Intruder Configurations in the A=33 Isobars: Mg33 and Al33

The β decay of Mg33 (N=21) presented in this Letter reveals intruder configurations in both the parent and the daughter nucleus. The lowest excited states in the N=20 daughter nucleus, Al33, are found to have nearly 2p-2h intruder configuration, thus extending the “island of inversion” beyond Mg. The allowed direct β-decay branch to the 5/2⁺ ground state of the daughter nucleus Al33 implies positive parity for the ground state of the parent Mg33, contrary to an earlier suggestion of negative parity from a g-factor measurement. An admixture of 1p-1h and 3p-3h configurations is proposed for the ground state of Mg33 to explain all of the experimental observables.