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

The$^{235}$U(n, f ) cross section was measured at n_TOF relative to$^{6}$Li(n, t) and$^{10}$B(n, $ \alpha$ ) , 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$^{235}$U(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\pm 1.4(\mathrm{stat}) \pm 0.94(\mathrm{syst})$ b·eV has been extracted for the cross section integral between 7.8 and 11eV, confirming the value of $ 247.5\pm 3$ b·eV recently established as a standard.