Ph.D. Thesis Defense by Lisa G. Chiang
Monday, March 18, 2002

(Dr. C-K Chris Wang, advisor)

"Passive Coincidence Technique to Determine the Shape of Plutonium Objects Using Second and Higher Order Statistics"

Abstract

A measurement technique was developed to extract the shape of a plutonium object containing Pu-240 from the second order cross correlations measured using passive nondestructive examination and correlation techniques. Under certain circumstances, the plutonium object can be modeled as a linear collection of point sources. Because the measured cross correlation scales linearly with the addition of point sources, one can generate a linear system, RX=B where R is the response matrix, X is the shape vector, and B is the measured cross correlation. The response matrix contains either a measured or analytically generated cross correlation for every grid point in the region of interest in the interior volume formed by the detectors. Once the response matrix has been formed, the cross correlation is measured for a plutonium object. Then, using nonnegative least squares, the linear system can be solved for the shape vector.

A set of four detectors were configured in a regular tetrahedron and Cf-252 sources were used to simulate the Pu-240. First, the sensitivity of the cross correlation to position, solid angle, and detector efficiency were demonstrated using a single point source. Then, a distributed source, modeled either by several point sources or a moving point source was used to demonstrate the shape extraction process.