High-precision quadrupole moment reveals significant intruder component in Al-33(13)20 ground state

The electric quadrupole moment of the Al20 13 33 ground state, located at the border of the island of inversion, was obtained using continuous-beam β-detected nuclear quadrupole resonance (β-NQR). From the measured quadrupole coupling constant νQ=2.31(4) MHz in an α−Al2O3 crystal, a precise value for the electric quadrupole moment is extracted: ∣∣Qs(Al33)∣∣=141(3) mb. A comparison with large-scale shell model calculations shows that Al33 has at least 50% intruder configurations in the ground state wave function, favoring the excitation of two neutrons across the N=20 shell gap. Al33 therefore clearly marks the gradual transition north of the deformed Na and Mg nuclei towards the normal Z≥14 isotopesstatus: publishe