Etude des noyaux riches en neutrons autour de N=28

For few years now, it has been suggested and demonstrated that deformation plays an important role in the evolution of the shell structure of the neutron-rich nuclei below $^{48}$Ca at N=28 [1]-[5]. This presence of deformation has been explained in S isotopes by a moderate reduction of the N=28 shell closure together with a proton induced collectivity originating from the near degeneracy of the PI d3/2 and PI s1/2 orbitals [6]. As a consequence, the observed deformation is the result of a subtle interplay between neutron and proton excitations and the importance of the N=28 shell closure reduction is then difficult to estimate. Since the proton configuration in the Si isotopes is expected to be more stable due to the Z=14 sub-shell gap [7], the $^{42}$Si was considered as a key nucleus in order to distinguish the different effects responsible of the structural changes observed at N=28. That is why, we performed at GANIL an in-beam gamma-spectroscopy experiment which aimed to extract the energy of the $^{42}$Si first 2$^{+}$ excited state, which traduces the difficulty or not of exciting this nucleus and gives information about the shell closure importance. The detailed analysis of these data and our understanding of the N=28 shell closure evolution up to the neutron drip-line will be presented. [1