Coulomb and nuclear breakup of a halo nucleus Be11

Breakup reactions of the one-neutron halo nucleus ¹¹Be on lead and carbon targets at about 700.3em0exMeV∕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=40.3em0exMeV of 1.05±0.060.2em0exe²0.2em0exfm². The energy weighted $E1$ strength up to Eₓ=40.3em0exMeV 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)0.3em0exfm, 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=1.780.3em0exMeV and Eₓ=3.410.3em0exMeV. Angular distributions for these states show the diffraction pattern characteristic of $L=2$ transitions, resulting in a J^π=(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.