Giant resonances in unstable oxygen isotopes

The electromagnetic and nuclear fragmentation of secondary neutron-rich Oxygen beams at energies of 600 MeV/u has been studied. We have shown that the experimental technique allows for an extraction of the electromagnetic excitation cross section, even though the cross sections for competing nuclear processes in peripheral reactions are of similar magnitude. The aim of the experiment is, in the first place, to extract the dipole strength function from electromagnetic excitation cross sections. Low-lying dipole components were observed in all neutron-rich Oxygen isotopes carrying about 5 % of the energy-weighted dipole sumrule for excitation energies up to 5 MeV above the threshold, while the maximum of the cross section shifts to lower energies going from ¹⁸O to ²²O. Coincident measurements of fragments and γ-rays after nuclear fragmentation, in particular knock-out processes, provide spectroscopic information on exotic nuclei. The experimental technique is also applicable to study the continuum response in nuclear reactions as demonstrated for the ⁶He halo breakup. Since the measurement is kinematically complete in the projectile rapidity domain, differential cross sections (with respect to scattering angle) for inelastic electromagnetic or nuclear scattering can be deduced as well. The availability of higher beam intensities in the near future at GSI makes a continuation of this experimental study to the dripline nucleus ²⁴O feasible, where the additional two neutrons occupy the 1s state, as well as an extension of such investigations to heavier exotic nuclei.