The ${}^{33}\mathrm{Na}$ β decay was studied online using mass separation techniques and a first description of the level structure of the neutron-rich isotope ${}^{33}\mathrm{Mg},$ with $N=21,$ has been obtained. The experiment involved the measurement of β-γ, β-γ-γ, and $\beta -n-\gamma$ coincidences as well as neutron spectra by time-of-flight technique. The first low energy level scheme for the daughter nucleus ${}^{33}\mathrm{Mg}$ is given with five bound states. Spin and parity assignments are proposed according to β feedings and γ-ray multipolarities. β-strength distribution is evaluated, taking into account $1n$- and $2n$-emission channels and it is compared with the calculated GT strength distribution. In particular, the $1p-1h$ and $2p-2h$ excitations are shown to result in substantial contribution to the low energy configurations. Allowed β branch to the ${}^{33}\mathrm{Mg}$ g.s. $[\log ft=5.27\mathrm{}(26\left)\mathrm{}\right]$ gives direct evidence for the inversion of $\nu {(f}_{7/2})$ and $\nu {(d}_{3/2})$ states.

• Received 16 May 2001

DOI:https://doi.org/10.1103/PhysRevC.64.054313