Physics Letters B 647 (2007) 93–97 www.elsevier.com/locate/physletb Measurement of the electric quadrupole moment of 32 Al D. Kameda a,∗ , H. Ueno a , K. Asahi a,b , M. Takemura b , A. Yoshimi a , T. Haseyama a , M. Uchida b , K. Shimada b , D. Nagae b , G. Kijima b , T. Arai b , K. Takase b , S. Suda b , T. Inoue b , J. Murata d , H. Kawamura d , Y. Kobayashi a , H. Watanabe c , M. Ishihara a a RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan c Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra AT 0200, Australia d Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan Received 6 November 2006; received in revised form 17 January 2007; accepted 23 January 2007 Available online 15 February 2007 Editor: D.F. Geesaman Abstract The electric quadrupole moment Q for the ground state of 32 Al has been measured using the β -NMR technique. Spin-polarized 32 Al nuclei were obtained from the fragmentation of 40 Ar projectiles at E/A = 95 MeV/nucleon, and were implanted in a single crystal α-Al 2 O 3 stopper. The quadrupole moment was deduced from the measured quadrupole coupling constant. The obtained value, |Q( 32 Al g.s. )|= 24(2) mb, is remarkably small as compared with other Al isotopes, but is well explained by shell model calculations within the sd shell. The present result indicates that 32 Al has a normal sd-shell structure, in contrast to the neighboring N = 19 isotone 31 Mg for which a strongly deformed intruder ground state has been reported to occur.  2007 Elsevier B.V. All rights reserved. PACS: 21.10.Ky; 24.70.+s; 23.20.En; 21.60.Cs Keywords: Electric quadrupole moment of 32 Al; Projectile fragmentation; β -NMR; Nuclear deformation; Island of inversion Among neutron-rich Ne, Na and Mg isotopes, those with neutron numbers around N = 20 are reported to show large de- formations that cannot be explained by the conventional shell model assuming the N = 20 shell closure [1–4]. The anom- alous deformations are reproduced by introducing conﬁgura- tions in which some neutrons are allowed to occupy the pf orbits via neutron excitations from the sd orbits [5–8]. Such a region where the sd-shell closure deteriorates and an intruder conﬁguration dominates the ground-state structure is known as the “island of inversion” [8]. Studies of the electromagnetic moments of nuclei in the vicinity of the island [9–13] have played an important role in elucidating the process of transi- tion from sd-normal to pf -intruder structures. In the case of neutron-rich Na isotopes, the magnetic moments μ of 30,31 Na, * Corresponding author. E-mail address: kameda@ribf.riken.jp (D. Kameda). and the electric quadrupole moments Q of 29–31 Na were re- ported to signiﬁcantly deviate from the conventional sd-shell model calculations, in contrast to the case of lighter Na iso- topes where the μ and Q moments are reasonably reproduced within the sd shell [9,10]. Later, these differences were consis- tently treated in Monte Carlo shell model (MCSM) calculations including the pf orbits [14]. According to the MCSM calcula- tions, the pf -intruder conﬁguration dominates the ground state of 31 Na (N = 20) and 30 Na (N = 19), while it substantially mixes with the sd-normal conﬁguration in the 29 Na (N = 18) ground state. The calculation on 29 Na further indicates that the value of the Q moment is appreciably affected by such a mid- way mixing of the pf orbits while that of the μ moment remains rather insensitive. The calculations in Ref. [14] also shed light on the role played by the effective energy gap between the sd and pf shells, that varies with Z and N and drives the varia- tion of the nuclear wave function from the normal to intruding conﬁgurations. 0370-2693/$ – see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2007.01.063