Evidence of a (1d _5/2) ² component to the ¹²Be ground state

Data have been obtained on exclusive single neutron knockout cross sections from ¹²Be to study its ground state structure. The cross sections for the production of ¹¹Be in its ground state (1/2 ⁺) and first excited state (0.32 MeV, 1/2 ^-) have previously been measured, indicating a strong (2s _{1 2}) ² component to the ¹²Be ground state. In the present experiment, performed at the GANIL laboratory, cross sections for the first (0.32 MeV, 1/2 ^-) and second (1.78 MeV, 5/2 ⁺, unbound) excited states in ¹¹Be were measured, which gives information on the admixture of (1p _{1 2}) ² and (1d _{5 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 event-by-event. The beam-like residues were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DéMoN array. The 1/2 ^- state of ¹¹Be was identified by measuring coincident 320 keV γ-rays, using four NaI detectors. Full kinematic reconstruction of unbound states in ¹¹Be was performed using coincident neutrons and ¹⁰Be ions. Detailed simulations were performed in order to interpret the data, and spectroscopic factors were calculated, using preliminary single particle removal cross sections calculated using a Glauber model.