(Dr. Chris Wang, advisor)

"Design of Gamma-Ray Collimators for Nondestructive Assay Applications"

Abstract

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.