Coulomb and nuclear breakup of a halo nucleus Be11

Abstract

Breakup reactions of the one-neutron halo nucleus $^{11}\mathrm{Be}$ on lead and carbon targets at about $70\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\text{nucleon}$ 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 $^{10}\mathrm{Be}({0}^{+})\ensuremath{\bigotimes}\ensuremath{\nu}2{s}_{1∕2}$ configuration in $^{11}\mathrm{Be}$ which is found to be $0.72\ifmmode\pm\else\textpm\fi{}\mathrm{0.04}$ with a $B(E1)$ strength up to ${E}_{\mathrm{x}}=4\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ of $1.05\ifmmode\pm\else\textpm\fi{}0.06\phantom{\rule{0.2em}{0ex}}{e}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{fm}}^{2}$. The energy weighted $E1$ strength up to ${E}_{\mathrm{x}}=4\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ explains $70%\ifmmode\pm\else\textpm\fi{}10%$ 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 $5.77(16)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, 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 ${E}_{\mathrm{x}}=1.78\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and ${E}_{\mathrm{x}}=3.41\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Angular distributions for these states show the diffraction pattern characteristic of $L=2$ transitions, resulting in a ${J}^{\ensuremath{\pi}}={(3∕2,5∕2)}^{+}$ 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.