n+12C Cross Sections from an R-matrix Analysis of Reactions in the 13C system

Abstract We have performed an R -matrix analysis of reactions in the 13 C system, resulting in a new evaluation of the neutron cross sections for 12 C at energies below about 6.5 MeV. Experimental data, including total cross sections, differential cross sections, and neutron analyzing powers, were fitted for the reactions 12 C( n , n ) 12 C and 12 C( n , n ′) 12 C* at neutron lab energies below 6.5 MeV. A good fit ( χ 2 / ν = 1.34 ) was obtained to the more than 6800 data points in the experimental data set with 37 free parameters. Representative fits to the experimental data and comparisons to the previous evaluation are shown. Changes in the integrated and total cross sections are much less than those in the Legendre coefficients representing the elastic and inelastic angular distributions. We also discuss the implications of the analysis for the level structure of 13 C at excitation energies below 10.75 MeV, and present covariance information for the cross sections. Further work on the n + 12,13 C reactions is planned or in progress.