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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.

Shape coexistence in the light krypton isotopes was studied in two low-energy Coulomb excitation experiments using radioactive 74 Kr and 76 Kr beams from the SPIRAL facility at GANIL. The ground-state bands in both isotopes were populated up to the 8 + state via multi-step Coulomb excitation, and several non-yrast states were observed. Large sets of matrix elements were extracted for both nuclei from the observed γ -ray yields. Diagonal matrix elements were determined by utilizing the reorientation effect. In both isotopes the spectroscopic quadrupole moments for the ground-state bands and the bands based on excited 0 + 2 states are found to have opposite signs. The experimental data are interpreted within a phenomenological two-band mixing model and model-independent quadrupole invariants are deduced for the relevant 0 + states using the complete sets of matrix elements and the formalism of quadrupole sum rules. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and are compared both with the experimental results and with recent calculations using the Skyrme SLy6 effective interaction and the full generator-coordinate method restricted to axial shapes.

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The pygmy dipole resonance has been studied in the proton-magic nucleus 124 Sn with the (α, α ′ γ) coincidence method at Eα = 136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α ′ γ) as well as in (γ, γ ′ ) reactions and a group of states at higher energies which is only excited in (γ, γ ′ ) reactions. Calculations with the self-consistent relativistic quasiparticle timeblocking approximation and the quasi-particle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.PACS numbers: 24.30.Cz, Collective phenomena are a common feature of strongly interacting many-body quantum systems directly linked to the relevant effective interactions. Atomic nuclei also show collective behavior. One example is given by the giant resonances, which have been investigated intensively using different experimental methods, see e.g., [1]. The isovector electric giant dipole resonance (IVGDR) has been the first giant resonance to be observed in atomic nuclei. Ever since it has been of particular interest, because collective E1 response is related to symmetry breaking between neutrons and protons. In recent years, the so-called pygmy dipole resonance (PDR) [2][3][4], a concentration of electric dipole strength energetically below the IVGDR, has been studied intensively in various nuclei. Within most modern microscopic nuclear structure models, this new excitation mode is related to the oscillation of a neutron skin against a symmetric proton-neutron core with isospin T = 0; for an overview see the recent review by Paar et al. [5]. Consequently, one expects an increase of the PDR strength approaching isotopes with extreme neutron-to-proton ratios. Experiments on radioactive neutron-rich nuclei seem to support this assumption [6][7][8][9][10][11]. If this picture holds, the strength of the PDR is related to the thickness of the neutron skin and the density dependence of the symmetry energy of nuclear matter [7,12]. The PDR thus permits experimental access to these properties. However, more consistent systematic investigations and especially more constraints on the structure of the PDR are mandatory, such as the experiments presented in this Letter, in order to confirm this picture.Up to now only experiments on stable nuclei allow more detailed investigations of the PDR which yield additional observables in order to understand the underlying structure of this new excitation mode. In nuclear resonance fluorescence (NRF) experiments the systematics of the PDR as well as its fragmentation and fine-structure can be studied [3,4,[13][14][15][16][17][18][19] up to the particle threshold. The mean excitation energy and the summed transition strength B(E1)↑ (of up to 1% of the isovector energy weighted sum rule) show a smooth variat...