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The neutron-rich nucleus 144 Ba (t 1/2 =11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive-and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multi-step Coulomb excitation of a postaccelerated 650-MeV 144 Ba beam on a 1.0-mg/cm 2 208 Pb target. The measured value of the matrix element, 3 − 1 M(E3) 0 + 1 = 0.65( +17 −23 ) eb 3/2 , corresponds to a reduced B(E3) transition probability of 48( +25 −34 ) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

Excited states in 38,40,42 Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of 40 S and 44 S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742 (8) 23.20.Lv, 27.40.+z, 29.38.Db Shell closures and collectivity are important properties that characterize the atomic nucleus. Interchange of their dominance along isotopic or isotonic chains has attracted much attention. The recent extension of the research frontier to nuclei far away from the valley of stability has revealed several new phenomena for neutronor proton-number dependent nuclear structure. For example, a weakening or even disappearance of shell closures occur in several neutron-rich nuclei at N = 8 [1][2][3] and N = 20 [4][5][6]. A well known example in the case of N = 20 is the so-called 'island of inversion ' [7] located around the neutron-rich nucleus 32 Mg. The low excitation energy of the first 2 + state E x (2 + 1 ) and large E2 transition probability [4][5][6] clearly indicate shell quenching in 32 Mg despite the fact that N = 20 is traditionally a magic number. The next magic number, N = 28, which appears due to the f 7/2 -f 5/2 spin-orbit splitting, has also been explored [8][9][10][11][12][13]. Weakening of the shell closure is seen by the decrease of the 2 With proton number Z = 14 and neutron number N = 28, the nuclear structure of 42 Si is of special interest. A simple but important question that arises is whether the weakening of the N = 28 shell closure continues, causing an enhancement of nuclear collectivity, or if shell stability is restored owing to a possible doubly magic structure. A study on 42 Si was made by a two-proton removal reaction experiment with radioactive 44 S beams at the NSCL [15]. The small two-proton removal cross sec-

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 ...

Despite the more than one order of magnitude difference between the measured dipole moments in 144 Ba and 146 Ba, the strength of the octupole correlations in 146 Ba are found to be as strong as those in 144 Ba with a similarly large value of B(E3; 3 − → 0 + ) determined as 48( +21 −29 ) W.u. The new results not only establish unambiguously the presence of a region of octupole deformation centered on these neutron-rich Ba isotopes, but also manifest the dependence of the electric dipole moments on the occupancy of different neutron orbitals in nuclei with enhanced octupole strength, as revealed by fully microscopic calculations.