Role of quadrupole deformation and continuum effects in the “island of inversion” nuclei 28,29,31F

• Published in: Physical review. C 2021-07, Vol.104 (1)
• Main Authors: Luo, Yu-Xuan, Fossez, Kévin, Liu, Quan, Guo, Jian-You
• Format: Article
• Language: English
• Subjects: 20 ≤ A ≤ 38; Nuclear forces; Nuclear many-body theory; NUCLEAR PHYSICS AND RADIATION PHYSICS; Nuclear structure & decays; Nucleon distribution
• ISSN: 2469-9985; 2469-9993

The peculiar properties of nuclei in the so-called “island of inversion” around Z = 10 and N = 20 are the focus of current nuclear physics research. Recent studies showed that 28F has a negative-parity ground state and thus lies within the southern shore of the island of inversion, and 29F presents a halo structure in its ground state, but it is unclear which effects, such as deformation, shell evolution due to tensor forces, or couplings to the continuum, lead to this situation. Here, we investigate the role of quadrupole deformation and continuum effects on the single-particle structure of 28,29,31F from a relativistic mean-field approach and show how both phenomena can lead to a negative-parity ground state in 28F and halo structures in 29,31F. We solve the Dirac equation in the complex-momentum (Berggren) representation for a potential with quadrupole deformation at the first order obtained from relativistic mean-field calculations using the NL3 interaction and calculate the continuum level densities using the Green’s function method. We extract single-particle energies and widths from the continuum level densities to construct the Nilsson diagrams of 28,29,31F in the continuum and analyze the evolution of both the widths and occupation probabilities of relevant Nilsson orbitals in 28F and find that some amount of prolate deformation must be present. In addition, we calculate the density distributions for bound Nilsson orbitals near the Fermi surface in 29,31F and reveal that, for a quadrupole deformation 0.3 ≤ β2 ≤ 0.45 (prolate), characteristic halo tails appear at large distances. Using the relativistic mean-field approach in the complex-momentum representation with the Green’s function method, we demonstrate that in neutron-rich fluorine isotopes, while in the spherical case the pf shells are already inverted and close to the neutron emission threshold, a small amount of quadrupole deformation can dramatically reduce the gap between positive- and negative-parity states and increase the role of continuum states, ultimately leading to the negative parity in the ground state of 28F and the halo structures in 29,31F.