Neutron cross sections for carbon and oxygen from new R-matrix analyses of the ^13,14C and ¹⁷O systems

Theoretical Division, Los Alamos National Laboratory, Los Alamos NM 87545, USA

^a e-mail: ghale@lanl.gov

Published online: 13 September 2017

Abstract

We report the latest results from R-matrix analyses of reactions in the ^13,14C and ¹⁷O systems that are of interest in reactor applications and nuclear astrophysics. These were done in order to provide separate cross sections for the stable isotopes (^12,13C) of natural carbon, and to contribute improved cross sections for ¹⁶O to the CIELO project. Although particular attention was paid to the data in the standards region (<2 MeV) for the carbon isotopes, and to the low-energy region for n+¹⁶O, the analyses extend to several MeV neutron energy for all the systems. The fits to the data included are generally quite good, in keeping with the unitary constraints of R-matrix theory. The cross sections for ^12,13C give results for natural carbon that are close to the previous evaluation by Fu et al. at energies below 1 MeV. Above that energy, the deviations become larger, especially near the narrow resonances. The thermal cross section for ¹⁶O is at the upper end of the range of recommended values, in excellent agreement with a high-precision measurement by Schneider. At higher energies, the ¹⁷O analysis follows in great detail high-resolution measurements of the total cross section, and agrees quite well with the ¹³C(α,n)¹⁶O cross section measurement of Bair and Haas at roughly their original normalization scale. We will discuss the implications of these new evaluations for critical benchmarks and astrophysical applications.