Fission fragment mass and energy distributions for neutron induced fission of 235U, 238U AND 239Pu measured with the NIFFTE time projection chamber

Abstract

Nuclear fission is a process vital to energy and defense applications. These applications rely heavily on predictive models which are informed by nuclear data. In order to improve the capabilities of fission models, new fission data is required with priorities including total kinetic energy (TKE) measurements and fission product yields (FPY). The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration developed a fission time projection chamber (fissionTPC) to measure fission cross section data with unprecedented precision. In this work the NIFFTE setup was used to determine fission fragment energies from neutron induced fission of 235U, 238U and 239Pu from in-beam data collected at the Los Alamos Neutron Science Center, which represents the first analysis of this kind with a TPC. Using the double energy (2E) analysis method on this data, fission fragment mass distributions were calculated for 235U and 238U and the average TKE (TKE) was determined for 235U, 238U and 239Pu at incident neutron energies from 0.1 to 100 MeV.Combining the fissionTPC data with available predictive fission codes, the 2E analysis procedure is used to calculate pre- and post-prompt neutron emission fission fragment masses and energies. This analysis expands upon previous 2E studies by accounting for fragment energy loss through the target and backing material as a function of the fragment’s mass as well as by taking into account the recently measured partial linear momentum transfer from neutrons to the target at high neutron energies. This is the first determination of pre-neutron emission TKE for 239Pu above an incident neutron energy of 5.5 MeV. For all three isotopes studied, the TKE shows an overall decrease with increasing incident neutron energy with some structure in TKE and σTKE at multi-chance fission energy thresholds, confirming the observations of previous experiments. Fission fragment mass distributions were determined with a mass resolution of ≈ 7 amu, limited by the energy resolution of the fissionTPC.