Nuclear structure in the neutron-deficient region investigated via lifetime measurements

Abstract:  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 behavior 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 which present a flatter trend of the B(E2; 2+→0+). 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 along the whole Sn isotopic chain and also to provide complementary information to the existing Coulomb excitation measurements, the lifetime of the 21+ and 41+ excited states were measured for the first time in 106,108Sn via the Recoil-Distance Doppler-Shift (RDDS) method, populating the nuclei of interest via a multi- nucleon transfer reaction. Thanks to the crucial role of the measured B(E2; 4+→2+) values, the comparison between the new experimental results with the state-of-the-art Large-Scale Shell-Model (LSSM) calculations has shown for the first time the limitation of the seniority truncation approach and the need of an extended valence space to correctly describe the region. 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 188Hg is of a great interest for a better comprehension of the evolution of the collective properties in this mass region. The preliminary experimental results have been compared with the state-of-the-art beyond mean-field calculations performed with the symmetry-conserving configuration-mixing method, which confirms the coexistence of two deformed structures for the nucleus of interest.