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The study of the N=28 shell closure : a way to probe nuclear forces

Olivier Sorlin, GANIL Caen
Mercredi 10 juin 2009 à 10h30 - Salle des Séminaires

The study of the evolution of the N=28 magic shell has been started about 15 years ago. A lot of experimental studies aimed at determining whether the N=28 shell closure is eroded in very neutron-rich nuclei through complementary methods. The very first hints of the vanishing of the N=28 gap were obtained by beta-decay, mass measurements and Coulomb-excitation experiments. Theoretical studies came progressively to the conclusion that the erosion of the N=28 shell gap should lead to shape coexistence in 44S and deformation in 42Si. Recent experimental campaigns were carried out worldwide to study atomic masses,beta-decay, nuclear spectroscopy, neutron single-particle energies to probe the onset of collectivity along the N=28 isotones. With these pieces of information in hand and new unpublished results, the progressive collapse of the N=28 shell closure is now established. Interestingly this study is also ideal for probing the nuclear forces such as the spin-orbit and tensor forces in nuclei. More generally, the shell-breaking mechanism discovered at N=28 should apply to other shell closures, such as the N=14, N=50 and N=82 ones, in which the same spin orbit forces are at play. However they seem to have various consequences with respect to shell erosion...

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