One-Neutron Knockout from Individual Single-Particle States ofB11e

The structure of the halo nucleus ${}^{11}\mathrm{Be}$ has been studied using the reaction ${}^9\mathrm{Be}{(}^{11}\mathrm{Be}{,}^{10}\mathrm{Be}+\gamma )X$ at 60 MeV/nucleon. The ground state structure of ${}^{11}\mathrm{Be}$ is determined by comparing the experimental cross sections to a calculation combining spectroscopic factors from the shell model with $l$-dependent single-particle cross sections obtained in an eikonal model. This experiment shows the dominant $1s$ single-particle character of the ${}^{11}\mathrm{Be}$ ground state and indicates a small contribution of $0d$ admixture in the wave function. After correction for the approximately $22\%$ intensity to excited levels, a clean and precise distribution of parallel momentum for knockout from the $1s$ halo wave function is obtained for the first time.