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 31Mg performed at GANIL to study the structure of 31Mg and of the core 30Mg. 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 02+ in 30Mg 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.