High-precision quadrupole moment reveals significant intruder component in 13 33Al20 ground state

The electric quadrupole moment of the Al201333 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 isotopesThis work was partly supported by the European Community FP6—Structuring the ERA—Integrated Infrastructure Initiative Contract EURONS No. RII3-CT-2004-506065, by the FWO-Vlaanderen, by the IAP programme of the Belgium Science Policy under Grants No. P6/23 and No. P7/12, by a grant of the MICINN (Spain) (FPA2011-29854), by the Nupnet network SARFEN (PRI-PIMMNUP-2011-1361), by MINECO (Spain) Centro de Excelencia Severo Ochoa Programme under Grant No. SEV-2012-0249, and by JSPS KAKENHI (Japan) Grants No. 21740204 and No. 15K05094. The experiment was carried out under Experimental Program E437