(Dr. Chris Wang, advisor)
"Design of Gamma-Ray Collimators for Nondestructive Assay Applications"
Nondestructive measurement of the radioisotope content of process material in a nuclear facility generally requires the use of a portable, collimated gamma-ray detector. The collimator is usually made of either lead or tungsten and is rigidly fixed with respect to the detector to form a detector-collimator assembly. This assembly can then be calibrated for the various measurement geometries for which it will be used. The suitability of a detector-collimator assembly to a particular application is determined by the collimator dimensions and its position relative to the detector, but there is no readily available information that defines a design process to optimize a collimator for a particular application. Collimators have generally been designed from experience, or by trial and error methods.
Design parameters are defined for a cylindrical detector-collimator
assembly, and the considerations involved in selection of values for these
parameters are discussed. A procedure for design of an optimized
collimator is provided. For example, design parameters Rd and Rf
determine the features of the detector-collimator off-axis response function.
This response function is the relative response of the detector as a function
of the position of a point source on a line normal to the detector-collimator
axis, and it indicates the degree of directionality of the detector-collimator
assembly. Detector response functions for various collimator designs
were modeled by Monte Carlo N-Particle and by ray analysis. This
involved computing the theoretical detector-collimator response functions
for various detector-collimator assemblies and comparing the results with
experimental data to validate the computer-generated response functions
and design parameters.