Identification of the crossing point at N=21 between normal and intruder configurations

The β− decay of 34Mg was used to study the 34Al nucleus through g-ray spectroscopy at the Isotope Separator On-Line facility of CERN. Previous studies identified two β-decaying states in 34Al having spin-parity assignments Jπ=4−dominated by the normal configurationπ (d5/2)−1⊗ν(f7/2) and Jπ=1+ by the intruder configuration π(d5/2)−1⊗ν(d3/2)−1(f7/2)2 . Their unknown ordering and relative energy have been the subject of debate for the placement of 34Al inside or outside the N=20 “island of inversion.” We report here that the 1+ intruder lies only 46.6 keV above the 4− ground state. In addition, a new half-life of T1/2=44.9(4)ms, that is twice as long as the previously measured 20(10) ms, has been determined for 34Mg. Large-scale shell-model calculations with the recently developed sdpf-u-mix interaction are compared with the new data and used to interpret the mechanisms at play at the very border of the N=20 island of inversion.