Photoneutron cross sections for unstable neutron-rich oxygen isotopes

The dipole response of stable and unstable neutron-rich oxygen nuclei of ma sses A = 17 to A = 22 has been investigated experimentally utilizing electr omagnetic excitation in heavy-inn collisions at beam energies about 600 MeV /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 si gma /dE were derived up to 30 MeV excitation energy. In contrast to stable nuclei, the deduced dipole strength distribution appears to be strongly fra gmented and systematically exhibits a considerable fraction of low-lying st rength.