Breakup reactions of the one-neutron halo nucleus ${}^{11}\mathrm{Be}$ on lead and carbon targets at about $70\mathrm{MeV}∕\text{nucleon}$ have been investigated at RIKEN by measuring the momentum vectors of the incident ${}^{11}\mathrm{Be}$, outgoing ${}^{10}\mathrm{Be}$, and neutron in coincidence. The relative energy spectra as well as the angular distributions of the ${}^{10}\mathrm{Be}+n$ center of mass system (inelastic angular distributions) have been extracted both for Pb and C targets. For the breakup of ${}^{11}\mathrm{Be}$ 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 ${}^{10}\mathrm{Be}\left(0^{+}\right)\otimes \nu 2s_{1∕2}$ configuration in ${}^{11}\mathrm{Be}$ which is found to be $0.72\pm \mathrm{0.04}$ with a $B\left(E1\right)$ strength up to $E_{\mathrm{x}}=4\mathrm{MeV}$ of $1.05\pm 0.06e^2{\mathrm{fm}}^2$. The energy weighted $E1$ strength up to $E_{\mathrm{x}}=4\mathrm{MeV}$ explains $70\%\pm 10\%$ 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 $5.77\left(16\right)\mathrm{fm}$, consistent with previously known values. In the breakup with the carbon target, we have observed the excitations to the known unbound states in ${}^{11}\mathrm{Be}$ at $E_{\mathrm{x}}=1.78\mathrm{MeV}$ and $E_{\mathrm{x}}=3.41\mathrm{MeV}$. Angular distributions for these states show the diffraction pattern characteristic of $L=2$ transitions, resulting in a $J^{\pi }={(3∕2,5∕2)}^{+}$ 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.

• Received 3 August 2004

DOI:https://doi.org/10.1103/PhysRevC.70.054606