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.
|Number of pages||8|
|Journal||Acta Physica Hungarica New Series Heavy Ion Physics|
|Publication status||Published - 2000|
- Coulomb excitation
- Dipole strength distribution
- Radioactive beams