Single-neutron knockout from intermediate energy beams of $^{30,32}$Mg: Mapping the transition into the ``island of inversion''
J. R. Terry, B. A. Brown, C. M. Campbell, J. M. Cook, A. D. Davies, D.-C. Dinca, A. Gade, T. Glasmacher, P. G. Hansen, B. M. Sherrill, H. Zwahlen, D. Bazin, K. Yoneda, J. A. Tostevin, T. Otsuka, Y. Utsuno, and B. Pritychenko

The breakdown of the $N=20$ magic number in the so-called ``island of inversion'' around \nuc{32}{Mg} is well-established. Recently developed large-scale shell model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of \nuc{30}{Mg} and \nuc{32}{Mg}, with the latter consisting nearly purely of $2p-2h$ $N=20$ cross-shell configurations. Single-neutron knockout experiments on \nuc{30,32}{Mg} projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative parity states in the knockout residues are deduced and compare well with shell model predictions.

© 2008 The American Physical Society.