Abstract
.The 235U(n, f ) cross section was measured at n_TOF relative to 6Li(n, t) and 10B(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 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 249.7pm 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 247.5pm 3 b·eV recently established as a standard.
Highlights
The 235U(n, f) cross section is one of the most important and widely used cross sections
The present results indicate that the cross section in the 9–18 keV neutron energy range is 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 International Atomic Energy Agency (IAEA) reference file
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
Summary
The 235U(n, f) cross section is one of the most important and widely used cross sections It is a standard at the 0.025 eV thermal neutron energy point and between 0.15 and 200 MeV, it is used as reference at all energies for a variety of purposes, such as for the measurement of the neutron fluence for various applications, or for the measurement of the fission cross section of other actinides. After properly normalizing the data to the respective standard cross sections at thermal energy, the flux extracted on the basis of the 6Li(n, t)4He and 10B(n, α)7Li reactions mostly agreed with each other and with the results of FLUKA simulations of the neutron beam [6]. The flux extracted on the basis of the 235U(n, f) reaction, determined independently with the PTB and Micromegas detectors, resulted several percent lower in the energy range 10–30 keV [1]. The 235U(n, f) cross section in that range is not a standard, such a large difference was unexpected, in particular since most of the evaluations available at that time quoted an uncertainty on this cross section below or around 1% (see for example the compilation of standard cross sections in refs. [7] and [8])
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