Magicity versus superfluidity around $^{28}$O viewed from the study of $^{30}$F

TL;DR

This study investigates the shell structure and superfluidity in neutron-rich fluorine isotopes near $^{28}$O, revealing the absence of a shell gap and suggesting potential halo nuclei and superfluid behavior.

Contribution

It provides the first measurement of the unbound isotope $^{30}$F and shows that the $N=20$ shell gap is not restored near $^{28}$O, challenging traditional magic number expectations.

Findings

$^{30}$F has a one-neutron separation energy of -472 keV.

The $N=20$ shell gap is not restored near $^{28}$O.

$^{29,31}$F are potential two-neutron halo nuclei.

Abstract

The neutron-rich unbound fluorine isotope $^{30}$F$_{21}$ has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasi-free proton knockout reaction of $^{31}$Ne nuclei at 235 MeV/nucleon. The mass and thus one-neutron-separation energy of $^{30}$F has been determined to be $S_n = -472\pm 58 \mathrm{(stat.)} \pm 33 \mathrm{(sys.)}$ keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in $S_n$($^{30}$F) shows that the ``magic'' $N=20$ shell gap is not restored close to $^{28}$O, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron $d$ and $fp$ orbitals, with the $1p_{3/2}$ and $1p_{1/2}$ orbitals becoming more bound than the $0f_{7/2}$ one. This degeneracy and reordering of orbitals has two potential consequences: $^{28}$O behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both $^{29,31}$F appear to be good two-neutron halo-nucleus candidates.