Exploring statistical properties of nuclei for explosive stellar nucleosynthesis

Descriptions of nuclear reaction cross sections are one of the main ingredients for the understanding of the nucleosynthesis in stars for two reasons: the extrapolation of measurements down to astrophysical relevant energy regions and the prediction of reaction rates on very exotic nuclei. Most of these reaction rates are calculated in the scope of the Hauser-Feshbach statistical model based on input-parameters like nuclear level densities, gamma-strength functions and optical-model potentials. This thesis deals with investigations of two of these statistical properties: the gamma-strength function and the alpha+nucleus optical-model potential. Radiative proton-capture reactions on stable copper isotopes have been investigated at the HORUS gamma-ray spectrometer to study the gamma-strength in 64Zn and 66Zn. A formalism has been worked out to deduce gamma-strength information from the ratios of primary gamma-ray intensities in a model-independent way. For the first time, these intensities have been measured by utilizing gamma-gamma coincidences and two-step gamma-ray cascades for (p, gamma) reactions. Latter technique has been also applied to the previously measured 92Mo(p, gamma) reaction to obtain more sensitive data on the gamma-strength in the unstable N=50 isotone 93Tc. In both cases, the results obtained extend the available experimental data and allows the comparison to other experimental techniques. This will be of utmost importance for the evaluation of the generalized Brink-Axel hypothesis which is widely utilized in nuclear astrophysics. For the investigations of the alpha optical-model potential for heavy nuclei at sub-Coulomb energies, cross sections of alpha-induced reactions on the p nucleus 108Cd have been measured. The complementary measurement of (alpha, n) and (alpha, gamma) cross sections via the activation method and the in-beam technique, respectively, increases the sensitivity onto the alpha optical-model potential. Comparison to statistical model calculations support previous modification of the Demetriou alpha optical-model potential. A systematic investigation of (alpha, n) reaction cross sections on heavy nuclei below the Coulomb barrier via the statistical model shows that the Demetriou potential is applicable for the mass region from A ~ 100 to A ~ 190.