Magicity versus Superfluidity around O28 viewed from the Study of F30

The neutron-rich unbound fluorine isotope ${}^{30}{\mathrm{F}}_{21}$ has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasifree proton knockout reaction of ${}^{31}\mathrm{Ne}$ nuclei at $235\mathrm{MeV}/\mathrm{nucleon}$. The mass and thus one-neutron-separation energy of ${}^{30}\mathrm{F}$ has been determined to be $S_n=-472\pm 58(\mathrm{stat})\pm 33(\mathrm{sys})\mathrm{keV}$ from the measurement of its invariant-mass spectrum. The absence of a sharp drop in $S_n({}^{30}\mathrm{F})$ shows that the “magic” $N=20$ shell gap is not restored close to ${}^{28}\mathrm{O}$, 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: ${}^{28}\mathrm{O}$ behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both ${}^{29,31}\mathrm{F}$ appear to be good two-neutron halo-nucleus candidates.