Extension of the N=40 Island of Inversion towards N=50 : Spectroscopy of Cr66 , Fe70,72

We report on the measurement of the first 2(+) and 4(+) states of Cr-66 and Fe-70,Fe-72 via in-beam gamma-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 DALI 2 gamma-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 the neutron number was observed for N >= 38 and N >= 40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N = 40 for more neutron-rich isotopes towards N = 50.