We report on the measurement of the first 2 + and 4 + states of 66 Cr and 70,72 Fe via in-beam γ-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 γ-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 N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified 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. Atomic nuclei are the place of a complex interplay between single-particle configurations and correlations which strongly determine their quantum coherent wavefunctions. All over the nuclear chart, the so-called magic numbers of nucleons define boundaries of large areas of deformation. This picture, mainly established for stable nuclei and neighbors, is re-examined at the light of new available nuclei with an unbalanced proton-to-neutron ratio, with the underlying question of the persistence or evolution of magic numbers [1,2]. Specific terms of the nuclear interaction can induce the formation of shell gaps or the lowering of relative orbital energies which, combined with correlations, sometimes lead to energetically favored intruder states as the ground state configuration [3][4][5][6][7][8]. Regions where two-particle two-hole (2p2h) configurations are favored over normally-filled orbitals by quadrupole correlations have been termed as Islands of Inversion (IoI) [9][10][11]. The N=20 IoI in the vicinity of 32 Mg has provided unique information on shell evolution [12]. This IoI does not show any decrease in collectivity for Mg isotopes at N > 24 and merges with the N=28 deformation region [8,13]

Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a 54 Ca beam striking on a liquid hydrogen target at ∼200 MeV=u. A significantly larger cross section to the p 3=2 state compared to the f 5=2 state observed in the excitation of 53 Ca provides direct evidence for the nature of the N ¼ 34 shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments.

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