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

¹ Institut de Physique Nucléaire, CNRS
² Unviersité Paris-Sud, 91406 Orsay, France
³ Facultad de Fisica, Universidade de Santiago de Compostela, 15782, Spain

^a e-mail: leong@ipno.in2p3.fr
^b e-mail: tassango@ipno.in2p3.fr

Abstract

The ²³⁷Np 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 ²³⁷Np, surrounded by enriched uranium ²³⁵U so as to approach criticality with fast neutrons. The multiplication factor ke 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 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 750 pcm. With compare to inelastic large distortion calculation, it is incompatible with existing measurements. Also we show that the v 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.