Abstract
Under a joint collaboration between TUNL-LANL-LLNL, a set of absolute fission product yield measurements has been performed. The energy dependence of a number of cumulative fission product yields (FPY) have been measured using quasi-monoenergetic neutron beams for three actinide targets, 235 U, 238 U and 239 Pu, between 0.5 and 14.8 MeV. The FPYs were measured by a combination of fission counting using specially designed dual-fission chambers and γ-ray counting. Each dual-fission chamber is a back-to-back ionization chamber encasing an activation target in the center with thin deposits of the same target isotope in each chamber. This method allows for the direct measurement of the total number of fissions in the activation target with no reference to the fission cross-section, thus reducing uncertainties. γ-ray counting of the activation target was performed on well-shielded HPGe detectors over a period of two months post irradiation to properly identify fission products. Reported are absolute cumulative fission product yields for incident neutron energies of 0.5, 1.37, 2.4, 3.6, 4.6, 5.5, 7.5, 8.9 and 14.8 MeV. Preliminary results from thermal irradiations at the MIT research reactor will also be presented and compared to present data and evaluations. This work was performed under the auspices of the U.S. Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and by Duke University and Triangle Universities Nuclear Laboratory through NNSA Stewardship Science Academic Alliance grant No. DE-FG52-09NA29465, DE-FG52-09NA29448 and Office of Nuclear Physics Grant No. DE-FG02-97ER41033.
Highlights
Precision measurements of fission observables such as fission product yields (FPY), kinetic energies, neutron emission and intrinsic angular momentum provides insights and constraints for theoretical modeling that attempts to understand this most collective of all nuclear phenomena
The energy dependence of the cumulative fission product yields of three actinide targets (235U, 238U and 239Pu) were studied in these measurements and to date have been measured at ten energies: thermal(preliminary), 0.56, 1.37, 2.37, 3.60, 4.56, 5.5, 7.5, 8.90 and 14.8 MeV
For each incident neutron energy reported on, neutron Time-Of-Flight measurements were performed by pulsing the incident proton/deuteron beams and the resulting neutron energy spectrum was measured with the BC-501A scintillator, mentioned above, positioned at 3 meters down stream of the target position
Summary
Precision measurements of fission observables such as fission product yields (FPY), kinetic energies, neutron emission and intrinsic angular momentum provides insights and constraints for theoretical modeling that attempts to understand this most collective of all nuclear phenomena. A joint collaboration between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL) and the Triangle Universities Nuclear Laboratory (TUNL) was formed. The goal of this collaboration was to measure the energy dependence of various high-yield fission products using monoenergetic neutrons in an accurate and self-consistent manner. This has been accomplished utilizing specially constructed dual-fission chambers and γ -spectroscopy. C The Authors, published by EDP Sciences.
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