Exploring the halo character and dipole response in the dripline nucleus $^{31}$F

Abstract

Background: Lying at the lower edge of the 'island of inversion', neutron-rich Fluorine isotopes ( 29−31 F) provide a curious case to study the configuration mixing in this part of the nuclear landscape. Recent studies have suggested that a prospective two-neutron halo in the dripline nucleus 31 F could be linked to the occupancy of the pf intruder configurations.Purpose: Focusing on configuration mixing, matter radii and neutron-neutron (nn) correlations in the groundstate of 31 F, we explore various scenarios to analyze its possible halo nature as well as the low-lying electric dipole (E1) response within a three-body approach.Method: We use an analytical, transformed harmonic oscillator basis under the aegis of a hyperspherical formalism to construct the ground state three-body wave function of 31 F. The nn interaction is defined by the Gogny-Pires-Tourreil potential that includes the central, spin-orbit and tensor terms, while the different two-body potentials to describe the core + n subsystems are constrained by the different possible scenarios considered. Results:The 31 F ground-state configuration mixing and its matter radius are computed for different choices of the 30 F structure coupled to the valence neutron. The admixture of p 3/2 , d 3/2 , and f 7/2 components is found to play an important role, favouring the dominance of inverted configurations with dineutron spreads for twoneutron halo formation. The increase in matter radius with respect to the core radius, ∆r 0.30 fm and the dipole distributions along with the integrated B(E1) strengths of 2.6 e 2 fm 2 are large enough to be compatible with other two-neutron halo nuclei. Conclusion:Three-body results for 31 F indicate a large spatial extension in its ground state due to the inversion of the energy levels of the normal shell model scheme. The increase is augmented by and is proportional to the extent of the p 3/2 component in the wave function. Additionally, the enhanced dipole distributions and large B(E1) strengths all point to the two-neutron halo character of 31 F.