Deformation of Nuclei Close to the Two-Neutron Drip Line in Mg Region

We perform the Hartree-Fock-Bogoliubov (HFB) calculations for ground states of even Mg isotopes using the Skyrme force and a density-dependent zero-range pairing force. The HFB equation is solved in a three-dimensional cartesian mesh, and a convergence of deformation is carefully examined with respect to a cut-off radius for a check of the calculations. We discuss systematics of the two-neutron separation energy, deformation and root-mean-square radius. We have found that 36,38,40Mg have appreciable static deformation, where 40Mg is a two-neutron drip-line nucleus in our calculation, and the deformations of the neutron and proton are different in these three nuclei. The deformation property is analyzed on the basis of the single-particle diagram. It is shown that N=28 is not a closed shell in Mg as well as Si.