Ph.D. Thesis Defense by Richard B. Oberer
Tuesday, October 22, 2002

(Dr. Chris Wang, advisor)

"Fission Multiplicity Detection with Temporal Gamma-Neutron Discrimination from Higher-Order Time Correlation Statistics"


Recently, there has been a renewed interest in using plastic scintillating detectors for the nondestructive assay of nuclear materials due to the higher count rates possible and the lower accidental coincidence rate. The major problem with the use of fast plastic detectors, however, is that both neutrons and gamma rays are detected. The pulses from the two are indistinguishable.

For this thesis, a new technique was developed to use higher-order time correlation statistics to distinguish combinations of neutron and gamma- ray detections in fast plastic detectors. A system of analysis to describe these correlations was developed based on simple physical principles. Other sources of correlations from non-fission events are identified and integrated into the analysis developed for fission events. A number of ratios and metrics are identified to determine physical properties of the source from the correlations. To account for detector dead-time, an alternative analytical technique was also be developed.

As a demonstration of this technique, an experiment based on a californium-252 source and three plastic scintillating detectors was conducted. The Nuclear Materials Identification System (NMIS), a system developed at ORNL, was used to analyze the time correlations for the experiment.