Varying shell gap and deformation in N ∼ 20 unstable nuclei studied by the Monte Carlo shell model -: art. no. 054315

The structure of neutron-rich nuclei in the N similar to 20 region is studied by the Monte Carlo shell model based on the quantum Monte Carlo diagonalization method. We present a comprehensive description of even-even isotopes of O, Ne, Mg, and Si. It is demonstrated that, for different nuclei, various particle-hole excitations from the sd to pf shell are mixed in different ways, producing distinct effects sometimes, for instance, in Ne-28. The monopole interaction is examined and modified, resulting in the shell gap changing from nucleus to nucleus. The drip line of O isotopes is then reproduced The interplay between the T=0 and T=1 monopole interactions is discussed from the Viewpoint of the potential energy surface and the effective single-particle energy. The extension of the neutron drip line of Ne isotopes is explained, and the boundary of the "island of inversion'' is shown to be rather indistinct. [S0556-2813(99)00211-3].