Criticality experiments for validation of cross sections: The neptunium case

Abstract

The 237Np neutron-induced fission cross section has been measured recently 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 simulate a criticality experiment performed at Los Alamos with a 6kg sphere of 237Np. This sphere was surrounded by enriched uranium 235U so as to approach criticality with fast neutrons. The simulation predicts a multiplication factor keff in better agreement with the experiment (the deviation of 750pcm is reduced to 250pcm) when we replace the ENDF/B-VII.0 evaluation of the 237Np fission cross section by the n_TOF data. We also explore the hypothesis of deficiencies of the inelastic cross section in 235U which has been invoked by some authors to explain the deviation of 750pcm. The large distortion that should be applied to the inelastic cross sections in order to reconcile the critical experiment with its simulation is incompatible with existing measurements. Also we show that the ν¯ of 237Np 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 237Np 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 237Np.