g factor of the exotic N=21N=21 isotope 34Al: probing the N=20N=20 and N=28N=28 shell gaps at the border of the “island of inversion”

For the first time the g   factor of an isotope beyond N=20N=20 near the ‘island of inversion’ has been measured. A 34Al radioactive beam was produced in a one-neutron pickup reaction on a 36S primary beam at 77.5 MeV/u, providing a large spin-polarization for application of the β-Nuclear Magnetic Resonance (β-NMR) method. The measured g factor of 34Al, |g|=0.539(2)|g|=0.539(2), combined with results from earlier β  -decay studies, allows to firmly assign a ground state spin/parity 4−4−. Comparison to large scale shell model calculations reveals that a dominant amount of intruder components is needed in the 34Al wave function to account for the observed large magnetic moment μ=(+)2.156(16)μNμ=(+)2.156(16)μN. This reveals Z=13Z=13 to be a true ‘transition number’ between the normal Z=14Z=14 Si isotopes and the abnormal Z=12Z=12 Mg isotopes. The sensitivity of this odd–odd ground state dipole moment to the N=20N=20, as well as the N=28N=28 gap, reveals that both are significantly reduced, despite Z=13Z=13 being outside the conventional island of inversion.