The Low-lying Levels of 13C and 13N

Abstract

An attempt is made to give a consistent account of observed properties of the low-lying levels of the mirror nuclei 13C and 13N. In the first stage of this analysis, least squares fits of the data are made using R-matrix formulae in the one- and two-level approximations; in the second stage, the resultant parameter values are compared with shell model predictions. Fitted properties include level widths, neutron scattering data, El radiative widths and El capture cross sections .. The R-matrix formulae include external contributions to the El transition matrix elements, calculated using wavefunctions with the correct asymptotic forms. Acceptable fits are obtained for channel radii in the range 4-6 fm, with the lower values preferred. The parameter values obtained in these fits agree with shell model predictions, except for quantities involving the t - levels. Level displacement energies are calculated from the fitted parameter values. A reasonable account is given of two notable asymmetries between 13C and 13N-the very different excitation energies of the first excited states and the very different strengths of the El decays of these states.