DATA: 06-04-2017

The way how the complex nuclear forces form atomic nuclei from their constituent protons and neutrons represents the core topic of nuclear structure research. The evolution of nuclear structure across the nuclear chart is dominated by the evolution of single-particle orbitals as a function of the numbers of protons and neutrons that make up the nuclei. The understanding of shell evolution has been pushed especially by the Tokyo group around T. Otsuka. It is supported by vast experimental evidence in particular from the structure of neutron-rich nuclei that have become accessible due to new facilities for intense beams of radioactive ions. Recently, the concept of Type II Shell Evolution has been introduced. It emphasizes, that the effective single-particle energies of nucleon orbitals systematically depend on the nucleonic configuration in each individual nuclear state due to the mutual interaction between the nucleons. This general mechanism can be particularly pronounced in key-nuclei with certain numbers of protons and neutrons, for which Type II Shell Evolution may lead to drastic nuclear structure effects such as even shape coexistence and first-order shape phase transitions. We report here on the first evidence [1] for shape coexistence caused by Type II Shell Evo- lution which is firmly based on the measurement of absolute E2 transition rates. The data have been obtained in high-resolution inelastic electron scattering spectroscopy of the nucleus 96Zr at the Superconducting Darmstadt Linear electron Accelerator (S-DALINAC). The data [1] will be presented and discussed. The neutron sub-shell closure at neutron number N = 56 plays a crucial role for the occurence of shape coexistence in 96Zr. We have studied the structure of exotic neutron-rich isotopes near the N = 56 neutron sub-shell closure using -ray spectroscopy and radioactive beams. The new data suggest the hitherto unknown existence of a region of pronounced nuclear triaxiality in neutron-rich Ge isotopes. We will present and discuss spectroscopic data on 84,86,88Ge taken [2] recently at RIBF, RIKEN. If time permits, other developments on precision studies of E2 collectivity, such as the evolution of E2 strength in Sn isotopes or the E2 strength of the 1+ scissors mode of heavy deformed nuclei will be reported, too.


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