Observation of a crossover of $S_{2n}$ in the island of inversion from precision mass spectrometry

Mass measurements have been performed of the neutron-rich Al isotopes near $N\approx 20$, along the island of inversion. In general, the precision of the mass values was improved by an order of magnitude. Based on these new and more accurate data, we find the two-neutron separation energy of ${}_{13}{}^{34}\mathrm{Al}$ to be 24.8(10.8) keV less than that of ${}_{12}{}^{33}\mathrm{Mg}$. This represents the first known crossover on the $S_{2n}$ surface of the chart of nuclides. Large-scale shell-model calculations show that the crossover can be attributed to exceptionally large energy gains of the Mg isotopes in the island of inversion.

Figure 1

The two-neutron separation energy $S_{2n}$ curves near the island of inversion. Data are taken from AME'12 [6] and this work. The difference between the two ($\sim 100$ eV to 100 keV) is not visible on this scale. The inset displays the singular crossover at $N=21$.

Figure 2

A TOF-ICR resonance of ${}^{34}{\mathrm{Al}}^{+}$ for an excitation time of 47 ms. The solid curve is an analytic fit [22] to the data.

Figure 3

The two-neutron separation energy $S_{2n}$ for the Mg and Al isotopes. The experimental values are taken from this work and [6]. The lines are the calculated values in the $sd-pf$ valence space.

Figure 4

(a) Contribution of the neutron excitations outside of the $sd$ shell to the two-neutron separation energies $S_{2n}$ for the Mg and Al isotopes and (b) the relative values as a function of $N$.