Measurement of the ²³⁶U(n, f ) cross section from 170 meV to 2 MeV at the CERN n_TOF facility

Abstract

The neutron-induced fission cross section of ²³⁶U was measured at the neutron Time-of-Flight (n_TOF) facility at CERN relative to the standard ²³⁵U(n,f) cross section for neutron energies ranging from above thermal to several MeV. The measurement, covering the full range simultaneously, was performed with a fast ionization chamber, taking advantage of the high resolution of the n_TOF spectrometer. The n_TOF results confirm that the first resonance at 5.45 eV is largely overestimated in some nuclear data libraries. The resonance triplet around 1.2 keV was measured with high resolution and resonance parameters were determined with good accuracy. Resonances at high energy have also been observed and characterized and different values for the cross section are provided for the region between 10 keV and the fission threshold. The present work indicates various shortcomings of the current nuclear data libraries in the subthreshold region and provides the basis for an accurate re-evaluation of the ²³⁶U(n,f) cross section, which is of great relevance for the development of emerging or innovative nuclear reactor technologies.