Measurement of the reaction 17O(α, n) 20Ne and its impact on the s process in massive stars

Best, A. and Beard, M. and Görres, J. and Couder, M. and deBoer, R. and Falahat, S. and Güray, R. T. and Kontos, A. and Kratz, K. -L. and LeBlanc, P. J. and Li, Q. and O'Brien, S. and Özkan, N. and Pignatari, Marco and Sonnabend, K. and Talwar, R. and Tan, W. and Uberseder, E. and Wiescher, M. (2013) Measurement of the reaction 17O(α, n) 20Ne and its impact on the s process in massive stars. PHYSICAL REVIEW C, 87 (4). ISSN 2469-9985

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Abstract

Background: The ratio between the rates of the reactions 17O(α, n) 20Ne and 17O(α, γ) 21Ne determines whether 16O is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by 16O(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 17O(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 20Ne. 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 channels 17O(α, n0) 20Ne and 17O(α, n1γ) 20Ne were measured in the energy range Eα = 800 keV to 2300 keV. A new 17O(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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