Scission neutron emission and prompt fission neutron spectrum

The mass, energy and angular integrated spectra of prompt fission neutrons for sup 2 sup 3 sup 5 U induced fission in the energy range from thermal to 5 MeV were analyzed. It allows assume that about 0.362+-0.025 neutrons per fission are emitted due to another mechanism then neutron emission from excited fragments after full acceleration. The spectrum of scission neutrons consists of two components with average energy 0.98 MeV and 2.74 MeV. The share of scission neutrons and their spectrum shape estimated in this work does not contradict to results of differential experiments analyzed in previous papers

New Evidence Intense Scission Neutron Source in the 252-Cf Spontaneous Fission.

Abstract not availableJRC.D-Institute for Reference Materials and Measurements (Geel

Prompt fission neutron spectra of

Model calculations were performed to interpret prompt fission neutron spectra (PFNS) of the 238 U(n, f) reaction for incident neutron energies $E_{\rm n} = 6$-18 MeV. Pre-fission (pre-saddle) ${}^{238}U(n, {\it x} nf)$ reaction neutron spectra were calculated with Hauser-Feshbach statistical model, 238 U fission and $(n, {\it x}n)$ reaction cross-section data being described consistently. The increase of the cut-off energy of (n, nf) reaction neutron spectra with excitation energy of fissioning nucleus is described. For $E_{\rm n} = 6$-9 MeV the low-energy PFNS component, which is due to the contribution of pre-fission (n, nf) neutrons, is compatible with measured data. Average energy of prefission (n, nf) neutrons is shown to be rather dependent on $E_{\rm n}$. For $E_{\rm n} = 13$-18 MeV, a decrease of measured PFNS average neutron energies is interpreted. Spectra of neutrons, evaporated from fission fragments, were approximated as a sum of two Watt distributions. The reduced fission fragment velocity is assumed for the neutron emission during fragment acceleration. Several interpretations of observed soft neutron excess are investigated, i.e., possible uncertainties of emissive fission contributions and additional neutron source. We claim the soft neutron excess cannot be attributed to the ${}^{238}U(n, {\it x} nf)$ pre-saddle neutrons contribution

Neutron data evaluation of sup 2 sup 3 sup 4 U

Consistent evaluation of sup 2 sup 3 sup 4 U measured data base is performed. Hauser-Feshbach-Moldauer theory, coupled channel model and double-humped fission barrier model are employed. Total, differential scattering, fission and (n,xn) data are calculated, using fission cross section data description as a major constraint. The direct excitation of ground state and higher band levels is calculated within rigid rotator and soft (deformable) rotator model, respectively. Average resonance parameters are provided, which reproduce evaluated cross sections in the range of 1.5 - 140 keV. This work is performed under the Project Agreement B-404 with the International Science and Technology Center (Moscow). The Financing Party for the Project is Japan

235U(N,F) Prompt Fission Neutron Spectra

A new prompt fission neutron spectrum matrix for the n+235U system is proposed. The thermal neutron spectrum is this matrix describes the newest data of JRC IRMM by Hambsch et al. (2009). The longstanding problem of inconsistency of integral thermal data testing and differential prompt fission neutron spectra data (PFNS) seems to be solved. It was mostly due to rather poor fits of differential PFNS data in major data libraries. The older measured database is updated here using modern standards like spontaneous fission neutron spectra of 252Cf(sf). That largely removes the inconsistency of older thermal neutron-induced PFNS data with the newest data. A phenomenological approach, developed by Kornilov et al. (1999), for the first-chance fission and extended for the emissive fission domain by Maslov et al. (2005) is normalized at Eth to predict both the PFNS average energy E and PFNS shape up to 20 MeV. In the first-chance and emissive fission domain evaluated PFNS are consistent with the E data by Ethvignot et al. (2005). A compiled ENDF/B-formatted file of the 235U(n,F) PFNS largely removes the inconsistencies of the evaluated differential PFNS with integral data benchmarks. Fast integral critical experiments like GODIVA or Flattop benchmarks are reproduced with the same accuracy as with the PFNS of the major data libraries. That reveals a rather delicate compensation effect, since present and previous PFNS shapes are drastically different from each other. Thermal assembly benchmarking reveals positive biases in keff, which might be attributed to the influence of a soft energy tail of the present PFNS. For some of Valduc¿s LCT benchmarks biases in keff are less than 20 pcm. Key Words: prompt fission neutron spectra, uranium-235, thermal and fast benchmarksJRC.DG.D.5-Nuclear physic

235U(n, F), 233U(n, F) and 239Pu(n, F) Prompt Fission Neutron Spectra

New prompt fission neutron spectra (PFNS) matrices for the n+235U, n+233U and n+239Pu systems are prepared. Present and previous PFNS shapes are drastically different from each other, the former being consistent with measured data base. A combined generalized least-squares fit of nth+235U, nth+233U, nth+239Pu and 252Cf(sf) measured data base is accomplished. The solution of the longstanding problem of inconsistency of 235U integral data testing and differential prompt fission neutron spectra data (PFNS) is proposed. Similar problems for the n+233U and n+239Pu are envisaged, revealing arbitrary tweaking of relevant neutron cross sections. Fast 235U integral critical assemblies like Godiva or Flattop benchmarks are reproduced with the same accuracy as with the PFNS of the major data libraries. That reveals a rather delicate compensation effect, tracking the success of the distorted PFNS of the major data libraries to the excessive number of neutrons at energies of 1-3 MeV. Thermal assembly benchmarking reveals positive biases in keff, which might be attributed to the influence of a soft energy tail of the present PFNS. For some of Valduc's LCT benchmarks biases in keff are less than 20 pcm. Fast 239Pu integral critical experiments like Jezebel reveal the influence of overestimated inelastic scattering for 239Pu, arbitrarily introduced previously to diminish the influence on the keff of the distortion of the PFNS.JRC.DG.D.5-Nuclear physic

Spectrum of Prompt Fission Neutrons from 235U(n, F)

New prompt fission neutron spectra (PFNS) matrices for the n+235U, n+233U and n+239Pu systems are prepared. Present and previous PFNS shapes are drastically different from each other, the former being consistent with measured data base. A combined generalized least-squares fit of nth+235U, nth+233U, nth+239Pu and 252Cf(sf) measured data base is accomplished. The solution of the longstanding problem of inconsistency of 235U integral data testing and differential prompt fission neutron spectra data (PFNS) is proposed. Similar problems for the n+233U and n+239Pu are envisaged, revealing arbitrary tweaking of relevant neutron cross sections. Fast 235U integral critical assemblies like Godiva or Flattop benchmarks are reproduced with the same accuracy as with the PFNS of the major data libraries. That reveals a rather delicate compensation effect, tracking the success of the distorted PFNS of the major data libraries to the excessive number of neutrons at energies of 1-3 MeV. Thermal assembly benchmarking reveals positive biases in keff, which might be attributed to the influence of a soft energy tail of the present PFNS. For some of Valduc's LCT benchmarks biases in keff are less than 20 pcm. Fast 239Pu integral critical experiments like Jezebel reveal the influence of overestimated inelastic scattering for 239Pu, arbitrarily introduced previously to diminish the influence on the keff of the distortion of the PFNS.JRC.D.4-Nuclear physic