Thermal and optical characteristics of radiation induced defect centers in alkali Halide crystals (TLD-100)

Abstract Thermal and Optical Characteristics of Radiation Induced Defect Centers in Alkali Halide Crystals (TLD-100) " YAZICI Ahmet Necmeddin Ph.D. in P.E., University of Gaziantep Supervisor: Asst.Prof.Dr. Zihni öztürk October 1996, 168 Pages The exact cause of sensitivity loss in TLD LiF:Mg,Ti has not been completely understood yet. If a used TLD is not annealed at an elevated temperature prior to reuse, residual deep defect centers still present in dosimeter. These centers will interact with new incoming radiations and produce further centers. This will introduce significant errors in low dose measurements. If the structure, thermal and optical characteristics of these defects were better understood, then a better model could be proposed to overcome sensitivity loss. Therefore, the samples were, firstly, irradiated by p-rays to produce sufficient concentration of defect centers, then optical absorption, phototransferred thermoluminescence, and thermoluminescence measurements were made to identify defect structures in LiF:Mg,Ti with appropriate heat treatments. The changes in the optical absorption band and mglow peak areas were determined as a function of various heat treatments using a computerised deconvolution program. The similarities in the variation of curves of glow peak 5 and OA band at 310 nm suggest that they may be belong to the same traps, namely Mg-trimer/Ti complex. The lack of correspondence between peak 2 and 380 nm band indicates that peak 2 is not related to Mg-dipoles as previously suggested, but related to Ti-related/F- center complex. Deconvolution analysis of OA and glow curves produced by annealing at and above 125 °C for long times before irradiation permits the identification of a new peak at 1 75 °C and OA band at 282 nm. Due to their similar thermal decaying properties, it is thought that they may be belong to the Z2-type centers. The effects of pre-irradiation annealing on the values of trapping parameters of LiF:Mg,Ti were determined as a function of durations at 125 °C by computerized glow curve analysis. The application of pre-annealing with different durations made significant changes on the activation energies of peaks 1 to 5, especially of peak 4. These variations are due to the interactions of defects with each other during heat treatments. A new simple model was developed to describe TL emission bands of luminescent centers in solids. TL emission spectra measurements of LiF:Mg.Ti showed one main emission band at 420 nm. The individual emission bands were separated using the developed model to investigate the success of the developed model. According to this model, the emission spectrum of whole glow peaks can be successfully fitted with two emission bands. Keywords: Alkali halide, LiF:Mg,Ti, thermoluminescence, optical absorbance, emission spectra, trapping parameters. IV