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

Abstract

The ²³⁵U(n, f) cross section was measured at n_TOF relative to ⁶Li(n,t) and ¹⁰B(n,α) , with high resolution (L = 183.49(2) m) and in a wide energy range (25 meV - 170 keV) 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 - 30 keV 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 - 18 keV 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 - 100 keV 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 11 eV, confirming the value of 247.5 ± 3 b·eV recently established as a standard.