Low-lying dipole strength in neutron-rich nuclei

The possibility of performing nowadays reactions with beams of unstable nuclei has opened a new field of study in nuclear physics. Using stable nuclei as targets and carrying out the experiments in inverse kinematics it becomes possible to have access to the excitation spectrum of the incoming short-lived projectiles. To this aim the most often used reactions are the Coulomb excitation and the inelastic scattering processes. Recently, Coulomb excitation measurements have been performed by Aumann et al. on Oxygen isotopes up to O. The (γ, xn) cross sections were measured in these nuclei and it is found that, as the neutron excess increases there appears an increasing fraction of E1 strength at energies below that of the giant dipole resonance (GDR). If one sums up the energy-weighted strength below 15MeV of excitation energy it is quite small in the stable nucleus O but it can be as much as 12% of the Thomas-Reiche-Kuhn (TRK) sum rule in O. Looking at the σ(γ, xn) cross sections one can see that this corresponds to the appearance of E1 strength in the so-called pygmy resonances located in the 10–12MeV region. They are known in heavy stable nuclei like Pb and it is interesting to investigate their nature in neutron-rich, unstable nuclei.