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 breaking of the N=8 shell-model magic number in the 12Be ground state has been determined to include significant occupancy of the intruder d-wave orbital. This is in marked contrast with all other N=8 isotones, both more and less exotic than 12Be. The occupancies of the [FORMULA: SEE TEXT]orbital and the [FORMULA: SEE TEXT], intruder orbital were deduced from a measurement of neutron removal from a high-energy 12Be beam leading to bound and unbound states in 11Be.

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The structure of 17−20 6 C nuclei was investigated by means of the in-beam γ-ray spectroscopy technique using fragmentation reactions of radioactive beams. Based on particle-γ and particle-γγ coincidence data, level schemes are constructed for the neutron rich 17−20 C nuclei. The systematics of the first excited 2 + states in the Carbon isotopes is extended for the first time to A=20 showing that in contrast to the case of the oxygen isotopes, the N =14 subshell closure disappears. Experimental results are compared with shell-model calculations. Agreement between them is found only if a reduced neutron-neutron effective interaction is used. Implications of this reduced interaction in some properties of weakly bound neutron-rich Carbon are discussed. The formation of nuclear shell gaps, as well as their collapse in certain regions of the chart of nuclides is largely being investigated worldwide. It impacts many unique features in nuclear physics as the abundance of the stable elements in the universe, the possible existence of an island of super heavy nuclei, the route of heavy nuclei to fission and the existence of cluster configurations.It is a remarkable fact that the shell (or subshell) gaps, such 14, 28, 50 and to a weaker extent 82 and 126 share a common origin. Taking for instance the neutron shell gaps, they are formed by the combined action of the spin-orbit (SO) force and by neutron-neutron interactions. The former force significantly over binds the orbit in which the angular momentum and intrinsic spin are aligned (denoted as ↑ ). In addition, the filling of neutrons inside this orbit amplifies its binding due to the attractive neutron-neutron V nn ↑ ↑

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