Coupled-channel description for mirror mass-11 nuclei compared to shell-model structures

The spectra of mass-11 nuclei are unusual, and so pose a challenge for nuclear-structure theory. Relating to nucleon emission, the set of isobars range from being well-bound (¹¹B,¹¹C) through weakly bound (¹¹Li, ¹¹Be), to being proton unstable (¹¹N,¹¹O). To add complexity, the weakly bound ¹¹Li takes the form of a two-nucleon halo nucleus. A self-consistent approach to understand this set of nuclei is especially important as the mirror pair ¹¹Be-¹¹N exhibit a parity-inverted ground state compared to their neighboring nuclei. Herein, the Multi-Channel Algebraic Scattering method (MCAS) has been used to describe the low excitation spectra of those isobars in terms of nucleon-nucleus clusters. A collective model description of the low-excitation states of the mass-10 mass-10 core nuclei has been used to form the coupled-channel interactions required in the method. For comparison, and to understand the underlying configurations, a shell model approach has been used to obtain those spectra with no-core (0 + 2 + 4) ħω and (0 + 2) ħω shell-model spaces for the mass 10 and mass 11 nuclei respectively. The results of the calculations suggest the need of a strong coupling in the collective coupled-channel vibrational model. In particular, the strong coupling of the collective 21+ state of ¹⁰Be to the valence neutron plays a decisive role in forming the positive parity ground state in ¹¹Be; an effect confirmed by the shell-model results.