Coulomb and nuclear breakup of a halo nucleus11Be

Breakup reactions of the one-neutron halo nucleus 11Be on lead and carbon targets at about 70 MeV/nucleon have been investigated at RIKEN by measuring the momentum vectors of the incident 11Be, outgoing 10Be, and neutron in coincidence. The relative energy spectra as well as the angular distributions of the 10Be+n center of mass system (inelastic angular distributions) have been extracted both for Pb and C targets. For the breakup of 11Be on Pb, the selection of forward-scattering angles, corresponding to large impact parameters, is found to be effective to extract almost purely the first-order E1 Coulomb breakup component and to exclude the nuclear contribution and higher-order Coulomb breakup components. This angle-selected energy spectrum is thus used to deduce the spectroscopic factor for the 10Be(0+) ⊗ ν2s1/2 configuration in 11Be which is found to be 0.72±0.04 with a B(E1) strength up to Ex=4 MeV of 1.05±0.06 e2 fm2. The energy weighted E1 strength up to Ex=4 MeV explains 70% ± 10% of the cluster sum rule, consistent with the obtained spectroscopic factor. The non-energy-weighted sum rule within the same energy range is used to extract the root-mean-square distance of the halo neutron to be 5.77(16) fm, consistent with previously known values. In the breakup with the carbon target, we have observed the excitations to the known unbound states in 11Be at Ex = 1.78 MeV and Ex=3.41 MeV. Angular distributions for these states show the diffraction pattern characteristic of L=2 transitions, resulting in a Jπ=(3/2,5/2)+ assignment for these states. We finally find that even for the C target the E1 Coulomb direct breakup mechanism becomes dominant at very forward angles.