Thermoluminescence of LiF TLD-100 dosimeter crystals

The unusually complex thermoluminescence obtained from Harshaw dosimeter type TLD-100 LiF crystals has been studied with apparatus for measuring emission intensity simultaneously as a function of both wavelength and sample temperature. A separate signal averaged spectrum is obtained at 5.5 C temperature intervals. Data reduction includes all corrections needed to obtain curves of absolute intensity vs photon energy. Numerous results were obtained from crystals exposed to Co⁶⁰ gamma-ray doses extending from 5 &time; 10² to 5 &time; 10⁷ R. All emission spectra could be resolved into Gaussian shaped bands using a computerized best-fit procedure. Below 10⁵ R all spectra contain three bands at roughly 3.01, 2.90, and 2.71 eV with full widths of 0.90, 0.72, and 0.96 eV; the precise values vary slightly with temperature. Above 10⁵ R additional bands appear at 2.98 and 2.50 eV, and with increasing exposure the 2.90 and 2.71 eV bands diminish in intensity. At doses greater than 4 or 5 &time; 10⁶ R additional bands appear at 4.0, 2.3, and 1.5 eV. In the range usually used for dosimetry, at least the 3 bands at 3.01, 2.90, and 2.71 eV contribute to the thermoluminescence. Glow curves for each of these emission bands were constructed from the resolved Gaussian shaped bands. They contain 8, 3, and 5 prominent peaks. In contrast, glow curves for unresolved spectra contain at least 8 glow peaks, and each peak can contain undetermined contributions from 1, 2, or 3 emission centers. All glow peaks analyzed to date can be fitted by the first-order expression dn/dt = − ns exp (-E/kT). However, the E and s values obtained for the 190 C peak and perhaps some of the other high temperature peaks are too large to be meaningful; thus the kinetics for this peak, and possibly other high temperature peaks, must be complex. To date, thirteen glow peaks have been identified. At low doses the emission is confined to the low temperature glow peaks. With increasing dose the emission intensity increases and progressively shifts to higher temperature peaks. The maximum intensity occurs at 10⁶ R. At higher doses the emission is reduced but occurs among the highest temperature peaks.