Three-Body Spectrum of 18C and its Relevance to r-Process Nucleosynthesis

The ¹⁸C spectrum has been studied in a three body n + n +¹⁶C model that includes deformation and the 2⁺ excitation of the ¹⁶C core as well as Pauli projection of forbidden states. The ¹⁶C – n interaction employed in this study has been fitted to reproduce the experimental spectrum of ¹⁷C. The calculations show that two neutron separation energy in ¹⁸C in consistent with three-body structure of this nucleus and predict more states bound with respect to three-body decay. The comparison of their position to known excited states in ¹⁸C is discussed. These calculations suggest also that a few states may exist in astrophysically relevant region between the ¹⁷C+n and ¹⁶C + 2n decay thresholds. The most important of them is 1⁻ as it can give a large E1 resonant contribution to ¹⁷C(n, γ)¹⁸C neutron capture. The calculations also suggest that a virtual s-wave state may exist above the ¹⁷C + n threshold that can give rise to non-negligible M1 contributions to the ¹⁷C(n, γ)¹⁸C reaction rate. The presence of these states in the ¹⁸C spectrum can lead to an increased ¹⁷C(n, γ)¹⁸C reaction rate, which can significantly influence the abundances of uranium and thorium synthesized in the r-process in the supernovae explosions.