Validation of Cross Sections with Criticality Experiment and Reaction Rates: the Neptunium Case

The Np-237 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 measurements the n\_TOF fission cross section appears to be higher by 5-7\% beyond the fission threshold. To check the relevance of the n\_TOF data, we considered a criticality experiment performed at Los Alamos with a 6 kg sphere of Np-237, surrounded by uranium highly enriched in U-235 so as to approach criticality with fast neutrons. The multiplication factor k(eff) of the calculation is in better agreement with the experiment when we replace the ENDF/B-VII. 0 evaluation of the Np-237 fission cross section by the n\_TOF data. We also explored the hypothesis of deficiencies of the inelastic cross section in U-235 which has been invoked by some authors to explain the deviation of 750 pcm. The large modification needed to reduce the deviation seems to be incompatible with existing inelastic cross section measurements. Also we show that the. of Np-237 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 Np-237 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 Np-237.