For the first time, we obtained the g factor for the ground state of 23 Al by use of a β-NMR measurement. 23 Al has a small proton separation energy and is a potential proton-halo candidate. The obtained g factor, |g| = 1.557 ± 0.088, clearly shows the spin and parity, J π = 5/2 + , for 23 Al, which is the same as that of its mirror partner, 23 Ne. The possible nuclear structure of 23 Al is also discussed. Studies of exotic nuclei far from the stability line have attracted much interest during the past two decades since the first observation of the neutron halo in 11 Li [1]. Further experiments have confirmed the neutron halo in 11 Li, and have also shown the existence of a neutron halo in other neutron-rich nuclei [2]. However, the number of experiments on the proton halo are relatively few compared with those on the neutron halo. Recently, experimental evidence for the proton halo in very proton-rich nuclei has become available: 8 B [3][4][5], 17 Ne [6,7], and 26,27,28 P [8]. 23 Al is one of the potential candidates for a proton-halo nucleus, because its proton separation energy is quite small (125 keV). Recently, a large reaction cross section of 23 Al was measured at the intermediate energy (∼36A MeV), although the error was still large [9,10]. The authors of Ref. [9,10] analyzed the data by a Glauber model with simply a core nucleus ( 22 Mg) plus a valence proton (p) configuration. According to their analysis, the density distribution of 23 Al should have a halolike long tail. Because the density of a loosely bound s-wave proton can be extended, it is suggested that the spin and parity (J π ) of 23 Al might be 1/2 + . However, the J π of its mirror partner, 23 Ne, is 5/2 + . Thus, if the ground-state J π of 23 Al is 1/2 + , the proton halo structure can be anticipated, but the mirror symmetry will be broken between the bound ground states of mirror pairs, which has not been observed for known nuclei so far. Because the magnetic moment of its mirror partner, 23 Ne, is known to be −1.077±0. 004 [11], the magnetic moment of 23 Al allows one to discuss the spin expectation value [12] of the A = 23 mirror pair with T = 3/2, where T is the isobaric spin. In p-shell nuclei, the 9 C-9 Li mirror moments (T = 3/2) yield an anomalous large spin expectation value, which suggests an exotic structure of proton-rich 9 C [13]. A similar anomaly may be found in the sd-shell region.Determining
