Study of 19C by One-Neutron Knockout Reaction with a Carbon Target

Abstract

The level structure of a neutron-rich nucleus 19C, which is known as a one-neutron halo, was studied via one-neutron (1n) knockout reaction with a 20C beam and a carbon target at 280 MeV/nucleon. Three prominent resonances were observed at E rel = 0.036(1), 0.84(4), and 2.31(3) MeV in the invariant mass spectrum constructed from momentum vectors of decay products of the knockout residue, a 18C isotope and a neutron. The resonances correspond to the neutron-unbound states in 19C at excitation energies of E x = 0.62(9), 1.42(10), and 2.89(10) MeV. The spin-parities of the 0.62-MeV and 2.89-MeV states were determined to be 5/2+1 and 1/2-1, respectively, by comparing the data with eikonal model calculations employing shell-model spectroscopic factors in terms of the 1n knockout cross section and the parallel momentum distribution.

We have clarified that the 0.62-MeV state is unbound and assigned its spin-parity with the direct evidence. The 1.42-MeV state is consistent with the 5/2+2 state, which is located at 1.46 MeV by the proton inelastic scattering measurement. The state at 2.89 MeV in 19C with a negative parity was observed for the first time, and its spin-parity was clearly determined. In addition, the shell structure involving the neutron 1s 1/2 and 0d 5/2 orbitals is discussed.