29Na:: Defining the edge of the island of inversion for Z=11 -: art. no. 162501

The low-energy level structure of the exotic Na isotopes Na-28,Na-29 has been investigated through beta-delayed gamma spectroscopy. The N=20 isotones for Z=10-12 are considered to belong to the island of inversion where intruder configurations dominate the ground state wave function. However, it is an open question as to where and how the transition from normal to intruder dominated configurations happens in an isotopic chain. The present work, which presents the first detailed spectroscopy of Na-28,Na-29, clearly demonstrates that such a transition in the Na isotopes occurs between Na-28 (N=17) and Na-29 (N=18), supporting the smaller N=20 shell gap in neutron-rich sd shell nuclei. The evidence for inverted shell structure is found in beta-decay branching ratios, intruder dominated spectroscopy of low-lying states, and shell model analysis.