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

Abstract

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 (^{11}B,^{11}C) through weakly bound (^{11}Li, ^{11}Be), to being proton unstable (^{11}N,^{11}O). To add complexity, the weakly bound ^{11}Li 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 ^{11}Be-^{11}N 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)hbar omega and (0+2)hbar omega 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 2^+_1 state of ^{10}Be to the valence neutron plays a decisive role in forming the positive parity ground state in ^{11}Be; an effect confirmed by the shell-model results.