Covariances from Light-Element R-Matrix Analyses

We review the method for obtaining covariance information for light-element reactions using R-matrix theory. The general LANL R-matrix analysis code EDA provides accurate covariances for the resonance parameters at a solution due to the search algorithm it uses to find a local minimum of the chi-square surface. This information is used, together with analytically calculated sensitivity derivatives, in the first-order error propagation equation to obtain cross-section covariances for all reactions included in the analysis. Examples are given of the covariances obtained from the EDA analyses for n-p scattering and for the n+6Li reactions used in the latest light-element standard cross section evaluation. Also discussed is a method of defining “pure theory” correlations that could be useful for extensions to higher energies and heavier systems.