Nuclear Collectivity Studied through High-precision Mass Measurements of Neutron-rich Argon and Chromium Isotopes

Due to their inherent relationship with the binding energy, nuclear masses are the fingerprint of all the interactions taking place within the nucleus. As such, precise and accurate mass values are an essential ingredient to the comprehensive understanding of nuclear phenomena in exotic regions of the chart of nuclides. In this thesis, two key regions exhibiting dramatic structural evolution are investigated by means of high precision mass measurements performed with the online mass spectrometer ISOLTRAP at ISOLDE/CERN. Numerous spectroscopy results indicate that the chromium isotopic chain exhibits the most dramatic structural changes within the region situated south of ⁶⁸Ni. This thesis reports on the first high-precision mass measurements of the neutron-rich ^58-63Cr isotopes using the well-established Penning trap mass spectrometry technique as well as the MRToF-MS technique pioneered at ISOLTRAP in recent years. The obtained mass values are up to 300 times more precise than the ones currently available in the literature. At odds with previous results, the new mass values exclude a sudden onset of ground state collectivity rather favouring a smooth transition towards deformation approaching N=40. The question of the persistence of the N=28 shell closure in the Argon chain is also studied in this PhD work through the measurement of the neutron-rich ^46-48Ar isotopes. The results of improved precision confirm the presence of a strong N=28 shell closure in the Argon chain. For both datasets, the detailed data analysis procedure will be presented. The implication of the obtained mass values for nuclear structure will be discussed through a phenomenological discussion of the binding energy trend. The results will also be discussed in the light of state of the art nuclear models including results from the promising valence-space formulation of the ab-initio IM-SRG formalism. (author)