Correlated Prompt Fission Data in Transport Simulations

Detailed information on the fission process can be inferred from the observation, modeling and theoretical understanding of prompt fission neutron and $\gamma$-ray~observables. Beyond simple average quantities, the study of distributions and correlations in prompt data, e.g., multiplicity-dependent neutron and \gray~spectra, angular distributions of the emitted particles, $n$-$n$, $n$-$\gamma$, and $\gamma$-$\gamma$~correlations, can place stringent constraints on fission models and parameters that would otherwise be free to be tuned separately to represent individual fission observables. The FREYA~and CGMF~codes have been developed to follow the sequential emissions of prompt neutrons and $\gamma$-rays~from the initial excited fission fragments produced right after scission. Both codes implement Monte Carlo techniques to sample initial fission fragment configurations in mass, charge and kinetic energy and sample probabilities of neutron and $\gamma$~emission at each stage of the decay. This approach naturally leads to using simple but powerful statistical techniques to infer distributions and correlations among many observables and model parameters. The comparison of model calculations with experimental data provides a rich arena for testing various nuclear physics models such as those related to the nuclear structure and level densities of neutron-rich nuclei, the $\gamma$-ray~strength functions of dipole and quadrupole transitions, the mechanism for dividing the excitation energy between the two nascent fragments near scission, and the mechanisms behind the production of angular momentum in the fragments, etc. Beyond the obvious interest from a fundamental physics point of view, such studies are also important for addressing data needs in various nuclear applications. (See text for full abstract.)Comment: 39 pages, 57 figure files, published in Eur. Phys. J. A, reference added this versio

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

Subcritical Neutron Multiplication Inference Measurements for Nuclear Data and Computational Methods Validation

Subcritical measurements have been continually performed since the 1940s, and simulation capabilities were developed alongside the measurements for comparison purposes. The accuracy of predictive radiation transport simulations is limited by the accuracy of the Monte Carlo simulation codes and underlying nuclear data. A subcritical benchmark measurement is a high-quality subcritical measurement in which all physical parameters and uncertainties are well characterized to a high degree of accuracy, and which is peer reviewed and compiled with other benchmark experiments into a database such as the International Criticality Safety Benchmark Evaluation Project (ICSBEP). Benchmark measurements are therefore trusted to provide accurate comparisons between experimental and simulated data, for nuclear data and radiation transport code validation purposes. Critical benchmarks are plentiful, but are not sensitive to correlated neutron parameters in the way that the handful of existing subcritical benchmarks are. This work demonstrates how we can apply subcritical neutron multiplication measurements and simulations to better validate relevant nuclear data and radiation transport computational methods currently used for nuclear nonproliferation and safety applications. The work encompasses the entire process of an advanced subcritical measurement, from the earliest planning stages to the final analysis and comparison to simulated results. Both the Critical and Subcritical 0-Power Experiment at Rensselaer (CaSPER) measurement, a novel advanced subcritical measurement, and the SCRaP measurement, a state-of-the-art subcritical benchmark measurement, campaigns have been completed. Simulations of LANL ICSBEP benchmark-quality reflected plutonium (BeRP) ball subcritical measurements have been conducted using various radiation transport codes that take into account the correlated physics of fission neutrons. Comparisons of both the results and the underlying neutron multiplicity models applied by the codes have been investigated, as well as new methods of applying comparisons of these subcritical neutron multiplication inference measurements and the associated simulations to nuclear data and computational methods validation. Optimization algorithm frameworks have been applied to both nuclear data evaluation based on subcritical neutron multiplication inference benchmarks, and the design of subcritical neutron multiplication inference benchmarks.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149790/1/jennannn_1.pd

Event-by-Event Simulation of Induced Fission

We are developing a novel code that treats induced fission by statistical (or Monte-Carlo) simulation of individual decay chains. After its initial excitation, the fissionable compound nucleus may either deexcite by evaporation or undergo binary fission into a large number of fission channels each with different energetics involving both energy dissipation and deformed scission prefragments. After separation and Coulomb acceleration, each fission fragment undergoes a succession of individual (neutron) evaporations, leading to two bound but still excited fission products (that may further decay electromagnetically and, ultimately, weakly), as well as typically several neutrons. (The inclusion of other possible ejectiles is planned.) This kind of approach makes it possible to study more detailed observables than could be addressed with previous treatments which have tended to focus on average quantities. In particular, any type of correlation observable can readily be extracted from a generated set of events. With a view towards making the code practically useful in a variety of applications, emphasis is being put on making it numerically efficient so that large event samples can be generated quickly. In its present form, the code can generate one million full events in about 12 seconds on a MacBook laptop computer. The development of this qualitatively new tool is still at an early stage and quantitative reproduction of existing data should not be expected until a number of detailed refinement have been implemented

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