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

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 $S_n=-472\pm 58(stat.)\pm 33(sys.)$ 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.

9 pages, 2 figures, accepted for publication in Physical Review Letters