Measurement of the 235U(n, f) cross section relative to the 6Li(n, t) and 10B(n, ?) standards from thermal to 170 keV neutron energy range at n_TOF

The 235U(n, f) cross section was measured at n_TOF relative to 6Li(n, t) and 10B(n,?) , with high resolution (L= 183. 49 (2) m) and in a wide energy range (25meV-170keV) with 1.5% systematic uncertainty, making use of a stack of six samples and six silicon detectors placed in the neutron beam. This allowed us to make a direct comparison of the yields of the 235U(n, f) and of the two reference reactions under the same experimental conditions, and taking into account the forward/backward emission asymmetry. A hint of an anomaly in the 10-30keV neutron energy range had been previously observed in other experiments, indicating a cross section systematically lower by several percent relative to major evaluations. The present results indicate that the cross section in the 9-18keV neutron energy range is indeed overestimated by almost 5% in the recently released evaluated data files ENDF/B-VIII.0 and JEFF3.3, as a consequence of a 7% overestimate in a single GMA node in the IAEA reference file. Furthermore, these new high-resolution data confirm the existence of resonance-like structures in the keV neutron energy region. The results here reported may lead to a reduction of the uncertainty in the 1-100keV neutron energy region. Finally, from the present data, a value of 249. 7 ± 1. 4 (stat) ± 0. 94 (syst) b·eV has been extracted for the cross section integral between 7.8 and 11eV, confirming the value of 247. 5 ± 3 b·eV recently established as a standard.