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Background: The sharing of excitation energy between the fission fragments is one of the key issues in studying nuclear fission. One way to address this is by studying prompt-fission neutron multiplicities as a function of other fission observables such as the mass, (nu) over bar (A). These are vital benchmark data for both fission and nuclear deexcitation models, putting constrains on the fragment excitation energy and hence on the competing prompt neutron/gamma-ray emission. Despite numerous detailed studies, recent measurements done at JRC-Geel with the SCINTIA array in the epithermal region show surprisingly strong discrepancies to earlier thermal fission data and the Wahl systematics. Purpose: The purpose was to perform measurements of the prompt-fission neutron multiplicity, as a function of fragment mass and total kinetic energy (TKE), in U-235(n(th), f) and Cf-252(sf), to verify and extend the SCINTIA results. Another goal was to validate the analysis methods, and prepare for planned investigations at excitation energies up to 5.5 MeV. Methods: The experiments were conducted at the former 7 MV Van de Graaff facility in JRC-Geel, using a Twin Frisch-Grid Ionization Chamber and two liquid scintillation detectors. A neutron beam with an average energy of 0.5 MeV was produced via the Li-7(p,n) reaction. The neutrons were thermalized by a 12 cm thick block of paraffin. Digital data acquisition systems were utilized. Comprehensive simulations were performed to verify the methodology and to investigate the role of the mass and energy resolution on measured (nu) over bar (A) and (nu) over bar (TKE) values. The simulation results also revealed that the partial derivative(nu) over bar/partial derivative A and partial derivative(TKE) over bar/partial derivative(nu) over bar are affected by the mass and energy resolution. However, the effect is small for the estimated resolutions of this work. Detailed Fluka simulations were performed to calculate the fraction of therma, QC 20210122

In this paper we present first results for prompt fission γ-ray spectra (PFGS) characteristics from the spontaneous fission (sf) of Pu240 and Pu242. For Pu242(sf) we obtained, after proper unfolding of the detector response, an average energy per photon ϵγ=(0.843±0.012) MeV, an average multiplicity Mγ=(6.72±0.07), and an average total γ-ray energy release per fission Ēγ,tot = (5.66 ± 0.06) MeV. The Pu240(sf) emission spectrum was obtained by applying a so-called detector-response transformation function determined from the Pu242 spectrum measured in exactly the same geometry. The results are an average energy per photon ϵγ=(0.80±0.07) MeV, the average multiplicity Mγ = (8.2 ± 0.4), and an average total γ-ray energy release per fission Ēγ,tot = (6.6 ± 0.5) MeV. The PFGS characteristics for Pu242(sf) are in very good agreement with those from thermal-neutron-induced fission on Pu241 and scales well with the corresponding prompt neutron multiplicity. Our results in the case of Pu240(sf), although drawn from a limited number of events, show a significantly enhanced average multiplicity and average total energy, but may be understood from a different fragment yield distribution in Pu240(sf) compared to that of Pu242(sf)., QC 20210128

Pu240 has recently been pointed out by a sensitivity study of the Organization for Economic Cooperation and Development (OECD) Nuclear Energy Agency (NEA) to be one of the isotopes whose fission cross section lacks accuracy to meet the upcoming needs for the future generation of nuclear power plants (GEN-IV). In the High Priority Request List (HPRL) of the OECD, it is suggested that the knowledge of the Pu240(n,f) cross section should be improved to an accuracy within 1-3 %, compared to the present 5%. A measurement of the Pu240 cross section has been performed at the Van de Graaff accelerator of the Joint Research Center (JRC) Institute for Reference Materials and Measurements (IRMM) using quasi-monoenergetic neutrons in the energy range from 0.5 MeV to 3 MeV. A twin Frisch-grid ionization chamber (TFGIC) has been used in a back-to-back configuration as fission fragment detector. The Pu240(n,f) cross section has been normalized to three different isotopes: Np237(n,f), 235U(n,f), and 238U(n,f). Additionally, the secondary standard reactions were benchmarked through measurements against the primary standard reaction 235U(n,f) in the same geometry. A comprehensive study of the corrections applied to the data and the associated uncertainties is given. The results obtained are in agreement with previous experimental data at the threshold region. For neutron energies higher than 1 MeV, the results of this experiment are slightly lower than the ENDF/B-VII.1 evaluation, but in agreement with the experiments of Laptev et al. (2004) as well as Staples and Morley (1998)., QC 20210201