Extension of the N=40 Island of Inversion towards N=50: Spectroscopy of 66Cr, 70,72Fe

We report on the measurement of the rst 2+ and 4+ states of 66Cr and 70;72Fe via in-beam -ray spectroscopy. The nuclei of interest were produced by (p; 2p) reactions at incident energies of 260 MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN using the DALI2 -ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2+ 1 and 4+ 1 energies as a function of neutron number was observed for N38 and N40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modied LNPS interaction in the pfg9=2d5=2 valence space reproduce the observations. Interpretation within the shell model shows an extension of the Island of Inversion at N=40 towards neutron-rich isotopes and hints at a smooth transition to collective N=50 Cr and Fe isotopes.