We report on the measurement of the first $2^{+}$ and $4^{+}$ states of ${}^{66}\mathrm{Cr}$ and ${}^{70,72}\mathrm{Fe}$ via in-beam $\gamma$-ray spectroscopy. The nuclei of interest were produced by $(p,2p)$ reactions at incident energies of $260\mathrm{MeV}/\text{nucleon}$. 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 $N\ge 38$ and $N\ge 40$ 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 $pf{g}_{9/2}{d}_{5/2}$ 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 $N=50$.

• Received 30 April 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.192501