The cross sections for single-neutron removal from the very neutron-rich nucleus ${}^{31}\mathrm{Ne}$ on Pb and C targets have been measured at $230\mathrm{MeV}/\mathrm{\text{nucleon}}$ 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 ${}^{31}\mathrm{Ne}$ occupies a low-$\ell$ orbital (most probably $2p_{3/2}$) with a small separation energy ($S_n\lesssim 0.8\mathrm{MeV}$), instead of being predominantly in the $1f_{7/2}$ orbital as expected from the conventional shell ordering. These findings suggest that ${}^{31}\mathrm{Ne}$ is the heaviest halo system known.

• Received 9 July 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.262501