La Fermeture de couche N=28 : coexistence de formes et contribution Spin-orbite

One of the fundamental questions, which emerge from the study of nuclei far from stability, concerns the persistance of the magic character of certain configurations of protons and neutrons. From previous measurements arond the N=28 magic number, it appears that this shell closure is especially weakening.In this context, a mass measurement experiment by a time of flight method arond N=28 (Z<20) with the SPEG spectrometer at GANIL has brought some new decisive observations to understand this phenomenum. More than thirty masses have been obtained in the region of interest, 8 of them measured for the first time. The analysis shows clearly the existence of a pseudo-shell closure at N=26. The comparison of these results with shell model and relativistic mean field calculations allow us to attribute the vanishing of the N=28 shell closure to deformation effects and shape coexistence predicted in this region. The evidence of an isomeric state in the 43S in the same experiment and its interpretation by a shell model calculation confirm the analysis of the masses and constitutes the first evidence of shape coexistence around N=28.At the same time, an estimation of the evolution of the contribution of the spin-orbit coupling far from stabilty, partially responsible of the magic numbers sequence, showed that, althought non-negligible, it is not sufficient to explain the vanishing of the shell closure. Through this study, it appeared extremely difficult to separate the contribution of the deformationfrom the one of the spin-orbit coupling in spectroscopic experiments. A feasibility study has thus been undertaken concerning a polarised proton and deuteron target to measure directly the evolution of the spin-orbit potential as a function of the isospin through elastic scattering experiments.Une des questions fondamentales qui émergent de l'étude des noyaux loin de la stabilité concerne la persistance du caractère magique de certaines configurations de protons et de neutrons. A la suite de mesures réalisées autour du nombre magique N = 28 pour des noyaux riches en neutrons, il apparaît que cette fermeture de couche semble particulièrement affaiblie. Dans ce contexte, une expérience de mesures de masses par temps de vol de noyaux autour de N = 28 (Z