The cross section of the reaction 112 Sn(α, γ ) 116 Te has been measured in the energy range of astrophysical interest for the p-process. Highly enriched self-supporting 112 Sn foils were bombarded with α beams in the effective center of mass energy range from 7.59 to 11.42 MeV at the Notre Dame FN Tandem Van de Graaff accelerator. The characteristic activity of 116 Te was counted with a pair of large volume Ge clover detectors in close geometry to maximize the detection efficiency. The cross section of the concurrent (α, p) reaction has also been measured. The results are compared with statistical model predictions for different global α-nucleus potentials.

Background:The ratio between the rates of the reactions 17 O(α, n) 20 Ne and 17 O(α, γ) 21 Ne determines whether 16 O is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by 16 O(n, γ) are recycled into the stellar environment. This ratio is of particular relevance to constrain the s process yields of fast rotating massive stars at low metallicity.Purpose: Recent results on the (α, γ) channel have made it necessary to measure the (α, n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. Method:The 17 O(α, n (0+1) ) reaction has been measured with a moderating neutron detector. In addition, the (α, n1) channel has been measured independently by observation of the characteristic 1633 keV γ-transition in 20 Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (α, n) and (α, γ) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. Results: The reaction channels17 O(α, n0) 20 Ne and 17 O(α, n1γ) 20 Ne were measured in the energy range Eα = 800 keV to 2300 keV. A new 17 O(α, n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. At typical He burning temperatures, the combination of the new (α, n) rate with a previously measured (α, γ) rate gives approximately the same ratio as current compilations. The influence on the nucleosynthesis of the s process in massive stars at low metallicity is discussed.Conclusions: It was found that in He burning conditions the (α, γ) channel is strong enough to compete with the neutron channel. This leads to a less efficient neutron recycling compared to a previous suggestion of a very weak (α, γ) channel. S process calculations using our rates confirm that massive rotating stars do play a significant role in the production of elements up to Sr, but they strongly reduce the s process contribution to heavier elements.

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The 106 Cd(α, γ) 110 Sn reaction cross section has been measured in the energy range of the Gamow window for the astrophysical p-process scenario. The cross sections for 106 Cd(α,n) 109 Sn and for 106 Cd(α,p) 109 In below the (α,n) threshold have also been determined. The results are compared with predictions of the statistical model code NON SMOKER using different input parameters. The comparison shows that a discrepancy for 106 Cd(α, γ) 110 Sn when using the standard optical potentials can be removed with a different α+ 106 Cd potential. Some astrophysical implications are discussed.