Direct evidence for the breakdown of the N = 8 shell closure in 12Be

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