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By deriving the angle between the proton and neutron spin vectors j ជ and j ជ in the shears bands in 198,199 Pb, we present a semiclassical analysis of the B(M 1) and B(E2) transition probabilities as a function of the shears angle. This provides a semiempirical confirmation of the shears mechanism proposed by Frauendorf using the tilted-axis-cranking model. In addition, we propose that the rotational-like behavior observed for these bands may arise from a residual proton-neutron interaction. ͓S0556-2813͑98͒51303-9͔

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 ...

We propose a phenomenological three-level mixing model of unperturbed 0p0h, 2p2h and 4p4h states, to describe the low-lying 0 + states in 32 Mg. Within this approach, self-consistent solutions exist that provides good agreement with the available experimental information obtained from the 30 Mg(t,p) 32 Mg reaction. The inclusion of the third state, namely the 4p4h configuration, resolves the "puzzle" that results from a two-level model interpretation of the same data. In our analysis, the 32 Mg ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level.

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