The effect of neutron excess and nuclear deformation on dipole strength functions below the neutron separation energy - nuclear resonance fluorescence experiments on 124,128,132,134 Xe at ELBE and HI gamma S

Within this thesis, nuclear resonance fluorescence experiments were analyzed which have been performed at the gamma ELBE facility of the Helmholtz-Zentrum Dresden-Rossendorf and the HI gamma S facility of the Triangle Universities Nuclear Laboratory. The dipole strength up to the neutron separation energy, its distribution as well as its split into electric and magnetic strength were determined. The influence of crucial nuclear parameters, like deformation and neutron excess, on the data was investigated. For the first time a whole set of enriched gaseous targets was measured in the energy region close to the neutron separation threshold. At ELBE the scattering of photons on four different isotopes 124, 128, 132, 134 Xe was investigated by irradiating containers with enriched target material with a broad bremsstrahlung distribution. The endpoint energies were chosen to be 12MeV. This ensures excitations up to the neutron separation threshold. The two isotopes 128, 134 Xe were measured in an additional campaign at HI gamma S facility. The region below the threshold was explored in detail in these experiments. A second, more model-independent determination of the cross section was possible. The work shows, how the measured spectra taken with high-purity germanium detectors, have to be corrected for several, partly overlapping effects in order to determine the complete excitation strength. The calculation of different backgrounds, detector response functions and the influence of inelastic scattering constitute the main part of the presented work. With the help of GEANT4 simulations the amount of not-nuclear scattered photons was estimated. GEANT4 was also used to test the influence of the extended targets on the detection efficiency and response. The code gamma DEX, which calculates deexcitation schemes based on statistical assumptions, was updated and finally used for the unfolding of the spectrum. The measured data is compared to different strength function models and a theoretical prediction based on a QRPA calculation. The summed strength is also set into comparison to other experimental data sets and a global trend for low-lying strength was found. This shows, that the nuclear deformation which has a large influence on the dipole strength above the threshold is only of minor impact for the strength at lower energies. Instead of this, the neutron excess seems to be the dominating factor for the strength in the investigated energy region. This work was supported by the German Research Foundation (DFG), Project No. SCHW883/1-1