On the Structure of Electric-Dipole Transitions

Inelastic α scattering at intermediate energies provides a tool for highly selective excitation of isoscalar Giant Resonances. If combined with coincident measurements of decay photons and decay particles the (micro-)structure of the resonances becomes transparent. We have performed neutron decay measurements of the ISGDR in 90Zr, 116Sn and 208Pb and photon decay measurements of the Pygmy resonance in 140Ce at kinematical conditions strongly favouring electric-dipole modes. In result, we got first-time evidence for the direct neutron decay component of the ISGDR in the nuclei under investigation and indications for a surprisingly clear splitting of the Pygmy E1 strength in 140Ce.