A study is made of the growth of the $F$ band in Mg${\mathrm{F}}_2$ crystals irradiated with ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ $\gamma$ rays as a function of irradiation time at room temperature. It is shown that a rapid initial-growth stage in the form of a saturating exponential occurs for crystals containing iron (up to 110 at. ppm), and that the saturation value increases monotonically with the Fe content. The initial growth rate, which corresponds to $\sim {10}^2 \mathrm{eV}/\left(F \mathrm{center}\right)$, is independent of the Fe concentration. Annealing experiments show that these first-stage $F$ centers anneal out at a relatively low temperature (∼ 110°C), while $F$ centers formed in later stages are more resistive to annealing. Electron-spin-resonance measurements show that during irradiation Fe is converted from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$. It is therefore concluded that the enhancement of coloration is due to the role of ${\mathrm{Fe}}^{2+}$ as a hole trap. A simple model is developed in which hole trapping enables available anion vacancies to capture electrons and become $F$ centers. This model quantitatively accounts for the essential features of the first-stage coloration behavior. Nevertheless, evidence exists to show that other traps are also involved in the actual process.

• Received 15 March 1978

DOI:https://doi.org/10.1103/PhysRevB.18.1840