Title: The determination of sensitizing centers and band gap of the photoconductive materials
by YAZICI A.Necmeddin
Supervisor: Doç.Dr.Ata SELÇUK
Title: Thermal and Optical Characteristics of Radiation Induced Defect Centers in Alkali Halide Crystal (TLD-100)
by YAZICI A.Necmeddin
Supervisor: Assist.Prof.Dr.Zihni ÖZTÜRK
October 1996, 168 pages.
 In this study, Selenium(Se) photoconductor was developed by mixing the power Se with the powder of Chlorine (Cl) under the conditions of high vacuum and high temperature. The production of ZnS photoconductor was also acoomplished from the mixture of ZnS and ZnCl powders under the same conditions. It was found that the developed Se and ZnS photoconductors were very sensitive to the light. The density of sensitizing centers in these photoconductors were determined by using Capacitance-Voltage (C-V) and Space-Charge-Limited methods. the energy value of band gap of Se and ZnS and the energy levels of sensitizing centers in the forbidden gap of Se and ZnS were evaluated by variation of photoconductivity excitation with wavelength method. 
    The density of sensitizing centers was found to chanege linearity with the light intensity; the energy value of band gap of Se and ZnS photoconductors were found to be in good agreement with theoretical values. 

KEYWORDS:    recombination Centers II, Sensitizing centers, Photoconductivity, Se, ZnS. 

  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 beta 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 glow peak areas were determined as a function of various heat treatments using a computerized 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 0C for long times before irradiation permits the identification of a new peak at 175 0C and OA band at 282 nm. Due to their similar  thermal decaying properties, it is thought that they may be belong to 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 0C 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 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 absobance, emission spectra, trapping parameters.