g factors of 31,32,33Al: Indication for intruder configurations in the 33Al ground state

The g factors of 31,32,33Al have been measured using the β-nuclear magnetic resonance (β-NMR) technique on spin-polarized beams produced in the fragmentation of a 36S (77.5 MeV/u) beam on a 9Be target. Nearly pure beams of Al (Z=13) isotopes were selected with the high-resolution fragment separator LISE at GANIL. An asymmetry as high as 6% has been observed in the β-NMR curve for 32Al implanted in a Si single crystal. The magnetic moment of the N=20 nucleus 33Al is obtained for the first time: μ(Al33,Iπ=5/2+)=4.088(5)μN, while those of 31,32Al are obtained with improved accuracy: μ(Al31,Iπ=5/2+)=3.830(5)μN and μ(Al32,Iπ=1+)=1.9516(22)μN. Comparison of the results to shell-model calculations in the sd and the sdpf shell-model spaces leads to the conclusion that 33Al must contain some contribution from 2p-2h intruder configurations in its ground-state wave function. This indicates a gradual transition from the normal sd shell Si (Z=14) isotopes to the intruder Mg (Z=12) isotopes.