New element synthesis and spectroscopic studies of nobelia 255 and 256

Understanding the nuclear structure and the underlying interactions is one of the major challenges in modern physics. The experimental information on the existence and the quantum characteristics of nuclei far from the stability valley places strong constraints on theoretical models. With new experimental devices allowing us to push back the current limits, this thesis addresses the question of the existence of superheavy elements beyond Oganesson (Z=118) and the study of nucleonic orbitals in the Nobelium region by fine spectroscopy. Since 2004, our team has been participating, in collaboration with the IJClab and FLNR teams, in multiple experimental campaigns with the SHELS separator and the GABRIELA detection array in Dubna. In this context, ²⁵⁶No was synthesized using the hot fusion reaction ²³⁸U(²²Ne, 4n)²⁵⁶No. Fifteen decays of an unknown high-K isomer were observed, allowing to measure a 7.8^+8.3_-2.6 μs half-life and an excitation energy of at least 1089 ± 74 keV. The study of ²⁵⁵No, synthesized using the ²⁰⁸Pb(⁴⁸Ca, 1n)²⁵⁵No cold fusion reaction, revealed four high-K isomers in cascade in this nucleus. Their characteristics and their interpretation, in connection with the neighboring nuclei systematics, are discussed in this manuscript. This spectroscopic study enabled to propose for the first time a level scheme for this nucleus. Since 2017, we have been trying to synthesize element 119 at RIKEN through the fusion-evaporation reaction ²⁴⁸Cm(⁵¹V, xn)^299-x119 as part of the nSHE collaboration (IPHC, ORNL, RIKEN) led by K. Morita. This thesis details the methods of filtering and chain search strategy developed specifically for this experiment. Since the data acquisition is still ongoing and the preliminary results are confidential, these analysis methods developed are illustrated by spectra from the calibration reactions and the validation run of the entire GARIS II device: the synthesis of ²⁵⁷Db. The analysis code was then used and refined during the seven experimental campaigns paving the way of this thesis. (author)