Radiative neutron capture cross section of $^{242}$Pu measured at n_TOF-EAR1 in the unresolved resonance region up to 600 keV

TL;DR

This paper reports a precise measurement of the neutron capture cross section of $^{242}$Pu from 1 to 600 keV, reducing uncertainties and resolving discrepancies among previous data and evaluations.

Contribution

The study provides the most accurate $^{242}$Pu(n,$$) cross section measurement up to 600 keV, employing a novel technique and systematic uncertainty reduction, improving data consistency for reactor applications.

Findings

Cross section uncertainty reduced to 8-12%

Results align with some previous measurements and evaluations

Discrepancies with DANCE data above a few keV are identified

Abstract

Accurate neutron capture cross sections are essential for the design and operation of fast reactors using MOX fuels. For $^{242}$Pu, the Nuclear Energy Agency (NEA) recommends 8--12% accuracy in the fast energy region (2--500 keV), compared to the current uncertainty of 35%. Moreover, integral experiments and previous measurements suggest the evaluated $^{242}$Pu(n,$\gamma$) cross section is overestimated, particularly in the JEFF-3.3 library, which shows a 14% overestimation between 1 keV and 1 MeV. Recent measurements from LANSCE reported a 20--30% reduction in the 1--40 keV range relative to evaluations. To solve these discrepancies, the $^{242}$Pu(n,$\gamma$) cross section was measured from 1 to 600 keV at CERN n_TOF-EAR1 facility using a 95(4) mg $^{242}$Pu target, enriched to 99.959%. Gamma rays from neutron capture were detected with an array of C$_6$D$_6$ scintillators and a novel application of the Pulse Height Weighting Technique was employed. The resulting cross section presents a systematic uncertainty between 8 and 12%, reducing the current uncertainties of 35\% and achieving the accuracy requested by the NEA. Analysis using FITACS produced average resonance parameters, consistent with the analysis of the resolved resonance region. Our data align well with Wisshak and Kaeppeler, and are 10--14\% lower than JEFF-3.3 in the 1--250 keV range, helping to achieve consistency with integral benchmarks. At higher energies, our results are in reasonable agreement with ENDF/B-VIII.1 and JEFF-3.3. In contrast, DANCE results appear to underestimate the cross section by a factor of 2--3 above a few keV.