Magicity versus Superfluidity around O28 viewed from the Study of F</mml:mrow…

The neutron-rich unbound fluorine isotope ^30F₂₁ 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 ^31Ne nuclei at 235 MeV/nucleon. The mass and thus one-neutron-separation energy of ^30F has been determined to be S₍=-472±58 (stat) ±33 (sys) keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in S₍ (^30F) shows that the "magic" N=20 shell gap is not restored close to ^28O, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron d and fp orbitals, with the 1p₃/₂ and 1p₁/₂ orbitals becoming more bound than the 0f₇/₂ one. This degeneracy and reordering of orbitals has two potential consequences: ^28O behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both ^29, 31F appear to be good two-neutron halo-nucleus candidates.