With the help of radioactive ion beam facilities, experimental studies on nuclei far from the beta-stability line have brought great opportunities to address the nuclear shell evolution as a function of proton-to-neutron ratio. Since the 1970s, the region around 32Mg (Z=12, N=20), which is called the island of inversion [1], has attracted much attention from both experimentalists and theorists because of the unexpected ground-state deformation observed at the neutron closed shell. According to the nuclear shell models, those deformed ground states arise from the neutron 2-particle-2-hole (2p2h) excitation across the N=20 shell gap [1]. To understand how the deformation, or the particle-hole excitation, develops towards the island of inversion, it is of great interest to investigate nuclei residing at the border of the island of inversion, such as the aluminum isotopes (Z=13) with N~20.
An experiment has been carried out at GANIL to measure the nuclear moments of the low-lying 1+ isomer in 34Al (N=21) using the beta-NMR technique. Unlike its 4- ground state, of which the wave function is strongly mixed with 0p0h and 2p2h components [2], the 1+ isomer is proposed to have a dominant 1p1h configuration [2, 3]. Even though the excitation energy of this isomer has been reported recently [4], its 1p1h nature remains to be verified experimentally. In this contribution, we will present the g factor and quadrupole moment of the isomer newly determined from our work [5]. The isomer's 1p1h character can be firmly identified by comparing the experimental g factor with either the effective single-particle g factor, or more complex shell-model calculations. The quadrupole moment, on the other hand, increases roughly by 50% with respect to that of the 32Al ground state (also a 1+ state), suggesting an enhanced deformation induced by the 1p1h excitation across N=20. In addition, shell-model calculations of 32-34Al using several different effective interactions are compared with available experimental data in various observables, to gain a deeper insight on the neutron particle-hole excitation in this transitional region.
