

Title of Thesis
Study of Gamma Ray Detector Efficiencies and Integral Parameters
Using Stochastic Simulations & Analytical Techniques. 
Author(s)
Shakeel ur Rehman 
Institute/University/Department
Details Department of Physics and Applied Mathematics /
Pakistan Institute of Engineering and Applied Sciences, Islamabad. 
Session 2010 
Subject Physics 
Number of Pages 163 
Keywords (Extracted from title, table of contents and
abstract of thesis) Predictions, Monte, Efficiency,
Variation, Simulations, Analytical, Computations, Axial, Range, Distance,
Gradual, Detector, Length, Ratio, Measurements, Gamma, parameters,
Stochastic 
Abstract In this work a novel
primaryinteraction 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
stateoftheart GEANT4 Monte Carlo simulation toolkit.The
methodology has been used for point isotropic sources and thin disk
sources at axial locations as well as offaxial 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 ƒray 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 efficiency for 137Cs as well as for 60Co sources
by increasing the axialoff 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 γray
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
γray energy.
Then Monte Carlo simulations were carried out for the determination
of photo peakto 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 peakto total ratio were derived for various
detector materials.

