  The onset of intruder ground states in Na isotopes is investigated by
comparing experimental data and shell-model calculations. This onset is one of
the consequences of the disappearance of the N=20 magic structure, and the Na
isotopes are shown to play a special role in clarifying the change of this
magic structure. Both the electromagnetic moments and the energy levels clearly
indicate an onset of ground state intruder configurations at neutron number
N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes
resulting from the spin-isospin dependence of the nucleon-nucleon interaction
(as compared to a wider gap in stable nuclei like 40Ca). It is shown why the
previous report based on the mass led to a wrong conclusion.
