New Evaluations of Neutron Cross Sections for 14N and 16O

Abstract

New evaluations of the neutron cross sections for 14N and 16O have been made for ENDF/B-VI. The evaluations are based at low energies on R-matrix analyses of reactions in the 15N and 17O systems, and at higher energies on GNASH calculations and experimental data evaluations, including covariance analyses. The 15N system R-matrix analysis includes data from reactions among the channels n+14N, p+14C, and α+11B at energies corresponding to excitations in 15N below Ex=13 MeV. The resonance structure of all cross sections in this energy range is fairly well reproduced. New data indicate a different J-value for the first resonance, however. Sub-threshold S-wave levels required to explain the large n+14N total and elastic cross sections near zero energy give scattering lengths that differ significantly from the previous values. The R-matrix analysis of the 17O system includes many new measurements of the n+16O total cross section, done primarily at Oak Ridge and at Karlsruhe. The resonance structure of all the cross sections [total, (n,n), (n,α), and (α,α)] is well represented by the fit in the region below En = 6.5 MeV. The new total cross section information gives different positions for some of the resonances and implies a different normalization for the (n,α) cross sections than that obtained in the ENDF/B-IV analysis. The evaluations at energies above the ranges of the R-matrix analyses incorporate results from a number of experiments performed since the previous ENDF/B evaluations. Especially important are new measurements of the total cross sections and differential elastic, inelastic, and gamma-ray production cross sections.