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

## TL;DR

This study investigates the unbound level structure of the halo nucleus $^{19}$C via single-neutron knockout from $^{20}$C, revealing detailed energy levels, spin-parity assignments, and valence neutron configurations consistent with shell-model predictions.

## Contribution

It provides new experimental data on $^{19}$C's level scheme and confirms shell-model predictions, including the significance of $0d^2_{5/2}$ contributions in $^{20}$C ground state.

## Key findings

- Identification of a narrow near-threshold state at 0.036 MeV.
- Observation of resonance-like features at 0.84 and 2.31 MeV.
- Spin-parity assignments of $5/2^+$ and $1/2^-$ for key levels.

## Abstract

The low-lying unbound level structure of the halo nucleus $^{19}\textrm{C}$ has been investigated using single-neutron knockout from $^{20}\textrm{C}$ on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity $^{18}\textrm{C}$ fragment and neutrons, was found to be dominated by a very narrow near threshold ($E_\textrm{rel}$ = 0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at $E_\textrm{rel}$ = 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 $^{20}\textrm{C}$ ground state is thus seen to include, in addition to the known $1s^2_{1/2}$ component, a significant $0d^2_{5/2}$ contribution. The level scheme of $^{19}\textrm{C}$, 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.