Study of the 112,120Sn(g,g') reaction and systematics of the pygmy dipole resonance at the z=50 shell closure

In this thesis the 112,120Sn(g, g?) reactions are studied at different endpoint energies ofthe incident bremsstrahlung spectrum below the neutron separation energies at the superconductingDarmstadt electron linear accelerator S-DALINAC. Dipole transition strengthdistributions are extracted for 112Sn up to 9.5MeV and for 120Sn up to 9.1MeV. A concentrationof dipole excitations is observed between 5 and 8 MeV. Furthermore a fluctuationanalysis is applied to the photon scattering spectra to estimate the amount of the unresolvedstrength hidden in background due to fragmentation of the strength. Together withexisting data for 116,124Sn (Govaert, et al., 1998) this provides a set of information on thestructure of the so-called pygmy dipole resonance (PDR) in the stable neutron-rich tinnuclei.The results are compared to microscopic quasiparticle-phonon model and relativisticquasiparticle RPA calculations.