Isobaric analog resonances of Mg31 and the border of the island of inversion

The evolution of the nuclear shell structure in the region of the neutron-rich shell-breaking nucleus Mg32 has been the subject of considerable interest. We present here the first determination of the overlap of the ground and two first excited states in Mg31 with a neutron coupled to the ground state in Mg30 based on studies of its isobaric analog resonances in Al31. The excitation function for proton resonant elastic scattering on Mg30 was measured close to 0∘ in the laboratory frame by bombarding a thick polyethylene target with a Mg30 beam at an energy of 2.92 MeV/nucleon at the REX-ISOLDE facility at CERN. Three resonances were successfully resolved, and angular momenta and total and proton resonance widths were determined by using R-matrix analysis. The deduced spectroscopic factor for the ground state in Mg31 is consistent with the shell-model calculation, whereas those for the first and second excited states could not be reproduced. These results show that a drastic change in structure occurs between Mg30 and Mg31 and that the onset of structural change in this region therefore occurs between these two isotopes.