Average energy of 237Np and 241Am prompt fission neutron spectra

Abstract

The use of 237 Np in nuclear reactors or electronuclear facilities requires substantially more accurate neutron data for this nucleus, especially, more accurate spectra and average energy of the prompt fission neutrons. Experimental spectra of prompt fission neutrons for the reaction 237 Np(n, ƒ) are available only for a limited set of incident-neutron energies [1-4] (Fig. 1). The theoretical methods used for analyzing and predicting such spectra [5, 6] cannot present a consistent and detailed estimate of the dependence of the prompt-neutron spectra on incident-neutron energy in the range E n = 0.5-20 MeV. The theoretical approach tested for 232 Th(n, F), 235 U(n, F), and 238 U(n, F) [7-9] holds great promise for neutron energies E n below and above the threshold of emissive fission, where together with the first-chance fission reaction (n, ƒ) the emissive fission reaction (n, xnƒ) with pre-fission emission of x (≥ 1) neutrons can also occur. Here (n, F) denotes the sum of the reactions (n, ƒ) and (n, xnƒ). Thus the spectra of prompt fission neutrons from the reaction 239 Pu(n, F) have been predicted [10], and detailed experimental investigations of these spectra are being planned in the US and France [11].In the energy range where emissive fission is excited, pre-and post-fission neutrons contribute to the observed spectra of prompt fission neutrons. The decrease of the average energy up the spectrum of prompt fission neutrons near the threshold of the second-chance reaction 237 Np(n, nƒ) and the third-chance reaction 237 Np(n, 2nƒ), as investigated in detail experimentally for 235 U and 238 U targets, depends strongly on the relative contribution of the first-chance fission reaction 237 Np(n, ƒ) and subsequent-chance fission reaction 237 Np(n, xnƒ) to the observed cross-section of the reaction 237 Np(n, F).Post-fission prompted neutrons are emitted by primary fission fragments. Analysis of the prompt neutron spectra of the reactions 232 Th(n, F), 235 U(n, F), and 238 U(n, F) showed that many of their features are correlated with the influence of pre-fission neutrons (n, xnƒ). For example, for E n > E n,2nƒ , where E n,2nƒ is the threshold of the reaction (n, 2nƒ); the stepped features of the prompt-neutron spectra of the reactions 238 U(n, F) and 232 Th(n, F) at energy ε~ 3-5 MeV are due to the first neutron reaction (n, 2nƒ) [7,8]. The features of the shape of the spectra at average energy 〈E〉 are closely associated with the chance structure (n, nxƒ) of the observed fission cross-sections. The fissibility of the initial compound nucleus 238 Np is much higher for the nuclei 239 U and 236 U than for the target nuclei 238 U and 235 U, respectively, so that the variations of the average energy are certainly smaller but still quite large, as one can see from the shape of the prompt neutron spectrum for E n = 7.8 MeV [2] near the threshold of the reaction 237 Np(n, nƒ).The ENDF/V VII.0 [12], , and BROND [14] evaluated neutron-nuclear data files for 237 Np ignore the effect of pre-fission neutrons (n, xnƒ) on the ave...

Current Inconsistencies in 238Pu, 241,243Am and 242Cm Evaluations and their Impact on Uncertainties

“…It is important to include evaluation of fission cross sections and prompt fission neutron spectra, as it was accomplished recently for 237 Np(n, F) and 241 Am(n,F) [37], using the approaches developed for 232 Th and 235,238 U (see [1,7,38] and references therein). These approaches were later supported by the measurement of prompt fission neutron spectra for 237 Np(n,F) by Taieb et al [39].…”