Abstract
235U neutron-induced fission cross-section is commonly used as reference for determining other isotope fission cross-section. However, below 150 keV this cross section is only included as Standard at the thermal point and recently its integral value between 7.8 eV and 11 eV [1]. The resolved resonance region, spanning up to 2.25 keV, has been reevaluated with high resolution in the last ENDF/B-VIII release [2] and a SAMMY resonance analysis was done by L. Leal et al. [3] including the CERN-nTOF experimental work of Paradela el al. [4] up to 10 keV, taken into account the IAEA Reference file. In this work the 235U(n,f) low-background and high-resolution experimental data obtained at the CERN-nTOF facility is combined with previous high-resolution experimental data, in order to produce a very fine grid dataset with normalisation to the IAEA Reference file. The extremelyhigh energy calibration required to reproduce the resonance sharp profiles is based on the nTOF DAQ system with a resolution below 0.1% with reference to the 8.78 eV resonance and to the sharp Al(n,g) capture dip at 5.904 keV. The comparison of the so-evaluated profile with the experimental data and with the evaluated ones will be discussed.
Highlights
Accurate data on the fission of heavy nuclei at energies beyond the resolved resonance region (RRR) are of a renewed interest for both fundamental and applied nuclear physics
2.25 keV the current ENDF/B-VIII shows a coarse energy resolution, notwithstanding its integral mean values are in fine agreement with the pointwise dataset in the IAEA Reference file
After inspection of the EXFOR library some selected datasets have been retained arriving to the conclusion that, besides the CERN-nTOF-PPAC results, the dataset of Weston and Todd [5] is the one having the best compromise between statistics and energy resolution, even though its bad energy calibration
Summary
Accurate data on the fission of heavy nuclei at energies beyond the resolved resonance region (RRR) are of a renewed interest for both fundamental and applied nuclear physics. Very few cross sections can be measured absolutely and in most cases cross sections are measured relative to the standards or to the data files retrieved from the evaluated libraries. Leal et al [3] after including the work of Paradela el al., presented at WONDER-2015 [4], and taken into account the IAEA Reference file. 235U(n,f) reaction has been amply measured and a bunch of entries can be retrieved at the EXFOR library, many of them consisting on single points, mean cross-section values in a broad energy interval or low energy resolution cross-section spectra. 3 keV, the cross-section spectrum is dominated by the grouping of the observed cross section peaks, pointing to the existence of resonance structures that have been so far not well resolved
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