Spectroscopy of ${}^{32}\mathrm{Ne}$ and the “Island of Inversion”

We report on the first spectroscopic study of the $N=22$ nucleus ${}^{32}\mathrm{Ne}$ at the newly completed RIKEN Radioactive Ion Beam Factory. A single $\gamma$-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a $226\mathrm{MeV}/u$ ${}^{32}\mathrm{Ne}$ beam on a carbon target and proton removal from ${}^{33}\mathrm{Na}$ at $245\mathrm{MeV}/u$. This transition is assigned to the deexcitation of the first $J^{\pi }=2^{+}$ state in ${}^{32}\mathrm{Ne}$ to the $0^{+}$ ground state. Interpreted through comparison with state-of-the-art shell-model calculations, the low excitation energy demonstrates that the “island of inversion” extends to at least $N=22$ for the Ne isotopes.

Figure 1

Particle identification before the secondary target.

Figure 2

Doppler corrected $\gamma$-ray energy spectra in coincidence ($\pm 5\mathrm{ns}$) with ${}^{32}\mathrm{Ne}$ (a),(b) and ${}^{30}\mathrm{Ne}$ (c),(d). Panel (a) shows the results for inelastic scattering of ${}^{32}\mathrm{Ne}$ and (b) the result for proton removal from ${}^{33}\mathrm{Na}$. The outcomes of the fitting procedure are shown by the solid (total) and dashed (background) curves. Here, both spectra were fitted simultaneously with the same peak position and peak width, but different peak areas and background parameters. The inset panels (c) and (d) show the results for inelastic scattering of ${}^{30}\mathrm{Ne}$ and for $p2n$ removal from ${}^{33}\mathrm{Na}$, respectively, populating the first $2^{+}$ state in ${}^{30}\mathrm{Ne}$.

Figure 3

Comparison of experimental $E(2_1^{+})$ in neutron-rich Ne and Mg isotopes [15, 17, 20, 30], indicated by horizontal lines (the present work is shown in bold), with the shell-model results of Utsuno et al. [32] ($+$) and Caurier et al. [34, 36] for the normal ($\mathcal{N}$, dashed) and intruder ($\mathcal{I}$, dash-dotted line) configurations, respectively. Their prediction for the configuration ($\mathcal{N}$ or $\mathcal{I}$) with the lowest energy $0^{+}$ state, i.e., the ground state, is marked by a circle. The $E(2_1^{+})$ are given relative to the $0^{+}$ state of the same configuration.