Extending the Southern Shore of the Island of Inversion to ${}^{28}$F

Abstract

Detailed spectroscopy of the neutron-unbound nucleus ${}^{28}$F has been performed for the first time following proton/neutron removal from ${}^{29}$Ne/${}^{29}$F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the ${}^{27}$F${}^{(\ast )}+n$ and ${}^{26}$F${}^{(\ast )}+2n$ coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the ${}^{28}$F ground state, with $S_n{(}^{28}$F$)=-199(6)$ keV, while analysis of the $2n$ decay channel allowed a considerably improved $S_n{(}^{27}$F$)=1620(60)$ 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 ${}^{28}$F. Importantly, in the case of the ground state, the reconstructed ${}^{27}$F$+n$ momentum distribution following neutron removal from ${}^{29}$F indicates that it arises mainly from the $1p_{3/2}$ neutron intruder configuration. This demonstrates that the island of inversion around $N=20$ includes ${}^{28}$F, and most probably ${}^{29}$F, and suggests that ${}^{28}$O is not doubly magic.

6 pages, 3 figures, Supplemental Material, accepted for publication in Physical Review Letters