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 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 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 v 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.
Highlights
IntroductionTo get a clarification on this problem we adopted the method of checking the cross sections within a critical experiment
The multiplication factor ke f f of the calculation is in better agreement with the experiment when we replace the ENDF/B-VII.0 evaluation of the 237Np fission cross section by the n_TOF data
To get a clarification on this problem we adopted the method of checking the cross sections within a critical experiment. It consisted in a spherical-like assembly made of a 237Np sphere surrounded by a spherical shell of Highly Enriched Uranium (HEU) in which the neutron multiplication factor ke f f is measured with an accuracy of 0.36%
Summary
To get a clarification on this problem we adopted the method of checking the cross sections within a critical experiment. Ke f f is an integral quantity depending on many neutron reaction parameters, many of them are accurately known, and its sensitivity to the 237Np fission cross section is high enough (1.5 10−3/%) to get a good test. Even if it cannot be considered as a definite test, due to its integral nature, it is a good indicator of the reliability of the 237Np(n, f ) cross section. We test the 237Np fission cross section against fission rate experiments in different neutron spectra
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