Recent experimental data on the structure of the very-neutron-rich isotopes of Na and Mg show sharp departures at $N=20\mathrm{}$ from the predictions of conventional $d_{\frac{5}{2}}-s_{\frac{1}{2}}-d_{\frac{3}{2}}$ shell-model calculations.

NUCLEAR STRUCTURE Isotopes of Na and Mg; calculated ground state energies, magnetic dipole moments, and excitations of $2^{+}$ states; $0d_{\frac{5}{2}}-1\mathrm{}{s}_{\frac{1}{2}}-0\mathrm{}{d}_{\frac{3}{2}}$ shell model, Chung-Wildenthal Hamiltonians.

• Received 31 March 1980

DOI:https://doi.org/10.1103/PhysRevC.22.2260