Abstract
We measured the neutron capture-to-fission cross-section ratio (α ratio) and the capture cross section of $^{235}$U between 0.2 and 200 eV at the n_TOF facility at CERN. The simultaneous measurement of neutron-induced capture and fission rates was performed by means of the n_TOF BaF$_{2}$ Total Absorption Calorimeter (TAC), used for detection of γ rays, in combination with a set of micromegas detectors used as fission tagging detectors. The energy dependence of the capture cross section was obtained with help of the $^{6}$Li(n,t) standard reaction determining the n_TOF neutron fluence; the well-known integral of the $^{235}$U(n,f) cross section between 7.8 and 11 eV was then used for its absolute normalization. The α ratio, obtained with slightly higher statistical fluctuations, was determined directly, without need for any reference cross section. To perform the analysis of this measurement we developed a new methodology to correct the experimentally observed effect that the probabilities of detecting a fission reaction in the TAC and the micromegas detectors are not independent. The results of this work have been used in a new evaluation of $^{235}$U performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, and are consistent with the ENDF/B-VIII.0 and JEFF-3.3 capture cross sections below 4 eV and above 100 eV. However, the measured capture cross section is on average 10% larger between 4 and 100 eV.
Highlights
The simultaneous measurement of neutron-induced capture and fission rates was performed by means of the n_TOF BaF2 Total Absorption Calorimeter (TAC), used for detection of γ rays, in combination with a set of micromegas detectors used as fission tagging detectors
Three new measurements have been performed following these recommendations: one of them at CERN, which is the one presented in this work, and two others at Los Alamos National Laboratory by Jandel et al [7] and at the Rensselaer Polytechnic Institute (RPI) by Danon et al [8]. The results of these three experiments have already been incorporated into a new evaluation of 235U performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, an initiative of the Organisation for Economic Co-operation and Development (OECD)/Nuclear Energy Agency (NEA) Data Bank under Working Party on Evaluation Co-operation (WPEC) Subgroup 40 [9,10,11] and the International Atomic Energy Agency (IAEA) Nuclear Data Section [12]
The measurement of the neutron capture cross section of fissile actinides is a challenging task due to the dominant time-correlated γ -ray background produced in the competing fission process
Summary
Three new measurements have been performed following these recommendations: one of them at CERN, which is the one presented in this work, and two others at Los Alamos National Laboratory by Jandel et al [7] and at the Rensselaer Polytechnic Institute (RPI) by Danon et al [8] The results of these three experiments have already been incorporated into a new evaluation of 235U performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, an initiative of the Organisation for Economic Co-operation and Development (OECD)/Nuclear Energy Agency (NEA) Data Bank under Working Party on Evaluation Co-operation (WPEC) Subgroup 40 [9,10,11] and the IAEA Nuclear Data Section [12]. Another is the fission tagging technique [18], used by Jandel et al [7] and in this work
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