Experimental measurement of N = 8 shell breaking in the 12Be ground state.

An experiment has been performed to measure exclusive single neutron knockout cross sections from 12Be to study its ground state structure. The yield to the first (0.32 MeV, ½-) and second (1.78 MeV, 5/2+, unbound) excited states in 11Be gives information on the admixture of (1p½)2 and (1d5/2)2 components in the ground state of 12Be. A fragmentation beam of 12Be of ~10000 pps (95% pure) was incident on a carbon target at 41 MeV/u. The beam particles were tracked onto the target, and their energies were measured from their time of flight. The beam-like residues 10,11Be were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DeMoN array. The ½- state of 11Be was identified by measuring coincident 320 keV gamma-rays, using four Nal detectors. Full kinematic reconstruction of unbound states in 11Be was performed using coincident neutrons and 10Be ions, which showed clear evidence for the 1.78 MeV state in 11Be. A strong background, due to reactions occurring in the zero degree telescope, was measured and subtracted by acquiring data with no target present. Neutron angular distributions in the laboratory frame were measured in coincidence with 10,11Be, and momentum distributions of neutrons and beam-like particles were measured for exclusive reaction channels. Detailed simulations were performed in order to quantitatively interpret the relative energy spectrum for the reconstructed 11Be, using the measurements of momentum distributions as constraints. The cross sections for the production of 11Be in its first (1/2-) and second (5/2+) excited states were measured to be 33.5(5.6) mb and 22.4(4.4) mb respectively. This is the first strong evidence for a (1d5/2)2 component in the 12Be ground state. Data were also acquired for the Coulomb excitation and breakup of 11Be on Pb and C targets