Extending the Southern Shore of the Island of Inversion to 28F

Detailed spectroscopy of the neutron-unbound nucleus 28F has been performed for the first time following proton/neutron removal from 29Ne/29F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the 27F(�) +n and 26F(�) + 2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28F ground state, with Sn(28F) = −199(6) keV, while analysis of the 2n decay channel allowed a considerably improved Sn(27F) = 1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28F. Importantly, in the case of the ground state, the reconstructed 27F+n momentum distribution following neutron removal from 29F indicates that it arises mainly from the 1p3/2 neutron intruder configuration. This demonstrates that the island of inversion around N = 20 includes 28F, and most probably 29F, and suggests that 28O is not doubly magic.