Extension of the N =40 Island of Inversion towards N =50 : Spectroscopy of <mml:mmultiscripts>Cr 66 </mml:mmultiscripts> , Fe,7270

We report on the measurement of the first 2⁺ and 4⁺ states of <mml:mmultiscripts>Cr 66 </mml:mmultiscripts> and ^Fe,72₇₀ via in-beam γ -ray spectroscopy. The nuclei of interest were produced by (p ,2 p ) reactions at incident energies of 260 MeV /nucleon . The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2 γ -ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2₁⁺ and 4₁⁺ 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 p f g_{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 .