Single-neutron knockout from ${}^{20}\text{C}$ and the structure of ${}^{19}\text{C}$

Abstract

The low-lying unbound level structure of the halo nucleus ${}^{19}\text{C}$ has been investigated using single-neutron knockout from ${}^{20}\text{C}$ on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity ${}^{18}\text{C}$ fragment and neutrons, was found to be dominated by a very narrow near threshold ($E_{\text{rel}}$ = 0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at $E_{\text{rel}}$ = 0.84(4) and 2.31(3) MeV, both of which exhibit characteristics consistent with neutron $p$-shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of $5/2_1^{+}$ and $1/2_1^{-}$ for the levels at $E_x$ = 0.62(9) and 2.89(10) MeV with $S_n$ = 0.58(9) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the ${}^{20}\text{C}$ ground state is thus seen to include, in addition to the known $1s_{1/2}^2$ component, a significant $0d_{5/2}^2$ contribution. The level scheme of ${}^{19}\text{C}$, including significantly the $1/2_1^{-}$ cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction.

Physics Letters B, in press