SHELL-MODEL STUDY OF THE NEUTRON-RICH ISOTOPES FROM OXYGEN TO SILICON

In this paper we extend the shell model calculations previously made f or the very neutron-rich nuclei with Z greater than or equal to 14 to Al, Mg, Na, Ne, F, and O, using the same valence space and effective i nteraction. Predictions are made for the neutron separation energies a nd for the location of the neutron drip line. We find that the isotope s of Ne, Na, and Mg are deformed for N greater than or equal to 22. In Mg-40, which is at the edge of the drip line, the N = 28 shell. closu re does not stand. By enlarging the valence space to include intruder states we are able to account for the vanishing of the N = 20 neutron shell closure in a small region around Na-31 that we delineate. The do minance of the intruders explains the collective features experimental ly found in this region. [S0556-2813(98)04309-X].