LABORATORI NAZIONALI DI LEGNARO
The recent technical upgrade of the large-scale experimental facilities combined with the boost in development of the advanced instrumentation (e.g. Ge tracking arrays), together with the quantitative and qualitative progress in the excellence of theoretical models, have resulted in remarkable gains in our understanding of the structure of the atomic nucleus. Thanks to the expected completion of the large RIB facilities in the USA and in Europe the following decade will bring us even closer to the frontiers of the nuclear physics. The essential information on the single-particle energies and two-body residual interactions can be derived from the experimental observables, such as excitation energies and reduced transition probabilities, and it can be used to estimate the nuclear structure of more complex configurations. Transition probabilities, especially B(E2) values, give particularly valuable insights into the nature of nuclear collectivity and its evolution with respect to the neutron and proton numbers. For almost all the even-even nuclei and in particular for those near the shell closures, while augmenting the number of valence nucleons from the shell closure, the reduced transition probability B(E2; 2+→0+) increases progressively until a maximum value is reached around the mid-shell, where the number of degrees of freedom is maximum. The deviation from this parabolic behaviour could be a fingerprint of the creation/disappearing of a shell closure or of the presence of other phenomena, such as shape coexistence. In this work the evolution of the collectivity has been studied via lifetime measurements in two crucial neutron-deficient regions. In the first case, the robustness of the proton shell closure Z=50 has been investigated in the exotic nuclei towards doubly-magic 100Sn. In order to overcome the experimental limitations induced by the presence of low-lying isomers present in the region and also to provide complementary information to the existing Coulomb excitation measurements, lifetime of the low-lying excited states were measured in light Cd and Sn nuclei via the Recoil-Distance Doppler-Shift (RDDS) method, populating the nuclei of interest via a multi-nucleon transfer reaction. In the second case, the shape coexistence has been studied in the neutron-deficient nuclei in the vicinity of Z=82 shell closure. In particular, the systematic studies of the low-lying structures of mercury isotopes have indicated 188Hg to be the heaviest isotope manifesting the fingerprints of this phenomenon. However, the information on the electromagnetic properties of low-lying states in this isotope is scarce or absent. Thus, the measurement of the lifetimes of excited states in light Hg is of a great interest for a better comprehension of the evolution of the collective properties in this mass region.

DATA: 08-07-2019