Intruder features in the island of inversion: The case of 33Mg

The ³³Na β decay was studied online using mass separation techniques and a first description of the level structure of the neutron-rich isotope ³³Mg, with N=21, has been obtained. The experiment involved the measurement of β-γ, β-γ-γ, and β-n-γ coincidences as well as neutron spectra by time-of-flight technique. The first low energy level scheme for the daughter nucleus ³³Mg 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 ³³Mg g.s. [log ft=5.27(26)] gives direct evidence for the inversion of ν(f_7/2) and ν(d_3/2) states.