Observation of a crossover of S2n in the island of inversion from precision mass spectrometry

Mass measurements have been performed of the neutron-rich Al isotopes near N approximate to 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 Al-34(13) to be 24.8(10.8) keV less than that of Mg-33(12). 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.