R-matrix analyses of 16O(n,n) scattering and 13C(α,n) reaction at astrophysical energies relevant to low-mass AGB stars

Abstract

The reaction [Formula: see text]C[Formula: see text]O is an important astrophysical process producing neutrons for [Formula: see text]-process nucleosynthesis of nuclei heavier than iron in low-mass AGB stars. The reaction rate of [Formula: see text]C[Formula: see text]O at relevant energies is dominated by a [Formula: see text] state in [Formula: see text]O near the [Formula: see text]-threshold. The adopted value for the excitation energy of the state is −3[Formula: see text]keV below the threshold at 6359[Formula: see text]keV. However, recent investigations have indicated that the state is, instead of a sub-threshold state, an above threshold resonance state. The observation is also corroborated by neutron scattering studies from [Formula: see text]O. The location of the state and its implication on the low energy behavior of the astrophysical [Formula: see text]-factor are of definite interest. The aim of this work is to ascertain the energy location of [Formula: see text] state in the excitation spectrum of [Formula: see text]O and to estimate its neutron and [Formula: see text]-partial widths. Subsequently, we look into the effect of the resultant set of parameters of the state on the [Formula: see text]-factor and reaction rate at important astrophysical energies. A multilevel, multichannel [Formula: see text]-matrix analysis has been performed for the existing [Formula: see text]O[Formula: see text], [Formula: see text]C[Formula: see text], [Formula: see text]C[Formula: see text] and [Formula: see text]O[Formula: see text] data. The excitation energy of the [Formula: see text] state in [Formula: see text]O is found to be at 6371.9[Formula: see text]keV, about 12.9[Formula: see text]keV above the threshold. The resulting [Formula: see text]-factor around the Gamow energy of 190[Formula: see text]keV [Formula: see text] is [Formula: see text][Formula: see text][Formula: see text]. Relatively, higher [Formula: see text]-factor value yields a larger reaction rate for [Formula: see text]C([Formula: see text])[Formula: see text]O at the required temperature window. Consequently, larger number of neutrons relevant to [Formula: see text]-process nucleosynthesis will be produced from [Formula: see text]C[Formula: see text]O reaction.