Thermoluminescence in Lithium Fluoride

Thermoluminescence has been studied in LiF crystals which were produced by a variety of growth conditions. X-ray, Co⁶⁰ gamma-ray, and/or optical excitation have been used to both populate and/or produce defects. Collectively, as many as 28 different glow peaks are observed in the range 20-550^∘{}K. However, in any one sample only about 15 of these peaks occur. The glow curves cover a range in intensity, the ratio of the strongest to weakest peaks is about 10⁴, and the strongest thermoluminescence occurs near 140^∘{}K. No one glow peak is common to all samples. Some of the factors which determine the presence or absence of a glow peak are discussed. The activation energies of the major peaks have been determined by several methods. The four peaks at 115, 147, 154, and 255^∘{}K, and possibly others, follow first-order kinetics. Typical peak temperatures (^∘{}K), pre-exponential factors (sec^-1), and activation energies (eV) are 119, 2.7×{}10¹⁰, 0.28; 133, 3.1×{}10³, 0.14; 147, 8.3×{}10³, 0.17; 158, 3.4×{}10⁷, 0.29; 194, 1.8×{}10⁶, 0.31; 255, 4.7×{}10⁸, 0.53. The peaks at 115, 158, 194, and 255^∘{}K, and possibly others, result from electron untrapping, since they can be populated by both x irradiation and by electrons released upon irradiating a colored crystal with F-band light. The peak at 68^∘{}K has been observed only after optical re-excitation. Several of the glow peaks have been corelated with impurities, ESR data, etc. These corelations suggest specific models for the defects associated with particular glow peaks. The peak at 115^∘{}K appears to result from the H center. The peak at 133^∘{}K is impurity-sensitive and is probably caused by the untrapping of a hole from the VK center. A Mg impurity produces peaks between 430 and 490^∘{}K, whereas Mn, Ti, Zr, or Ce merely enhance the 133^∘{}K peak.