There is strong circumstantial evidence that the shape of atomic nuclei with particular values of Z and N prefers to assume octupole deformation, in which the nucleus is distorted into a pear shape that loses the reflection symmetry of a quadrupole-deformed (rugby ball) shape prevalent in nuclei. Recently, useable intensities of accelerated beams of heavy, radioactive ions have become available at the REX-ISOLDE facility at CERN. This has allowed electric octupole transition strengths, a direct measure of octupole correlations, to be determined for short-lived isotopes of radon and radium expected to be unstable to pear-like distortions. The data are used to discriminate differing theoretical approaches to the description of the octupole phenomena, and also help restrict the choice of candidates for studies of atomic electric-dipole moments, that provide stringent tests of extensions to the Standard Model.

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.

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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.

The quadrupole collectivity of low-lying states and the anomalous behavior of the 0 + 2 and 2 + 3 levels in 72 Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2. A total of forty six E2 and M 1 matrix elements connecting fourteen low-lying levels were determined using the least-squares search code, gosia. Evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, is presented. These are interpreted using a simple two-state mixing model as well as multistate mixing calculations carried out within the framework of the triaxial rotor model. The results represent a significant milestone towards the understanding of the unusual structure of this nucleus.The structure of low-lying states in even-even Ge isotopes has been the subject of intense scrutiny for many years due to the inherent challenge of interpreting their systematics as a function of mass A. These nuclei possess at least one excited 0 + state in their low-energy spectrum that differs from the ground state in its properties. Systematically, the energy of the 0 + 2 level varies parabolically with A and reaches a minimum in 72 Ge, where it becomes the first excited state. The existence of even-mass nuclei with a J π = 0 + first excited state is an uncommon phenomenon which, to date, has been observed in only a few nuclei located near or at closed shells: 16 O [1], 40 Ca [2, 3], 68 Ni [4, 5], 90 Zr [6], 180,182 Hg [7-9], 184,186,188,190,192,. There are also examples of such nuclei where a subshell appears to play a role similar to a closed shell such as 96,98 Zr [16,17]. These cases have all been explained as resulting from shape coexistence due to the presence of intruder configurations; i.e., configurations involving the excitation of at least one pair of nucleons across a shell or subshell energy gap [18].The structure of 72 Ge is highly unusual in that this nucleus is far from closed shells and, yet, possesses a 0 + first excited state. It shares this distinction with only two other known nuclei: 72 Kr [19] and 98 Mo [20]. It should be noted that while 68 Ni might be doubly magic [4,21] with a presumed subshell closure at N = 40 [22], there is no evidence to date of spherical ground-state configurations in any of the other N = 40 isotones [23]. There are, however, strong experimental indications of enhanced collective behavior in or near the ground states of the N = 40 neutron-rich Fe and Cr isotones [24]. In the Ge isotopes, the absence of a subshell closure at N = 40 can be inferred from the fact that the 2 + 1 level with the highest energy appears in 70 Ge rather than 72 Ge. Thus, understanding the nature and origin of this anomalous 0 + 2 state in 72 Ge has been a major challenge for collective model descriptions.The theory of collectivity in nuclei is predominantly focused on models with quadrupole degrees of freedom. The simplest of these models, based on the quantization of a liquid drop [25,26], describes quadrupole collectivity as ei...

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Many-body nuclear theory utilizing microscopic or chiral potentials has developed to the point that collectivity might be dealt with in an ab initio framework without the use of effective charges; for example with the proper evolution of operators, or alternatively, through the use of an appropriate and manageable subset of particle-hole excitations.We present a precise determination of E2 strength in 22 Mg and its mirror 22 Ne by Coulomb excitation, allowing for rigorous comparisons with theory. No-core symplectic shell-model calculations were performed and agree with the new B(E2) values while in-medium similarity-renormalization-group calculations consistently underpredict the absolute strength, with the missing strength found to have both isoscalar and isovector components.