Study of Gamma Ray Detector Efficiencies and Integral Parameters Using Stochastic Simulations and Analytical Techniques

Shakeel ur Rehman, PhD, Department of Physics & Applied Mathematics, PIEAS, July 2010. “Study of gamma-ray detector efficiencies and integral parameters using stochastic simulations and analytical techniques”; Supervisor: Dr. Nasir M. Mirza; Co-Supervisor: Dr. Sikander M. Mirza; Department of Physics & Applied Mathematics, PIEAS, Nilore 45650, Islamabad. In this work a novel primary-interaction based methodology has been developed for Monte Carlo based calculations of the detector efficiency which accelerate the computations and yields converged results with much shorter computer time. The proposed methodology has been first implemented in a MATLAB based computer program PIMC and later implemented in the state-of-the-art GEANT4 Monte Carlo simulation toolkit. The methodology has been used for point isotropic sources and thin disk sources at axial locations as well as off-axial locations with respect to the detector axis, excellent agreement has been found between the predictions of the PIMC code with the corresponding results obtained by using hybrid Monte Carlo as well as by analytical techniques and experimental measurements over a wide range of γ-ray energy values. Using the PIMC program, the energy dependent detector efficiency has been found to approach an asymptotic profile by increasing either thickness or diameter of scintillator while keeping the other fixed. The variation of energy dependent total efficiency of a 3" × 3" NaI(Tl) scintillator with axial distance has been studied using the PIMC code. About two orders of magnitude change in detector efficiency has been observed for zero to 50 cm variation in the axial distance. For small values of axial separation, a similar large variation has also been observed in total xviiefficiency for 137 Cs as well as for 60 Co sources by increasing the axial-off set from zero to 50 cm. The results for disk sources show a gradual decreasing trend in the total efficiency as disk source radius increases. For coaxial thin disk sources, the total detection efficiency has been found to approach a corresponding maximum value as length is increased in 1–20 cm range for various values of radii of disk sources and γ-ray energy while keeping the diameter of the detector fixed at 3” (7.62 cm). A similar behavior is observed when the radius of the detector is increased from 1–20 cm while keeping the length of detector fixed at 3” (7.62 cm), for various values of radii of disk sources and γ-ray energy. Then Monte Carlo simulations were carried out for the determination of photo peak- to-total ratio using the GEANT4 based detector model for various gamma ray energies and detector materials. The effect of variation of length and radius of the detector, while keeping the other fixed, was studied for various gamma ray energies. Mathematical curves for peak- to-total ratio were derived for various detector materials.