Ground-state electric quadrupole moment of Al31

levels are lowered [3, 4] and theirB(E2) values are enhanced [5] sizably in these isotopes,and the possibility of deformation has been proposed.Theoretical analyses [6] discussed the importance of 2p-2h excitations from the sd shell to the upper pf shell,and concluded it plausible that an inversion of ampli-tudes between the sd normal and pf intruder configura-tions would lead to deformation of the ground states.The region of nuclei where such a phenomenon occurs iscalled the island of inversion. In elucidating the under-lying mechanism for the inversion, the measurements ofthe electromagnetic moments have played an importantrole. For example in a series of neutron-rich Na isotopes,it has been found that, once entering the island of in-version, the ground-state magnetic dipole moment µ andelectric quadrupole moment Q [7, 8] show clear devia-tions from the conventional shell-model predictions [9],indicating that µ and Q are sensitive to changes in thenuclear configuration [10]. Also in the recent study of Mgisotopes, anomalous ground-state properties have beenrevealed through the µ-moment measurements [11, 12].In the present work, the ground-state Q moment of