Search

To describe the nuclear structure of one or a group of nuclei several observables should be consulted. The excitation energies of states with given spin and parity is often known and gives a good first impression. Nevertheless transition strengths of excited states are a much better probe, since they depend quadratically on the wavefunction. These can be determined in a model-independend way from the corresponding level lifetime. Within this work a total of four experiments were performed to measure the lifetime of excited states in 180Pt and 178Hg. In the neutron-deficient mass region close to the shell closure at lead (Z=82) interesting phenomena such as shape coexistence and shape transitions are observed. The most prominent example is 186Pb, which exhibits three different shapes. The ground state as well as two excited 0+ states together with the corresponding rotational bands are associated to multiparticle-multihole excitations resulting in the different shapes. A similar situation is established for the Hg isotopes near the neutron N=104 midshell, where the lightest isotope for which transition strengths are known is 180Hg. Going further away from the shell closure the 176,178,180Os isotopes are good candidates for the X(5) symmetry, i.e. the transition from spherical to axially deformed. Two questions arise: How can the Pt isotopes just between Os and Hg be described and what is the evolution of shape coexistence in the Hg isotopes leaving neutron midshell? Both questions are addressed within this thesis. 180Pt shows no remains of a X(5) like shape transition and multiparticle-multihole excitations are evident. This structure reaches the ground state and a new so-called island of inversion is found. This simultaneously answers the longlasting question whether configuration mixing is needed to describe the Pt isotopes or not. From the excitation energies 178Hg is expected to be at the edge of the shape-coexistence region. The measured transition strengths confirm the shift of the structure with different shape to higher-lying states, but also a new phenomenon of intermediate deformation (between normal and superdeformation) shows up with decreasing neutron number. The interpretation is given by comparison to several existing calculations as well as particular calculations using geometrical models and the interacting boson model. The experiments were carried out at different accelerator facilities employing the recoil distance Doppler-shift (RDDS) method, except one experiment when electronic-timing techniques were used to measure long lifetimes. Further a dedicated plunger apparatus was commissioned during one experiment. The GALILEO -ray spectrometer is the successor of GASP, which necessitated a completely new mechanical design of the plunger. This fragile device newly allows the combination with charged-particle detectors, e.g. EUCLIDES.