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Fission product mass yields in uranium-233 and plutonium-239 from fast-neutron-induced fission

Montoya, Kristina Brown
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2023
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Precise knowledge of nuclear fission is a critical component for nuclear energy and stockpile stewardship research. Despite having been discovered over 80 years ago, fission is not fully described by a predictive theory, so thorough and accurate nuclear data libraries are necessary. These libraries provide information about the fission process in studied isotopes that is used in scientific and engineering models. Modern fission models are semi-empirical and so depend on measured quantities for example mass yield and total kinetic energy (TKE) of the fission fragments. Data acquired at high neutron energies like those measured here can be used to test and refine fission models that cannot be distinguished in their outputs at thermal energies. A neutron beam on the 90L flight path at the Weapons Neutron Research (WNR) facility of the Los Alamos Neutron Science Center (LANSCE) accelerator impinged on a thin-backed actinide target inside a dual-sided Frisch-gridded ionization chamber (FGIC). The masses and energies of the resulting fission fragments were extracted using the double energy (2E) method. Mass yields, TKEs, and ∆TKEs of 233U(n,f) and 239Pu(n,f) were determined at energies up to 100 MeV and compared to literature values at the few energies available. Calibration yields of 252Cf(sf) were measured in order to benchmark the experimental setup. An improved quality of the 233U target allowed for measurement of ∆TKE values lower than previous experiments, however, also lower than provided by presumably predictive models. The 239Pu target was of inferior quality, leading to reduced experimental resolution, however, due to increased statistics, the 239Pu TKE data was still able to refute speculation in the literature from a recent experiment which claimed existence of TKE structure associated with the onset of third-chance and higher chance fission. The measurement also for the first time provided fission fragment mass distributions above 20 MeV incident neutron energies. In both the 233U and 239Pu data the onset of 2nd chance fission was observed clearly in the TKE.
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