Erosion of N=20 shell in Al-33 investigated through the ground-state electric quadrupole moment

Electric quadrupole moment Q of the 33Al ground state has been measured by means of β-NMR spectroscopy using a spin-polarized 33Al beam produced in a projectile fragmentation reaction. The obtained Q moment, |Qexp(33Al)| = 132(16) emb, shows a significant excess from the prediction of shell model calculations within the sd shell. The result indicates sizable admixing of pf intruder configurations in the ground state, demonstrating that the N = 20 shell closure certainly erodes in 33Al, a nucleus located on the border of the island of inversion. Comparison was made with predictions of the Monte Carlo shell model, and also a particle-vibration coupling model treating the neutron pairing correlations in the ground state of 33Al. Again, a significant admixture of pf intruder configurations to the 33Al ground state was needed in both theoretical approaches to explain the observed large Q .