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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.

Background Over the last 60 years, a large amount of experimental nuclear data has been obtained for reactions which probe the 16 O compound nucleus near the alpha and proton separation energies, the energy regimes most important for nuclear astrophysics. Difficulties and inconsistencies in R-matrix fits of the individual reactions prompt a more complete analysis.Purpose Determine the level of consistency between the wide variety of experimental data using a multiple entrance/exit channel R-matrix framework. Using a consistent set of data from multiple reaction channels, attain an improved fitting for the 15 N(p, γ0) 16 O reaction data.Methods Reaction data for all available reaction channels were fit simultaneous using a multichannel R-matrix code.Results Over the wide range of experimental data considered, a high level of consistency was found, resulting in a single consistent R-matrix fit which described the broad level structure of 16 O below Ex = 13.5 MeV. The resulting fit was used to extract an improved determination of the low energy S-factor for the reactions 15 N(p, γ) 16 O and 15 N(p, α) 12 C. ConclusionThe feasibility and advantages of a complete multiple entrance/exit channel R-matrix description for the broad level structure of 16 O has been achieved. A future publication will investigate the possible effects of the multiple channel analysis on the reaction 12 C(α, γ) 16 O.

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Background The CNO cycle is the main energy source in stars more massive than our sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. In our sun about 1.6% of the total solar neutrino flux comes from the CNO cycle. The largest uncertainty in the prediction of this CNO flux from the standard solar model comes from the uncertainty in the 14 N(p, γ) 15 O reaction rate, thus the determination of the cross section at astrophysical temperatures is of great interest.Purpose The total cross section of the 14 N(p, γ) 15 O reaction has large contributions from the transitions to the Ex = 6.79 MeV excited state and the ground state of 15 O. The Ex = 6.79 MeV transition is dominated by radiative direct capture, while the ground state is a complex mixture of direct and resonance capture components and the interferences between them. Recent studies have concentrated on cross section measurements at very low energies, but broad resonances at higher energy may also play a role. A single measurement has been made that covers a broad higher energy range but it has large uncertainties stemming from uncorrected summing effects. Further, the extrapolations of the cross section vary significantly depending on the data sets considered. Thus new direct measurements have been made to improve the previous high energy studies and to better constrain the extrapolation.Methods Measurements were performed at the low energy accelerator facilities of the nuclear science laboratory at the University of Notre Dame. The cross section was measured over the proton energy range from Ep = 0.7 to 3.6 MeV for both the ground state and the Ex = 6.79 MeV transitions at θ lab = 0• , 135• and 150• . Both TiN and implanted 14 N targets were utilized. γ-rays were detected using an array of high purity germanium detectors.Results The excitation function as well as angular distributions of the two transitions were measured. A multi-channel Rmatrix analysis was performed with the present data and is compared with previous measurements. The analysis covers a wide energy range so that the contributions from broad resonances and direct capture can be better constrained. ConclusionThe astrophysical S-factors of the Ex = 6.79 MeV and the ground state transitions were extrapolated to low energies with the newly measured differential cross section data. Based on the present work, the extrapolations yield

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