Evolution of the N=20 and N=28 shell closures neutron-rich nuclei

The isospin dependence of shell closure phenomena is studied for light neutron-rich nuclei within a microscopic self-consistent approach using the Gogny force. Introducing configuration mixing, Mg-32 is found to be dynamically deformed, although the N = 20 spherical shell closure persists at the mean-field level for all N = 20 isotones. In contrast, the N = 28 spherical shell closure is found to disappear for N-Z greater than or equal to 10 whereas deformed shell closures are preserved and lead to shape coexistence in S-44. Configuration mixing shows that the ground state of this nucleus is triaxially deformed. The first 2(+) excitation energy E-x = 1.46 MeV and the reduced transition probability B(E2; 0(g)(s+), --> 2(1)(+))= 420 e(2) fm(4) obtained with our approach are in good agreement with experimental data.