11/08/2024
By Hadrick Green
Candidate Name: Hadrick Green
Degree: Doctoral
Defense Date: Thursday, November 21, 2024
Time: 10 a.m. - Noon
Location: Online, Email Hadrick_Green@student.uml.edu for the link
Dissertation Title: Studies of Correlations in Prompt Fission Radiation
Advisor: Marian Jandel, Ph.D, Department of Physics, University of Massachusetts Lowell
Committee Members: Erno Sajo, Ph.D, Department of Physics, University of Massachusetts Lowell; Mark Tries, Ph.D, Department of Physics, University of Massachusetts Lowell
Brief Abstract:
Nuclear fission is the process in which the nucleus of an isotope splits into two or more smaller nuclei while releasing an average of 200 MeV per fission reaction. Fission results in hundreds of fission products, most of which are neutron rich nuclei. In addition to the multitude of fission products, the short timescale that fission occurs at complicates acquiring all details that describe the fission process; for example, how the excitation and angular momentum is shared between the two fission fragments. In this work we reanalyzed past experimental data on gamma-ray emission from neutron induced fission of 235U and spontaneous fission of 252Cf that were obtained at Los Alamos National Laboratory (LANL) in the Los Alamos Neutron Scattering Center (LANSCE) using the Detector for Advanced Neutron Capture Experiments (DANCE) array. The multiparameter experimental data on gamma-ray emission from DANCE are compared to the results of theoretical calculations. The calculations include detailed fission fragment de-excitation within the framework of the Hauser-Feshbach statistical model of compound nucleus reactions. The two codes used in these studies are Cascading Gamma-ray Multiplicity code for Fission (CGMF) (Los Alamos National Laboratory) and Fission Reaction Event Yield Algorithm (FREYA) (Lawrence Livermore National Laboratory). Both codes use Monte-Carlo sampling of the initial configurations of primary fission fragments followed by the emission of prompt radiation. The results of gamma-ray and neutron emission are then used with the radiation transport Monte-Carlo simulation in Geant4. The Geant4 model of the DANCE array was validated using the experimental results from the standard gamma-ray calibration sources to a high degree of accuracy. The simulations of CGMF and FREYA resulted in the following differences from experimental data for two fission reactions we studied. For 235U(n,f), average total gamma ray energy differed by 7-12% with CGMF and by 2-7% with FREYA; average gamma ray energy differed by 8-14% with CGMF and by 6-53% with FREYA. For 252Cf(sf), average total gamma ray energy differed by 3-49% with CGMF and by 4-50% with FREYA; average gamma ray energy differed by 16-44% with CGMF and by 8-43% with FREYA. Post Geant4 simulations, both codes were found to match experimental results but have deficiencies, specifically at higher energies and multiplicities. To narrow these differences, various configurations of the angular momentum distribution were exercised, and the distribution of total kinetic and total excitation energy was also altered, to no avail in CGMF. The time dependence of gamma-ray emission was also studied for twelve different time intervals and compared to experimental data sorted by identical time intervals. While the magnitudes of the trends between the two differed, the direction agreed when studying gamma-ray energy and multiplicity. Gating data by time and energy also enabled the identification of known fission products, displaying potential applications to nuclear defense and safety. Further studies, both experimental and theoretical, are needed to improve the description of γ-ray de-excitation from the fission fragments.