Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic li
nearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (alpha,alphagamma) experiment at E_{alpha}=136 MeV gave cross sections for an isoscalar probe. The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character. A strong isoscalar state at 7.6 MeV was identified for the first time supporting a recent theoretical prediction.
The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (a,ag) coincidence method at E=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structu
re: one group of states which is excited in (a,ag) as well as in (g,g) reactions and a group of states at higher energies which is only excited in (g,g) reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.
