Systematics of the pygmy dipole resonance in stable tin isotopes from resonant photon scattering

The 112,120Sn(γ,γ') reactions were studied for different endpoint energies of the incident bremsstrahlung spectrum below the neutron separation energies at the superconducting Darmstadt electron linear accelerator S-DALINAC. Dipole transition strength distributions have been extracted for 112Sn; the analysis of 120Sn is still in progress. In the case of 112Sn a concentration of dipole excitations is observed between 5 and 8MeV. Combining with existing data for 116,124Sn provides a systematics of the pygmy dipole resonance (PDR) in stable Sn nuclei which serves as a benchmark test of recent microscopic calculations aiming at a description of the PDR in exotic neutron-rich Sn isotopes