge about the chemical nature of a solution of irradiated NaCl. According to the interpretation favored in this article, the stored energy associated with one $F$ center in this coloration range is 9.2±0.3 eV. This value for the stored energy is in good agreement with a simple energy level scheme involving the $F$ center, so that if there are any defects produced by x rays at room temperature which have not yet been discovered, they probably do not involve much stored energy. Crystals which have been irradiated and then thermally bleached no longer have any stored energy, from which we conclude that chlorine does not leave the crystals during irradiation or subsequent heating.