Abstract:
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.