High-sensitivity investigation of low-lying dipole strengths in Sn-120

Background: The term Pygmy Dipole Resonance (PDR) denotes an accumulation of electric dipole excitations below and around the neutron separation threshold. It may be important, e.g., for the nucleosynthesis of heavy nuclei or the symmetry energy in the Equation of State (EoS). For a deeper understanding of the PDR, systematic studies are essential. Purpose: The tin isotopic chain is a well-suited candidate to investigate the systematics of the PDR, and the (gamma, gamma') reactions on Sn-112,Sn-116,Sn-120,Sn-124 have already been measured in experiments using bremsstrahlung. It was claimed that the extracted electric dipole transition strengths of these isotopes increase with increasing neutron-to-proton ratio with the exception of Sn-120. Furthermore, previous results from elastic photon scattering experiments on Sn-120 are in disagreement with corresponding (p, p') Coulomb excitation data. To examine this discrepancy an additional high-sensitivity bremsstrahlung experiment on Sn-120 was performed. Method: The Nuclear Resonance Fluorescence (NRF) method is used, which is based on the scattering of real photons. The bremsstrahlung experiment presented in this work was performed with a maximum energy of E-gamma,E-max = 9.5 MeV at the gamma ELBE facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Besides a state-to-state analysis, the quasicontinuum was investigated as well. Results: Above E-x = 4 MeV 228 dipole transitions were clearly identified; 163 were observed for the first time. Assuming that all identified dipole transitions have electric dipole character the summed electric dipole strength equals Sigma B(E1)up arrow=369(49) x 10(-3) e(2)fm(2) [which amounts to 0.58(8)% of the Thomas-Reiche-Kuhn sum rule] for transitions from 4 MeV to S-n = 9.1 MeV. This is an enhancement of a factor 2.3 compared to the previously published Sn-120(gamma, gamma') results. This increase can be explained by the contribution of many weak, previously not included transitio