Systematics of the Electric Dipole Response in Stable Tin Isotopes

The present thesis reports on the systematics of the electric dipole response in stable even mass tin isotopes. Inelastic proton scattering experiments were performed at the Research Center for Nuclear Physics (RCNP) in Osaka, Japan. Using a 295 MeV proton beam, scattered protons were detected under spectrometer angles of 0°, 2.5° and 4.5°. In two experimental campaigns data were taken in an excitation energy region of 5-26 MeV. Double differential cross sections were determined and by means of a multipole decomposition analysis the main contributions, namely E1 and M1, were extracted. In order to determine the electric dipole strength, the obtained double differential cross sections were converted to photoabsorption cross sections using the equivalent photon method. The photoabsorption cross sections are in fair agreement with (γ,x) results around the maximum of the giant dipole resonance. However, deviations towards the neutron threshold were observed. The most recent (γ,n) data, available for Sn-116, Sn-118, Sn-120 and Sn-124 are in a good agreement with results obtained in this work. B(E1) strength distributions were determined and compared to available data from nuclear resonance fluorescence experiments for Sn-112, Sn-116 and Sn-124 below the neutron threshold. The present results show considerably more strength, confirming previous findings for Sn-120. Furthermore, an accumulation of strength is found around 6.5 MeV, which is also observed in experiments with isoscalar probes. The dipole polarisability in even-even stable tin isotopes Sn-112, Sn-114, Sn-116, Sn-118, Sn-120 and Sn-124 was extracted. The polarisability of Sn-120 was found to be lower than in a previous work, but compatible within the uncertainties, if corrected for the quasideuteron effect. Some of the model calculations based on the nuclear energy density functional theory able to reproduce the polarisability of Sn-120 are not compatible with the new value. A systematic comparison to one of the model calculations was carried out for different parametrisations of the symmetry energy parameters. Using the so-called unit cross section technique electromagnetic B(M1) strength distributions and features of the spin M1 resonance are provided for the first time for stable even-even tin isotopes Sn-112, Sn-114, Sn-116, Sn-118, Sn-120 and Sn-124. The total gamma strength function was determined including E1 and M1 contributions. In the giant dipole resonance region fair agreement with photoabsorption experiments is found. In the pygmy dipole resonance region data on Sn-116 and Sn-118 are available from (³He,³He'γ) and (³He,αγ) experiments. Results for Sn-116 are in excellent agreement. In Sn-118 good agreement is found within the uncertainties. However, a prominent peak is found in all isotopes around 6.5 MeV which is not seen in (³He,³He'γ) and (³He,αγ) experiments, indicating a possible violation of the Brink-Axel hypothesis. Finally, using a fluctuation analysis, level densities were extracted for Sn-124 in an excitation energy region of 6-15 MeV for 1⁻ states. These level densities were converted to the total level density and compared to results from (³He,³He'γ) and (³He,αγ) experiments, where level densities for Sn-116, Sn-118 and Sn-122 are available. A fair agreement between these level densities is found