Evolution of the N = 20 and N = 28 shell closures in 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, $^{32}$Mg 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 $N-Z$ $\geq$ 10 whereas deformed shell closures are preserved and lead to shape coexistence in $^{44}S$. 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 $B(E2;0^+_{\rm gs}\rightarrow 2^+_{1})$= 420 $e^2 {fm}^4$ obtained with our approach are in good agreement with experimental data