Coulomb excitation of the one-neutron halo nucleus 11Be

An exotic beam of ${}^{11}\mathrm{Be}$ at 57–60 MeV/nucleon has been used for a study of the excitation cross section of the 320 keV first excited state in collisions with targets of Pb, Au, C, and Be. The deexcitation $\gamma$ rays were observed in an array of position-sensitive NaI(Tl) detectors in coincidence with the scattered fragment. The reduced transition strengths $B$($E$1) extracted for the heavy targets suggest a predominantly electric excitation mechanism. Although the $B$($E$1) value is marginally in agreement with that obtained from the lifetime of the state, the deviation is of the size expected from higher-order Coulomb and nuclear effects. Excitations on the lighter targets have sizable nuclear contributions.