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.

The Coulomb excitation experiment to study electromagnetic properties of the heaviest stable Mo isotope, 100 Mo, was performed using a 76 MeV 32 S beam from the Warsaw cyclotron U-200P. Magnitudes and relative signs of 26 E1, E2, E3, and M1 matrix elements coupling nine low-lying states in 100 Mo were determined using the least-squares code GOSIA. Diagonal matrix elements (related to the spectroscopic quadrupole moments) of the 2 + 1 , 2 + 2 , and 2 + 3 states as well as the 4 + 1 state were extracted. The resulting set of reduced E2 matrix elements was complete and precise enough to obtain, using the quadrupole sum rules approach, quadrupole deformation parameters of 100 Mo in its two lowest 0 + states: ground and excited. The overall deformation of the 0 + 1 and 0 + 2 states in 100 Mo is of similar magnitude, in both cases larger compared to what was found for the neighboring isotopes 96 Mo and 98 Mo. At the same time, the asymetry parameters obtained for both states strongly differ, indicating a triaxial shape of the 100 Mo nucleus in the ground state and a prolate shape in the excited 0 + state. Low-energy quadrupole excitations of the 100 Mo nucleus were studied in the frame of the general quadrupole collective Bohr Hamiltonian model (GBH). The potential energy and inertial functions were calculated using the adiabatic time-dependent Hartree-Fock-Bogoliubov (ATDHFB) method starting from two possible variants of the Skyrme effective interaction: SIII and Sly4. The overall quadrupole deformation parameters resulting from the GBH calculations with the SLy4 variant of the Skyrme interaction are slightly closer to the experimentally obtained values than those obtained using SIII.

Article:Bree, N., Wrzosek-Lipska, K., Petts, A. et al. (67 more authors) (2014) Shape coexistence in the neutron-deficient even-even 182-188Hg isotopes studied via Coulomb excitation.

The x-ray cascade from antiprotonic atoms was studied for 208 Pb and 209 Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modern antiproton-nucleus optical potentials, the neutron densities in the nuclear periphery were deduced. Assuming two-parameter Fermi distributions (2pF) describing the proton and neutron densities, the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii r np equal to 0.16 ± (0.02) stat ± (0.04) syst fm for 208 Pb and 0.14 ± (0.04) stat ± (0.04) syst fm for 209 Bi were determined, and the assigned systematic errors are discussed. The r np values and the deduced shapes of the neutron distributions are compared with mean field model calculations.