New measurement of $^{50}$Cr and $^{53}$Cr (n,$\gamma$) cross sections at n_TOF: a call for chromium nuclear data revision

Abstract

$^{50}$Cr and $^{53}$Cr are very relevant in criticality safety benchmarks related to nuclear reactors. The discrepancies of up to 30% between the neutron capture cross section evaluations have an important effect on the $k_{eff}$ and $k_{\infty}$ in criticality benchmarks particularly sensitive to chromium. In this work, the $^{50,53}$Cr(n,$\gamma$) cross sections are to be determined between 1 and 100 keV with an 8-10% accuracy following the requirements of the NEA High Priority Request List (HPRL) to solve the current discrepancies. We have measured these reactions by the time-of-flight technique at the EAR1 experimental area of the n_TOF facility, using an array of four C$_6$D$_6$ detectors with very low neutron sensitivity. The highly-enriched samples used are significantly thinner than in previous measurements, thus minimizing the multiple-scattering effects. We have produced, and analysed with the R-matrix analysis code SAMMY, capture yields featuring 33 resonances of $^{50}$Cr and 51 of $^{53}$Cr with an accuracy between 5% and 9%, hence fulfilling the requirements made by the NEA. The differential and integral cross sections have been compared to previous data and evaluations. The new measured $^{50,53}$Cr(n,$\gamma$) cross sections provide a valuable input for upcoming evaluations, which are deemed necessary given that the results presented herein do not support the increase in both cross sections proposed in the recent INDEN evaluation.