Coulomb excitation of low-lying states in 66Zn with the SPIDER array

Low-energy Coulomb excitation provides a well-understood method of exciting atomic nuclei, allowing the study of the electromagnetic properties of their low-lying excited states. In particular, for collective excitations, these properties are very sensitive to the underlying nuclear shape and their experimental study contributes significantly to the understanding of the nuclear many-body problem. The development of the low-energy Coulomb excitation technique at the INFN Legnaro National Laboratories is particularly important in view of the realization of the radioactive ion beam facility SPES, since the large cross-section of the Coulomb excitation process can compensate the typical low intensity of the beams. An experimental setup for low-energy Coulomb excitation measurements typically consists of an array of γ-ray detectors coupled to a segmented heavy ion detector, thereby allowing the reconstruction of the scattering kinematics for particle-γ coincidence events. The subject of this thesis is the set up of a new heavy-ion detector for low-energy Coulomb excitation measurements, SPIDER (Silicon PIe DEtectoR), and its first use in an experiment aimed to investigate the structure of the low-lying states in 66Zn. Some of the electromagnetic properties of the low excited states in this nucleus were already known to a high accuracy, offering an important benchmark to test the performances of the new setup. The experiment also provided a set of new experimental data, helping to solve the open questions about the structure of the 41+ and 22+ states in 66Zn and allowing the first study of the properties of the 02+ state. Reduced transition probabilities and spectroscopic quadrupole moments have been deduced from the experimental γ-ray yields using the GOSIA code. The results have been compared to the predictions of state-of- the-art “beyond mean field” and shell model calculations, showing that 66Zn cannot be simply considered as a vibrational or rotational nucleus, as previously claimed. Indeed, microscopic degrees of freedom have to be included in its description and triaxiality plays an important role in determining its low-lying structure.