Americium-241 phase I. Reevaluation for JEFF-3.1.1 and a step forward

This article reviews the low energy neutron cross section revision and the capture isomeric ratio evaluation of the 241Am neutron data performed for JEFF-3.1.1. It covers the history of the anterior evaluation made for JEF-2.2 and gives the consistent path used for re-evaluating the resolved resonance parameters from differential measurements and 'integral' data feedback. This article highlights the procedure pursued to evaluate the 241Am capture ISOmeric ratio (ISOγ) from 0 to 20 MeV in parallel with a valuable estimation of the associated variances. Monte Carlo-type calculations have been made which predict ISOγ fluctuations over the energy region of the resolved resonances due to the presence of 2 s-wave types exhibiting well separated individual ISOγJ values (estimated to (90 ± 3)% and (76 ± 3)%, respectively, for a 2- and a 3- resonance) and well-resolved γ-multiplicities (νγ2-=(3.43 ± 0.3) and νγ3-=(3.67 ± 0.3)). Arguments are developed for using directly pointwise ISOγ data in reactor codes in the form of resonant partial capture cross sections, possibly from extended multi-level Breit-Wigner resonance parameterization, allowing systematic ISOγ Doppler broadening for a better comparison between experimental, differential, and integral measurements. This work corrects the fission widths of the lowest resonances inadequately treated during the JEFF-3.1.1 revision with an attempt of spin assignment supported by the present ISOγ study. Part of this work was also the preamble for the design of a new differential data program at the IRMM/Geel. This article shows the present status on the 241Am evaluated neutron data in terms of low energy cross sections and ISOγ data and, proposes some directions for the next JEFF-3.2 stage. (author)