Nuclear structure studies with radioactive beams at GSI

The electromagnetic excitation process at beam energies around 500 MeV/u is utilized to probe the dipole response of light neutron-rich nuclei. The secondary beams are produced in fragmentation reactions and separated by the GSI fragment separator FRS. By measuring the neutron decay after inelastic scattering on a Pb target in a kinematically complete experiment, the differential electromagnetic excitation cross sections dσ/dE* are derived. The oxygen isotope chain is studied systematically ranging from A = 17 to A = 22. Low-lying dipole strength is observed, exhausting about 10% of the energy-weighted dipole sum rule for excitation energies below 15 MeV. Even though this soft dipole excitation is present in all neutron-rich oxygen isotopes investigated, the cluster sum rule associated with the dipole motion of the valence nucleons against the ¹⁶O core is only fully exhausted for ¹⁸O. For the halo nuclei ¹¹Be and ^15,17C it is shown that detailed spectroscopic information on the ground state single particle structure can be deduced from the 'threshold' dipole strength.