The $\beta$ decay of ${}^{33}\mathrm{Mg}$ ($N=21$) 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, ${}^{33}\mathrm{Al}$, are found to have nearly $2p-2h$ intruder configuration, thus extending the “island of inversion” beyond Mg. The allowed direct $\beta$-decay branch to the $5/2^{+}$ ground state of the daughter nucleus ${}^{33}\mathrm{Al}$ implies positive parity for the ground state of the parent ${}^{33}\mathrm{Mg}$, contrary to an earlier suggestion of negative parity from a $g$-factor measurement. An admixture of $1p\mathrm{\text{−}}1h$ and $3p\mathrm{\text{−}}3h$ configurations is proposed for the ground state of ${}^{33}\mathrm{Mg}$ to explain all of the experimental observables.

• Received 2 July 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.142504