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Intermediate-energy Coulomb excitation measurements are performed on the N ! 40 neutron-rich nuclei 66;68 Fe and 64 Cr. The reduced transition matrix elements providing a direct measure of the quadrupole collectivity BðE2; 2 þ 1 ! 0 þ 1 Þ are determined for the first time in 68 Fe 42 and 64 Cr 40 and confirm a previous recoil distance method lifetime measurement in 66 Fe 40 . The results are compared to state-ofthe-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed. DOI: 10.1103/PhysRevLett.110.242701 PACS numbers: 25.70.De, 27.50.+e For many decades the nuclear shell structure originally proposed by Mayer [1] and Jensen and coworkers [2], where energy gaps are predicted at specific nucleon numbers, was a paradigm of nuclear physics, as it was consistent with the experimental findings at or near the valley of beta stability. However, with the possibility of producing more exotic nuclei, the traditional magic numbers have been observed to be weakened or to disappear while new subshell gaps have emerged. In particular, the role of the proton-neutron tensor interaction has been recognized as driving changes in the shell structure [3]. Alterations to the effective single-particle orbital gaps can lead to enhanced particle-hole excitations, which are supported by deformation and pairing effects, and may give rise to new regions of well-developed nuclear deformation.A region of recent interest is that of the neutron-rich isotopes near N ¼ 40, below the 28 Ni isotopes. In many ways structurally similar to the ''island of inversion '' nuclei near N ¼ 20 [4], the Fe and Cr isotopes in this region have been experimentally observed to exhibit increasingly collective behavior, rather than the near-magic behavior naively expected assuming a robust N ¼ 40 subshell gap. In a schematic way, the development of collectivity moving from 28 Ni to 26 Fe and 24 Cr is understood as a result of a narrowing of the N ¼ 40 subshell closure and the enhancement of quadrupole collectivity through promotion of neutron pairs across the subshell gap. With the removal of protons from the 1f 7=2 orbital, the attractive tensor and central parts of the p-n interaction between 1f 7=2 proton holes and neutrons in the 1g 9=2 and 2d 5=2 orbits pull these neutron single-particle levels down in energy. At the same time, the repulsive tensor ð1f 7=2 Þ À1 À 1f 5=2 interaction dominates over the central attractive p-n interaction and drives the neutron 1f 5=2 orbital up, effectively quenching the N ¼ 40 gap. Looking at it another way, adding 12 neutrons to 48 Ca produces a gapless 60 Ca; as protons are added in the 1f 7=2 orbit, the repulsive interaction between the 1f 7=2 protons and the 1g 9=2 and 2d 5=2 neutrons and the strongly attractive 1f 7=2 -1f 5=2 interaction opens the N ¼ 40 gap up to its value in 68 Ni. The disappearance of the N ¼ 40 gap towards 60 Ca supports ...

The structure of the neutron-rich silicon isotopes 36,38,40 Si was studied by one-neutron and oneproton knockout reactions at intermediate beam energies. We construct level schemes for the knockout residues 35,37,39 Si and 35,37,39 Al and compare knockout cross sections to the predictions of an eikonal model in conjunction with large-scale shell model calculations. The agreement of these calculations with the present experiment lends support to the microscopic explanation of the enhanced collectivity in the region of 42 Si. We also present an empirical method for reproducing the observed low-momentum tails in the parallel momentum distributions of knockout residues.