Shell evolution approaching the N=20 island of inversion: Structure of Mg29

The island of inversion for neutron-rich nuclei in the vicinity of N = 20 has become the testing ground par excellence for our understanding and modeling of shell evolution with isospin. In this context, the structure of the transitional nucleus Mg-29 is critical. The first quantitative measurements of the single-particle structure of Mg-29 are reported, using data from the d(Mg-28, p gamma )(29) Mg reaction. Two key states carrying significant l= 3 (f-wave) strength were identified at 2.40 +/- 0.10 (J(pi) = 5/2(-)) and 4.28 +/- 0.04 MeV (7/2(-)). New state-of-the-art shell-model calculations have been performed and the predictions are compared in detail with the experimental results. While the two lowest 7/2(-) levels are well described, the sharing of single-particle strength disagrees with experiment for both the 3/2(-) and 5/2(-) levels and there appear to be general problems with configurations involving the P-3/2 neutron orbital and core-excited components. These conclusions are supported by an analysis of the neutron occupancies in the shell-model calculations.