Measurement of the 235 U(n,f) cross section at n_TOF from thermal to 170 keV

Abstract

The U-235(n, f) cross section plays a key role for nuclear physics due to its widespread use as a standard reference for neutron cross section measurements and for neutron flux measurements. Recent experimental data of the fission cross section have suggested the presence of discrepancies around 6-8% with respect to the most used libraries, precisely in the range between 10 keV and 30 keV. In order to shed light on this disagreement, an accurate measurement of the U-235(n,f) fission cross section has been performed at n_TOF facility @CERN, using the standard reactions Li-6(n,t) and B-10(n,alpha) as reference. A custom experimental setup based on a stack of silicon detectors sandwiched between pairs of U-235, Li-6 and B-10 targets, has been installed along the neutron beam line to intercept the same neutron flux, allowing the detection of the fission fragments and the products of the reference reactions at the same time. Such a technique allows calculation of the cross section via the "ratio method", by normalizing the U-235(n,f) reaction yields with respect to the reference reactions and to the recommended data in the IAEA libraries; in particular, the integral between 7.8 and 11 eV has been chosen. Accurate Monte Carlo simulations have allowed evaluation of the neutron absorption in the different layers, as well as the detection efficiency of each detector. The data are in excellent agreement with the standard values and highlight the overestimation of the U-235(n,f) cross section between 9 and 18 keV in the most recent libraries.