Real-time investigation of temporal and spatial correlations in fast neutron assay from spontaneous and stimulated fission

Abstract

A study of the use of digital techniques for the real-time, fast neutron coincidence analysis of time- and space-correlated radiations emitted by californium-252 and uranium-235 is described. These radiations have been measured with detectors based on the organic liquid scintillant, EJ-309. Time-synchronized neutron and gamma-ray event-trains, separated with pulse shape discrimination, have been sampled with a field-programmable gate array programmed with an algorithm developed in this research. This approach has been used to extract the interval time distribution of this event-train, with a time resolution of 5 ns, to investigate the temporal correlation between the neutrons and/or gamma rays emitted in the spontaneous fission of californium-252. The established model for the characterization of the interval-time distributions of correlated thermal neutron events, used widely in thermal neutron coincidence assay, has been extended to fast neutrons. The influence of geometry and the surroundings on these distributions has been investigated and quantified: the temporal coefficients for the die-away of the distributions for neutrons and gamma rays are 3.18\pm0.09 ns and 1.49\pm0.06 ns, respectively. It has been observed that 99.7% of the correlated neutrons and gamma rays are detected within 27 ns and 21 ns of each other, respectively, when a low-scatter geometry is examined. The spatial distribution of fast neutrons emitted in spontaneous fission (californium-252) has also been investigated to yield the evidence for the angular distribution of higher-order, correlated neutrons presented in this thesis; this infers a dipolar trend for third (triplet) and fourth (quadruplet) neutrons consistent with that known for second (doublet) neutrons. The gamma-ray emission has been used to provide time-of-flight information and hence the neutron spectrum for fission neutrons from californium-252. A technique for the determination of the foreground and background coincidence distribution of the emitted fast neutrons and/or gamma rays for passive and active neutron coincidence counting methods has been developed. Finally, two models have been developed to correct for erroneous coincidence events which might otherwise limit the use of organic scintillators in coincident assay: one for photon breakthrough and one for detector crosstalk. These models have been validated using californium-252 indicating that photon-breakthrough constitutes a 20% increase in the neutron count rates whilst crosstalk can result in increases of 10% and 35% on first-and second-order coincident events, respectively, for the investigated geometries. The instrumentation, techniques and results reported in this thesis extend our understanding of the fundamental temporal characteristics of nuclear fission, and are of direct relevance to the application of organic scintillators with pulse shape discrimination to nuclear safeguards and non-proliferation verification.