Speaker
Description
{Evaluated nuclear data files are the basis for representing radiation interactions with nuclei in most modern transport and diffusion codes. It is therefore important to verify and update isotopic evaluations periodically to ensure they incorporate up-to-date experimental data and reaction theory in the hope to accurately model systems with low uncertainty. Recently, great interest has been generated in using lead as a coolant for fast neutron systems and as a result it is important to investigate the ENDF/B-VIII.0\cite{ENDF} isotopic evaluations that comprise stable lead. To this end, resonance parameters for $^{206}$Pb, $^{207}$Pb, and $^{208}$Pb were re-evaluated because their resolved resonance regions extend beyond 0.5 MeV meaning resonance parameters used in reconstructing cross sections and elastic scattering angular distributions impact fast systems. The impact of resonance parameters is demonstrated by the differences between the evaluations in predicting experimental results from the fully modeled RPI Quasi-Differential Scattering Experiment\cite{RPIscatter} in MCNP\cite{MCNP}, Figure \ref{fig:RPIScat2}. In addition, MCNP KCODE calculations of lead-sensitive fast spectra critical benchmarks showed variations of k$_{\mathit{eff}}$ on average of 200 pcm, caused solely from differences in elastic scattering angular distributions in $^{208}$Pb. Re-evaluation entailed fitting data with the program SAMMY\cite{SAMMY}. Resonance energies as well as neutron and radiation widths were fit sequentially to previously available high resolution transmission measurements and new capture yield experiments made available after the release of ENDF/B-VIII.0. Constraints for resonance parameters included SAMMY's reduced chi-square, resonance statistics (Wigner, Porter-Thomas, and summed strength plots), and resonance properties (scattering lengths, thermal quantities, and resonance integrals). Preliminary validation, done via a suite of critical benchmarks containing lead as well as MCNP simulations of the RPI scattering experiment, shows better agreement from the RPI evaluation for fast spectra criticality and scattering simulations than ENDF/B-VIII.0, JENDL-4.0 \cite{JENDL}, and JEFF-3.3\cite{JEFF}.