Unresolved resonance parameter evaluation and uncertainty quantification of n+181Ta reactions

Nuclear technology applications, including reactor modeling, accelerator design, and isotope production, strongly depend on evaluated nuclear data libraries and their uncertainty information for the assessment of predictive accuracy of calculated quantities. Major nuclear data libraries such as JENDL-5, JEFF-3.3, and ENDF/B-VIII.0 lack uncertainty information for $n+$¹⁸¹Ta reactions. In addition to the lack of evaluated uncertainty information even in major nuclear data library releases, the most current US ENDF/B-VIII.0 evaluation of the unresolved resonance region (URR) does not extend to high enough energies to appropriately account for resonance self-shielding effects. This work addresses these shortcomings through a new evaluation of the URR, performed with the SAMMY evaluation tool, which extends the evaluation of the URR to encompass neutron energies of 2.5 keV to 100 keV. This study reports evaluated covariances and includes newly measured data in the evaluation analysis that were unavailable to previous evaluators. The new evaluation was designed to be closely coupled to the resolved resonance region evaluation to improve consistency across multiple evaluation regions. The updated cross sections in the URR have reduced capture and total cross sections, which improve agreement with differential measurements compared to ENDF/B-VIII.0, but they deviate slightly further from integral benchmarks.¹