How sharp is the transition into the N=20 island of inversion for the Mg isotopes ?

The N=20 island of inversion is an excellent playground for testing shell model calculations. The Mg chain is a region of shell evolution still far from being well understood. In this paper we present preliminary results of a single-neutron knockout experiment from Mg-31 performed at GANIL to study the structure of Mg-31 and of the core Mg-30. The level scheme and longitudinal momentum distributions were mesured and spectroscopic factors were deduced. Negative parity states arise at low energy and the spectroscopic factor for the isomeric 0(2)(+) in Mg-30 was determined to be smaller than foreseen in the standard picture. The preliminary experimental results are compared to state-of the art shell model calculations revealing opposed interpretations.