Measurements of the dipole response with radioactive beams

We have measured the differential Coulomb dissociation cross sections for neutron-rich nuclei in the mass range around A ≈ 20. For the halo nuclei ¹¹Be and ^15,17C the observed 'threshold strength' is analyzed aiming to deduce the single particle structure. For the known cases ¹¹Be and ¹⁵C the extracted spectroscopic factors are in good agreement with other experiments. The importance of a γ coincidence measurement, which was not applied in earlier experiments on Coulomb dissociation of halo nuclei, is obvious in the ¹⁷C case. There, we find that the dominant ground state configuration involves a mixture of s and d wave neutrons coupled to the 2⁺ excited ¹⁶C core state. This excludes a ground state spin of 1/2 for ¹⁷C. For the oxygen isotope chain we have measured systematically the evolution of dipole strength in the neutron-rich nuclei A = 17 to A = 22. Excitation-energy differential cross sections were derived up to an excitation energy of 30 MeV. For all neutron-rich oxygen isotopes investigated, soft dipole excitations, well below the giant dipole resonance are observed. This low-lying dipole strength is found to exhaust up to about 10% of the energy weighted classical dipole sum rule if integrated up to 15 MeV excitation energy. The cluster sum rule limit with ¹⁶O taken as the core, however, is almost exhausted only for ¹⁷O and ¹⁸O, while for the more neutron-rich isotopes the experimentally observed strength below 15 MeV decreases with respect to that limit to about 40 % for ²²O.