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

Abstract

Detailed spectroscopy of the neutron-unbound nucleus F28 has been performed for the first time following proton/neutron removal from Ne29/F29 beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the F(∗)27+n and F(∗)26+2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the F28 ground state, with Sn(F28)=-199(6) keV, while analysis of the 2n decay channel allowed a considerably improved Sn(F27)=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 F28. Importantly, in the case of the ground state, the reconstructed F27+n momentum distribution following neutron removal from F29 indicates that it arises mainly from the 1p3/2 neutron intruder configuration. This demonstrates that the island of inversion around N=20 includes F28, and most probably F29, and suggests that O28 is not doubly magic.