R-Matrix Analysis of the ²⁴⁰Pu Neutron Cross Sections in the Thermal to 5700-eV Energy Range

A Bayesian R-matrix-fitting code called SAMMY is used in a new analysis of the ²⁴⁰Pu neutron cross sections in the 0.02- to 5700-eV energy range. This work aims to resolve the discrepancies revealed by the JEF2 validation studies. A set of experimental data, suitable for the analysis, is determined from preliminary SAMMY analyses of the available experimental data treated individually. Finally, the sequential analysis of the selected transmission, total, and fission cross-section measurements gives an accurate set of resonance parameters. For the 1.056-eV resonance, the analysis of a recent transmission measurement gives parameters very close to those adopted in ENDF/B-VI. In the energy range above 200 eV, the average capture cross section calculated from the present evaluation is in agreement with ENDF/B-VI but is 25% lower than the values of JEF-2 and JENDL-3. A rigorous study of the statistical properties of the resonance parameters is done during this work. Although 158 new resonances are identified by the careful examination of the experimental fission cross-section and transmission data, the value of the average spacing above 2750 eV (D₀ = 16.10 eV) shows a 25% loss of resonances compared with the lower energy region (D₀ = 12.06 eV). Finally, for the s-wave resonance parameters, this work recommends the following average values: resonance spacing, D₀ = 12.06 ± 0.60 eV; strength function, S₀ = (1.032 ± 0.071) × 10⁻⁴ eV; and capture width, = 31.92 ± 1.6 meV.