First attempt toward a quasi-Pandemonium free β-delayed spectroscopy of ⁸⁰Ge using PARIS at ALTO

The β-decay, a precisely described process at the most elementary (quark) level, becomes a rich and complex probe when involved in the strongly interacting multi-nucleon medium which is the atomic nucleus. In this correlated system the transformation of a neutron into a proton (or vice versa) described by the Fermi and Gamow-Teller operators gives rise to collective states involving coherently a large fraction of the nucleons under the form of a giant resonance. Close to stability, for neutron-rich nuclei, these giant resonances lie far above the energy window accessible in the β-decay (Q_β), and this decay must proceed through tail components of these resonances in competition with first-forbidden transitions. However, the advent of a new generation of Radioactive Ion Beam facilities making possible β-decay studies of nuclei with ever larger Q_β windows, beyond 10 MeV, now changes this perspective. For instance β-decay of exotic nuclei can provide a new and unique method to investigate the more neutron-skin-sensitive collective motion Pygmy Dipole Resonance (PDR) and also an opportunity for a deeper understanding of exotic phenomena involved in nuclear β-decay through precise measurement of the Gamow-Teller transition strength distribution, B(GT). The objective of this thesis is to improve the understanding of the structure in the neutron threshold (S_n) region of exotic nuclei and the influence of this structure on the B(GT) by studying a mixed ^80g+mGa source collected at the BEDO decay station of the ALTO Isotope Separation On Line facility in Orsay. The ^80gGa and ^80mGa β-decaying states having spin-parity 6^- and 3^- respectively, this source has the advantage that it can populate an unusually large spin-range of daughter states in the nucleus ⁸⁰Ge. The goal of this work therefore covers three aspects: investigate the PDR in ⁸⁰Ge, disentangle the decay level schemes fed by the two β-decaying states of ⁸⁰Ga and achieve a precise determination of the associated B(GT) in the whole Q_β window. To achieve this goal, a hybrid gamma spectrometer composed of PARIS NaI/LaBr₃ phoswiches associated to high resolution Ge detectors was used. A detailed analysis is presented in this manuscript, the results suggest a manifestation of PDR in ⁸⁰Ge. Separated decay level schemes and B(GT) distributions of ^80g+mGa are proposed for the first time. In addition, through comparison with state-of-the-art nucleons-blocked quasiparticle random-phase approximation (QRPA) calculations, the microscopic mechanism of PDR in ⁸⁰Ge is discussed. (author)