Halo Structure of the Island of Inversion Nucleus 31Ne

Abstract : The cross sections for single-neutron removal from the very neutron-rich nucleus 31Ne on Pb and C targets have been measured at 230 MeV=nucleon using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft E1 excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of 31Ne occupies a low-‘ orbital (most probably 2p3=2) with a small separation energy (Sn & 0:8 MeV), instead of being predominantly in the 1f7=2 orbital as expected from the conventional shell ordering. These findings suggest that 31Ne is the heaviest halo system known.