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 γ spectroscopy at the Isotope Separator OnLine facility of CERN. Previous studies identified two β-decaying states in 34Al having spin-parity assignments J π = 4− dominated by the normal configuration π(d5/2) −1 ⊗ ν(f7/2) and J π = 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.