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Measurement of the reaction 17O(,n)20Ne and its impact on the s process in massive stars
Journal
Physical review. C, Nuclear physics
Volume
87
Number
4
Pages / Article-Number
045805
Abstract
Background: The ratio between the rates of the reactions O-17(alpha,n)Ne-20 and O-17(alpha,gamma)Ne-21 determines whether O-16 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by O-16(n,gamma) 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 (alpha,gamma) channel have made it necessary to measure the (alpha,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars.
Method: The O-17(alpha, n((0+1))) reaction has been measured with a moderating neutron detector. In addition, the (alpha, n(1)) channel has been measured independently by observation of the characteristic 1633 keV gamma transition in Ne-20. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (alpha, n) and (alpha, gamma) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature.
Result: The reaction channels O-17(alpha, n(0))Ne-20 and O-17(alpha, n(1)gamma)Ne-20 were measured in the energy range E-alpha = 800 keV to 2300 keV. A new O-17(alpha, 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 (alpha, n) rate with a previously measured (alpha,gamma) 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 (alpha,gamma) 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 (alpha,gamma) 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.
