Direct Evidence for the Breakdown of theN=8Shell Closure inB12e

Partial cross sections and corresponding momentum distributions have been studied in the one-neutron knockout reaction ( ${}^{12}\mathrm{Be}{,}^{11}\mathrm{Be}+\gamma$) on a ${}^9\mathrm{Be}$ target at 78 MeV/nucleon. The resulting spectroscopic factors for the only two bound states of ${}^{11}\mathrm{Be}$ are $0.42\pm 0.06$ ( $1/2^{+}$) and $0.37\pm 0.06$ ( $1/2^{-}$), where the errors are experimental only. This result shows that $N=8\mathrm{}$ is not a good closed shell in the neutron-rich ${}^{12}\mathrm{Be}$ and that the last neutron pair is two-thirds in the ( ${1s}^2{+0d}^2$) intruder configuration.