Thermoluminescence (TL), kinetic parameters and dosimetric features of Pakistani limestone.

Thermoluminescence (TL), kinetic parameters and dosimetric features of Pakistani limestone (CaCO3) is reported in this study. Both compositional and structural analyses reveal that the material has a crystalline nature with rhombohedral structure and non-uniform crystallite size having major content of CaCO3. A powdered limestone sample of 30 mg is found to be the optimized weight for TL and other dosimetric studies. After irradiating the samples with a test dose of 100 Gy using a ß source three composite glow peaks termed as P1, P2 and P3 are visible at 100, 230 and 330 °C respectively using a linear heating rate of 1 °C/s during the TL readout. The Coefficient of Variation (COV) is found to be about 4%. Kinetic parameters (i.e., frequency factor (f), activation energy (E), and the kinetic order (b)) are estimated using both first and second Order of kinetics using an in-house Computerized Glow Curve Deconvolution (GCD) software. The figure-of-merit (FOM) is found to be 2.12%. The distribution of continuum traps with activation energy in the range of 0.77-2.59 eV is observed in the kinetic parameter analysis of the glow peaks of the sample. The TL response in the dose range of 1-5 Gy (not reported previously) and linearity in the dose response in the dose range of 1-10 Gy is observed in samples of Pakistani limestone. The Minimum Detectable Dose (MDD) is 1.01 Gy clearly resembling the experimentally linear fitted results. After a fading study for a period of thirty days, only the first peak i.e., P1 majorly fades while no major change is observed in the amplitude of peaks P2 and P3. In addition, P1 is the main contributor fading by 92% within the first 24 h of irradiation while P2 fades by 30 %. However, P3 shows stability with a very minor fading of 0.05% within 24 h of irradiation. This study concludes that Pakistani limestone can be further assessed as a potential radiation dosimeter for various applications.

Computerized glow curve deconvolution (CGCD): A comparison using asymptotic vs rational approximation in thermoluminescence kinetic models.

In this study an open-source tool GCD Analyzer, based on Microsoft® Excel, for Computerized Glow Curve Deconvolution analysis (CGCD) of thermoluminescence (TL) glow peak has been developed using a more accurate rational approximation. It is capable of deconvolution of glow curves having discrete or continuous trap distribution and can be used for routine as well as emergency radiation dosimetric analysis. This tool has a unique feature of TL glow curve analysis by combining discrete and continuous energy distributions for crystalline, amorphous, and mixed materials. To obtain best values of trap parameters like activation energy (E), frequency factor (s), order of kinetics (b), GCD Analyzer has the capability of analyzing glow curve using selectable individual peak of various physical kinetic models i.e. FOK, SOK, GOK, MOK, and Continuous Traps Distribution (CTD) with subtraction of optional background signal. The residual graph gives a clear visual understanding of the Figure of Merit (F.O.M). A comparison of asymptotic and rational approximation to the built-in second order exponential integral function E2(E/KT) for E/KT < 600 is also presented. The results are verified by deconvolution of test synthetic glow curves with F.O.M up to 0.0005%, experimental glow curve for CTD with F.O.M of 0.9781%, and in the case of GLOWCANIN project glow curves, the F.O.M is comparable to the least values achieved by inter-comparison participants.

Thermoluminescence study of pellets prepared using NaCl from Khewra Salt Mines in Pakistan.

In this study, the thermoluminescence characteristics of naturally occurring salt (NaCl) were assessed for the development of a radiation dosimeter. For this purpose, mined crystalline samples of salt were procured directly from Khewra salt mines in Pakistan. The samples were hand crushed, sieved, and compressed to pellets comparable in size to standard TLD chips, and irradiated to gamma radiation doses in the range of 5 mGy and 5000 mGy. Thermoluminescence (TL) response showed three main peaks in the glow curve around 115-130 °C, 150-170 °C, and 220-240 °C. A linear TL response was observed for the dose range of 5-100 mGy. The TL response became supra-linear for the dose ranges of 100-1000 mGy and 1000-5000 mGy. The Tm-Tstop method was applied to identify the overlapping peaks of the glow curve. Computerized glow curve deconvolution (CGCD) was then employed for the characterization of electron trap parameters such as frequency factor (s), activation energy (E), and the kinetic order (b), using General Order (GO) kinetics. The figure-of-merit (FOM) was found to be 1.08%, 0.94%, 0.77%, and 0.75%, at 500 mGy, 1 Gy, 2 Gy, and 5 Gy, respectively. The TL intensity faded by 20% within the first 24 h after irradiation and finally stabilized after two weeks. In addition, structural, morphological, and elemental analyses, were also performed using various analytical techniques. X-ray diffraction (XRD) showed that the salt crystallizes in a face-centered cubic structure. Scanning electron microscope (SEM) micrographs indicated that the crystallites are closely packed and cubic-shaped with non-uniform size, and mostly found in the agglomerated form. Similarly, the elemental analysis confirmed the presence of impurities such as Mg, Sr, S, K, O, and Ca, in the samples. The present study concludes that the pellets made from salt samples from Khewra mines have a potential for use as radiation dosimeters.

Study of CdTe detection efficiency for medical applications using Geant4-based stochastic simulations.

The determination of detection efficiency and peak-to-total ratios has been performed for rectangular CdTe detectors for various x-ray and low-energy γ-ray source configurations including parallel beams, point, and cylindrical sources. The dependence of efficiency values on axial and off-axial distances, detector thickness and area, and source dimensions has been studied. The detector model developed in this work has been validated by comparing the Monte Carlo simulated values of detector efficiency for a parallel incident beam with the available published data and good agreement has been found with discrepancies remaining within 2% throughout the energy range. Geant4 simulations show nearly 100% photopeak and total efficiency with peak-to-total ratios approaching a maximum value of 1.0 for photons in the 4-70 keV energy range. Similar high values of detection efficiency have been obtained for brachytherapy I-125 seed sources having cylindrical geometries which indicates the suitability of CdTe detectors for the calibration of sources used in therapy. The logistic power curve was found excellent for empirically fitting the photopeak efficiency variations with axial displacement of the I-125 brachy source in the horizontal configuration. Geant4 simulations clearly show that small thicknesses, of the order of 0.5 mm, of CdTe material are sufficient for attaining almost 100% detection efficiency for low-energy photons having energies up to 100 keV.