Measured and Simulated Prompt Fission Neutron and Photon Correlations

An accurate understanding of fission is critical to characterization of special nuclear material (SNM) for nonproliferation and safeguards applications. Noninvasive and nondestructive techniques rely primarily on highly penetrating and relatively abundant fission emissions. Spontaneously and under particle interrogation, SNM emits neutrons and photons from fission, which are characteristic of the fissioning isotopes. Characteristic neutrons and photons are emitted from nuclear fission when a deformed, neutron-rich nucleus divides into two fragments that then de-excite. During de-excitation, neutrons are emitted first, followed by photons; this process gives rise to correlations. New, event-by-event, physics-based models, CGMF (Los Alamos National Laboratory) and FREYA (Lawrence Livermore National Laboratory), predict correlations in prompt fission emissions. Current safeguards and nonproliferation systems do not utilize angular or multiplicity correlations. Little data exist to validate these models; correlated quantities have been measured only for 252Cf(sf). My work provides measured correlation data to validate models useful for future system design. Previous correlation measurements have been limited by the acquisition challenges of a many-detector array and therefore have used simple detector systems. Additionally, few detection methods exist that are simultaneously efficient to neutrons and photons. In this work, I show a many-detector array of pulse-shape-discrimination-capable organic scintillators, sensitive to both fast neutrons and photons, to measure correlations in neutron energy, photon energy, multiplicity, and emission angle. This work is achieved through MCNPX-PoliMi simulations and through use of time-synchronized, high-throughput, multiple-digitizer acquisition systems. I performed experiments sensitive to correlations with a large array of organic scintillators. I performed measurements of 252Cf(sf) at both the University of Michigan and the Los Alamos National Laboratory; and of 240Pu(sf) at the Joint Research Centre in Ispra, Italy, and at the Los Alamos National Laboratory. I measured the 240Pu(sf) neutron-neutron angular distribution and found it to be less anisotropic than the 252Cf(sf) neutrons. 240Pu(sf) and 252Cf(sf) neutron-neutron angular distribution simulation results indicate that fission models capture the general trend of neutron anisotropy. 240Pu(sf) and 252Cf(sf) experimental multiplicity results suggest weak neutron-photon competition during fragment de-excitation. The measured correlations were compared with MCNPX-PoliMi simulations using the built-in model and two new event-by-event fission models, CGMF and FREYA, which predict correlations in prompt emissions from fission. Simulation results from CGMF and FREYA predict a stronger negative correlation than the experiment result.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147560/1/mmarcath_1.pd

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

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

Fission Reaction Event Yield Algorithm FREYA 2.0.2

FREYA (Fission Reaction Event Yield Algorithm) is a fission event generator which models complete fission events. As such, it automatically includes fluctuations as well as correlations between observables, resulting from conservation of energy and momentum. The purpose of this paper is to present the main differences between FREYA versions 1.0 and 2.0.2 : additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction. FREYA 2.0.2 has been integrated into the LLNL Fission Library 2.0.2, which has itself been integrated into MCNP6.2, TRIPOLI-4.10, and can be called from Geant4.10. The previous version of this program (AEVS_v1_0) may be found at http://dx.doi.org/10.1016/j.cpc.2015.02.002