Thermoluminescence and Color Centers in LiF

Abstract

Thermoluminescence emission above room temperature and optical absorption have been studied in LiF crystals irradiated by gamma rays, with emphasis on conditions which are of greatest interest for applications to radiation dosimetry. The specimens were Harshaw crystals of differing purity, and the purpose was to correlate the electron and hole traps and recombination centers involved in the thermoluminescence with optical‐absorption bands produced by the trapped carriers. The dominant glow peak in suitably treated crystals occurs at about 200°C, and this is correlated with an absorption band at 310 mμ, through thermal and optical bleaching of the thermoluminescence and optical absorption. A smaller glow peak at 100°C is correlated with an absorption band at 380 mμ. The F center absorbing at 250 mμ appears to be the electron trap associated with both glow peaks. It is concluded that the 310‐ and 380‐mμ centers are trapped‐hole centers, and the thermoluminescence occurs after they are thermally ionized. The recombination center which produces the visible‐light emission is believed to be not the F center, however, but an impurity center which absorbs at 196 mμ. An energy‐level diagram is suggested, which can account for the observed optical and thermal transitions. It depends on electron tunneling from the F center to the 196‐mμ center. Available evidence indicates that the 310‐ and 380‐mμ centers involve Mg; the impurity responsible for the 196‐mμ center may be Al.