Unbound States in ¹⁷C Probed via Single-Neutron Removal from ¹⁸C at 245 MeV/u

A study of unbound states in ¹⁷C by means of one-neutron knockout from a fast moving ¹⁸C beam at an energy of 245 MeV/nucleon on a carbon target was performed at the RIKEN-RIBF laboratory utilizing SAMURAI. The energy spectrum of unbound ¹⁷C constructed from momentum vectors of the forward going beam velocity ¹⁶C fragment and neutron covered an energy range of the decay neutron below about 6 MeV in the center of mass of ¹⁷C*. The relative energy E(rel) spectrum was characterized by two resonances at about 1.92 and 3.24 MeV in E(rel) and a possible multiplet at around E(rel) similar to 0.7 MeV. Momentum distributions of the knockout residue for the E(rel)= 1.92 and 3.24-MeV states well matched to those of p-wave neutron removal, providing a model independent confirmation of negative parity assignments for these states. The proposed level scheme of ¹⁷C was compared with a range of different shell-model predictions; it turned out that the YSOX shell-model interaction, employing the monopole based universal interaction for the cross-shell two-body matrix elements and incorporating larger (h) over bar omega excitations, gave a consistent account of it, including the locations of the 1/2₁^- and 3/2₁^- cross-shell states. (authors)