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