Background: Statistical model calculations have to be used for the determination of reaction rates in large-scale reaction networks for heavy-element nucleosynthesis. A basic ingredient of such a calculation is the α-nucleus optical model potential. Several different parameter sets are available in literature, but their predictions of α-induced reaction rates vary widely, sometimes even exceeding one order of magnitude.Purpose: This paper presents the result of α-induced reaction cross-section measurements on gold which could be carried out for the first time very close to the astrophysically relevant energy region. The new experimental data are used to test statistical model predictions and to constrain the α-nucleus optical model potential.Method: For the measurements the activation technique was used. The cross section of the (α,n) and (α,2n) reactions was determined from γ-ray counting, while that of the radiative capture was determined via X-ray counting. Results:The cross section of the reactions was measured below Eα = 20.0 MeV. In the case of the 197 Au(α,2n) 199 Tl reaction down to 17.5 MeV with 0.5-MeV steps, reaching closer to the reaction threshold than ever before. The cross section of 197 Au(α,n) 200 Tl and 197 Au(α,γ) 201 Tl was measured down to Eα = 13.6 and 14.0 MeV, respectively, with 0.5-MeV steps above the (α,2n) reaction threshold and with 1.0-MeV steps below that. Conclusions:The new dataset is in agreement with the available values from the literature, but is more precise and extends towards lower energies. Two orders of magnitude lower cross sections were successfully measured than in previous experiments which used γ-ray counting only, thus providing experimental data at lower energies than ever before. The new precision dataset allows us to find the best-fit α-nucleus optical model potential and to predict cross sections in the Gamow window with smaller uncertainties.

Background: The 3 He(α, γ ) 7 Be reaction is a widely studied nuclear reaction; however, it is still not understood with the required precision. It has a great importance both in Big Bang nucleosynthesis and in solar hydrogen burning. The low mass number of the reaction partners makes it also suitable for testing microscopic calculations. Purpose: Despite the high number of experimental studies, none of them addresses the 3 He(α, γ ) 7 Be reaction cross sections above 3.1-MeV center-of-mass energy. Recently, a previously unobserved resonance in the 6 Li(p, γ ) 7 Be reaction suggested a new level in 7 Be, which would also have an impact on the 3 He(α, γ ) 7 Be reaction in the energy range above 4.0 MeV. The aim of the present experiment is to measure the 3 He(α, γ ) 7 Be reaction cross section in the energy range of the proposed level. Method: For this investigation the activation technique was used. A thin window gas-cell target confining 3 He gas was irradiated using an α beam. The 7 Be produced was implanted into the exit foil. The 7 Be activity was determined by counting the γ rays following its decay by a well-shielded high-purity germanium detector. Results: Reaction cross sections have been determined between E cm = 4.0 and 4.4 MeV with 0.04-MeV steps covering the energy range of the proposed nuclear level. One lower-energy cross-section point was also determined to be able to compare the results with previous studies. Conclusions: A constant cross section of around 10.5 μb was observed around the 7 Be proton separation energy. An upper limit of 45 neV for the strength of a 3 He(α, γ ) 7 Be resonance is derived.

Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favor a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion timescales, and (α,n) reactions become critical to move matter toward heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the α+nucleus optical potential. There are several sets of parameters to calculate the α+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the 96Zr(α,n)99Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV ≤ Ec.m. ≤ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for Z = 44–48 isotopes under different neutrino-driven wind conditions.