Experimental cross sections of 165Ho(α,n)168Tm and 166Er(α,n)169Yb for optical potential studies relevant for the astrophysical γ-process

Abstract

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 165Ho(α,n) and 166Er(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 165Ho(α,n) and 166Er(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.