The dipole response of stable and unstable neutron-rich oxygen nuclei of masses $A=17\mathrm{}$ to $A=22\mathrm{}$ has been investigated experimentally utilizing electromagnetic excitation in heavy-ion collisions at beam energies about $600\mathrm{MeV}/\mathrm{nucleon}$. A kinematically complete measurement of the neutron decay channel in inelastic scattering of the secondary beam projectiles from a Pb target was performed. Differential electromagnetic excitation cross sections $d\sigma /dE$ were derived up to 30 MeV excitation energy. In contrast to stable nuclei, the deduced dipole strength distribution appears to be strongly fragmented and systematically exhibits a considerable fraction of low-lying strength.

• Received 19 December 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.5442