Magicity versus Superfluidity around O 28 viewed from the Study of F 30

Abstract

The neutron-rich unbound fluorine isotope F2130 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 Ne31 nuclei at 235 MeV/nucleon. The mass and thus one-neutron-separation energy of F30 has been determined to be Sn=-472±58(stat)±33(sys) keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in Sn(F30) shows that the "magic"N=20 shell gap is not restored close to O28, 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: O28 behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both F29,31 appear to be good two-neutron halo-nucleus candidates.