One-neutron knockout reaction from 20C

Recent researches in neutron-rich nuclei have demonstrated that the depth of each single-particle level varies from that in stable nuclei : some of the well-known magic numbers disappear and new shell closures develop. Cross-shell excitation, transition of a nucleon across a shell gap, can be exploit to probe changes in shell structure. The present work aims at exploration of neutron-unbound states of $^{19}$C, especially a hole- state populated by cross-shell excitation, via a one-neutron knockout reaction. The experiment was performed at the RIBF facility in RIKEN. A $^{20}$C secondary beam produced by BigRIPS with an energy of 280 MeV/nucleon impinged on a carbon target placed before the SAMURAI spectrometer. By taking full advantage of the analyzer system comprised of a large-acceptance super-conducting dipole magnet, associated tracking detectors, and a large volume neutron detector system, an invariant mass spectrum for the system of $^{18}$C$ + n$ was reconstructed. Three unbound excited states in $^{19}$C were identified including the unknown $1/2^{+}_{1}$ state at 2.90 MeV in excitation energy. Details of the measurement and analysis along with results will be presented.