Abstract: The transverse momentum distribution of 11;12Be from the breakup of 12;14Be on a proton target at 700.5, and 700 MeV/u, respectively, have been mea-sured. The ground state configuration of 12Be was studied within the framework of the distorted wave impulse approximation (DWIA) using quasi-free scattering as-sumption (QFS). The result shows the existence of a d5/2+ single-particle state with the spectroscopic factor of 2.73(30) in 12Be which is interpreted as evidence for an
intruder state in this nucleus.