Measurement of total kinetic energy and fragment mass distribution in neutron induced fission of thorium-232 and uranium-233

Abstract

A significant amount of energy is released in nuclear fission and the majority of that energy is in the form of kinetic energy of the fission fragments. The total kinetic energy (TKE) released by neutron induced fission of actinides is typically on the order of 180 MeV and is well represented by a normal distribution with a typical full-width half-maximum of 25 MeV. The average TKE of fission events within a specific neutron energy range has been shown in past studies to have a dependence on incident neutron energy and although it has been well measured at thermal energy, it has not been well characterized at fast energies. In this work, properties of fission in Th-232 and U-233 were studied at the Los Alamos Neutron Science Center (LANSCE) at neutron energies from thermal to 40 MeV. Fission fragments were observed in coincidence using a twin ionization chamber with Frisch grids. Prompt neutron emission was determined based on novel applications of the General Description of Fission Observables (GEF) fission model. The average TKE released from fission and fragment mass distributions were calculated using the double energy (2E) analysis method based on conservation of mass and momentum. The results showed an overall decrease in average TKE with increasing incident neutron energy for both isotopes at a rate similar to other actinides. Also, this experiment confirmed a region below 5.5 MeV of increasing average TKE for Th-232 observed by past experiments. The fragment mass yield results revealed a bimodal distribution with an increasing prevalence of symmetric fission with increasing neutron energy for both isotopes, although the symmetric component was more pronounced for Th-232. These results were compared to past experiments at limited energy ranges and reasonable agreement was found. Fission events with symmetric mass splits were found to be associated with lower average TKE and the increasing prevalence of these events at higher energy is the primary driver of the overall decrease in average TKE. Accurate experimental measurements of these parameters are necessary to better understand the fission process in isotopes relevant to the thorium fuel cycle, in which Th-232 is used as a fertile material to generate the fissile isotope U-233. This process mirrors the uranium breeder process used to produce Pu-239 with several potential advantages including an inherent nuclear weapons proliferation resistance and reduced actinide production. For these reasons, there is increased interest in the thorium fuel cycle to meet future energy demands and improve safety and security while increasing profitability for the nuclear power industry.