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Background: Optical potentials are crucial ingredients for the prediction of nuclear reaction rates needed in simulations of the astrophysical γ process. Associated uncertainties are particularly large for reactions involving α particles. This includes (γ,α) reactions which are of special importance in the γ process. Purpose: The measurement of (α,n) reactions allows for an optimization of currently used α-nucleus potentials. The reactions 165 Ho(α,n) and 166 Er(α,n) probe the optical model in a mass region where γ process calculations exhibit an underproduction of p nuclei which is not yet understood. Method: To investigate the energy-dependent cross sections of the reactions 165 Ho(α,n) and 166 Er(α,n) close to the reaction threshold, self-supporting metallic foils were irradiated with α particles using the FN tandem Van de Graaff accelerator at University of Notre Dame. The induced activity was determined afterwards by monitoring the specific β-decay channels. Results: Hauser-Feshbach predictions with a widely used global α potential describe the data well at energies where the cross sections are almost exclusively sensitive to the α widths. Increasing discrepancies appear towards the reaction threshold at lower energy. Conclusions: The tested global α potential is suitable at energies above 14 MeV, while a modification seems necessary close to the reaction threshold. Since the γ-and neutron width show non-negligible impact on the predictions, complementary data are required to judge whether or not the discrepancies found can to be solely assigned to the α width.

The existing Nuclear Resonance Fluorescence (NRF) setup at the HIγS facility at the Triangle Universities Nuclear Laboratory at Duke University has been extended in order to perform γ-γ coincidence experiments. The new setup combines large volume LaBr 3 :Ce detectors and high resolution HPGe detectors in a very close geometry to offer high efficiency, high energy resolution as well as high count rate capabilities at the same time. The combination of a highly efficient γ-ray spectroscopy setup with the mono-energetic high-intensity photon beam of HIγS provides a worldwide unique experimental facility to investigate the γ-decay pattern of dipole excitations in atomic nuclei. The performance of the new setup has been assessed by studying the nucleus 32 S at 8.125 MeV beam energy. The relative γ-decay branching ratio from the 1 + level at 8125.4 keV to the first excited 2 + state was determined to 15.7(3) %.

Abstract. Activation experiments are a perfect tool to perform systematic studies due to their high sensitivity and selectivity. Exemplary applications to understand the nucleosynthesis of the p nuclei -such as the optimization of optical particle-nucleus potentials and investigations of (γ,n) reactions in a broad mass range -are presented. Some recent and partly preliminary results are briefly discussed.