Criticality experiments and benchmarks for cross section evaluation: the neptunium case

Abstract

Abstract.The 237 Np neutron-induced fission cross section has been recently measured in a large energy range (from eV to GeV) at the n_TOF facility at CERN. When compared to previous measurement the n_TOF fission cross section appears to be higher by 5-7% beyond the fission threshold. To check the relevance of n_TOF data, we apply a criticality experiment performed at Los Alamos with a 6 kg sphere of 237 Np, surrounded by enriched uranium 235 U so as to approach criticality with fast neutrons. The multiplication factor k e f f of the calculation is in better agreement with the experiment (the deviation of 750 pcm is reduced to 250 pcm) when we replace the ENDF/B-VII.0 evaluation of the 237 Np fission cross section by the n_TOF data. We also explore the hypothesis of deficiencies of the inelastic cross section in 235 U which has been invoked by some authors to explain the deviation of 750 pcm. With compare to inelastic large distortion calculation, it is incompatible with existing measurements. Also we show that the ν of 237 Np can hardly be incriminated because of the high accuracy of the existing data. Fission rate ratios or averaged fission cross sections measured in several fast neutron fields seem to give contradictory results on the validation of the 237 Np cross section but at least one of the benchmark experiments, where the active deposits have been well calibrated for the number of atoms, favors the n_TOF data set. These outcomes support the hypothesis of a higher fission cross section of 237 Np.

Neutron induced fission of 237Np – status, challenges and opportunities

“…While being in good agreement with previous measurements and evaluated data files at energies below and around the fission threshold, the data exhibit an excess of about 6-7% above E n = 1 MeV [8][9][10][11][12][13]. The more recent measurements [14] were not able completelly to resolve the situation.…”

“…1 and Ref. [8][9][10][11][12][13][14]). Similar discrepancy between different data libraries exists for fissionresonance parameters below the fission threshold [10].…”

“…Although some data sets for fast-neutron induced fission are consistent within the claimed accuracy, usually about 3-4%, some significant discrepancies show up between more recent data, when compared to several previous measurements. For example, several experiments based on the time-of-flight (TOF) technique were dedicated to the measurement of fission cross section data in a wide neutron energy range (0.7 eV-1 GeV), using different fission-fragment detectors [12][13][14].…”

“…6 [12]) and in the neutron energy range between 1.5 MeV and 9 MeV (Fig. 7 [14]); the depicted data are from different experiments, as well as from different evaluated nuclear data libraries (see the references in [12][13][14]). …”