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

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

Figure 1

Left: relative-energy spectra of the ${}^{27}\mathrm{F}+n$ system populated from the reactions (a) ${}^{29}\mathrm{Ne}(-1p)$ and (b) ${}^{29}\mathrm{F}(-1n)$. Right: same for the ${}^{26}\mathrm{F}+2n$ system populated from (e) ${}^{29}\mathrm{Ne}(-1p)$ and (f) ${}^{29}\mathrm{F}(-1n)$. The fit in red corresponds to a sum of resonances (blue, with the resonance energy in keV) plus a non-resonant distribution (dashed black). Center: same as left, obtained in coincidence with the 915 keV excited state of ${}^{27}\mathrm{F}$ (after background subtraction) populated from (c) ${}^{29}\mathrm{Ne}(-1p)$ and (d) ${}^{29}\mathrm{F}(-1n)$. The energy axis $E$ on the top right is given with respect to $S_n({}^{28}\mathrm{F})$, and orange dots mark resonances in coincidence with the corresponding fragment $\gamma$ rays (see Fig. 2).

Figure 2

Energies of the resonances observed in ${}^{28}\mathrm{F}$ in the $1n$ (black) and $2n$ (blue) decay channels compared to shell-model calculations (the g.s. energies are normalized to experiment). The resonances observed in the $2n$ channel are placed in the level scheme according to $S_n({}^{27}\mathrm{F})=1.620(60)\mathrm{MeV}$ (see text). The grey and blue bands represent the uncertainty in the resonance energies. Levels with an orange dot decay to excited states of ${}^{27}\mathrm{F}$ ($1n$ decay) or ${}^{26}\mathrm{F}$ ($2n$ decay).

Figure 3

Experimental transverse-momentum distributions of the ${}^{27}\mathrm{F}+n$ system following the ${}^{29}\mathrm{F}(-1n)$ reaction compared to eikonal-model calculations for the following: (a) removal of a neutron with $\ell =1$, 3 (respectively, blue, brown lines) for the g.s. at $E_{\mathrm{rel}}=198\mathrm{keV}$; (b) a pure $\ell =2$ distribution for the resonance at 406 keV (corresponding to the state at 1321 keV); (c) a mixture of $\ell =0$, 2 (respectively, purple, green lines) for the state at $E_{\mathrm{rel}}=996\mathrm{keV}$. In (a),(c) the red line is the total fit.