Abstract
The N=20 and N=28 “islands of inversion” are described by large scale shell model calculations which mix configurations with different Nħ ω or equivalently with different number of particles promoted from the sd -shell to the pf -shell. We pay particular attention to the properties of the states at fix Nħ ω which turn out to be the real protagonists of the physics at N=20. In particular we study the appearance of spherical, deformed and superdeformed structures in 32 Mg at fixed 0p-0h, 2p-2h and 4p-4h configurations. Finally we submit that the islands of deformation at N=20 and N=28 merge in the Magnesium isotopes
Highlights
At the neutron rich edge, the structure of the spherical mean field is at variance with the usual one at the stability line
If the shell gaps get reduced, intruder configurations, usually two neutron excitations across the neutron closure, take advantage of the availability of open shell protons to build highly correlated states that are more bound than the closed shell configuration
In the framework of the shell model, the deformation in the region was soon associated with the dominance of two-particle -two-hole (2p − 2h) excitations across the N = 20 shell gap between the normally occupied neutron d3/2 orbit and the valence f7/2 and p3/2 orbits [7]
Summary
At the neutron rich edge, the structure of the spherical mean field is at variance with the usual one at the stability line. If the shell gaps get reduced, intruder configurations, usually two neutron excitations across the neutron closure, take advantage of the availability of open shell protons to build highly correlated states that are more bound than the closed shell configuration.
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