Measurement of the reaction17O(α,n)20Ne and its impact on thes

Abstract

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.

Uncertainty of the astrophysical O17,18(α,n)Ne20,21Mohr1 2017Phys. Rev. C9070View full textAdd to dashboardCite

“…The Best results are slightly lower than NACRE, but the deviation does not exceed a factor of two for 0.5 < ∼ T 9 < ∼ 2.0. The significantly lower N A σv at very low temperatures below T 9 = 0.5 results from the fact that the lowest resonance in the Denker data at 888 keV is considered as spurious and has been assigned to the 17 O(α,n) 20 Ne reaction by Best et al [23]. The results are shown in Fig.…”

“…This confirms the TALYS calculations for the branchings to individual final states in 20 Ne. Again similar to the 18 O case, Best et al [23] have extended the neutron counting experiments by an additional measurement of the 17 O(α,n 1 ) 20 Ne reaction by γ-spectroscopy of the 1634 keV γ-ray from the decay of the first excited state in 20 Ne to the ground state. The (α,n 1 ) data at low energies are on average well reproduced by the TALYS calculations.…”

“…This possibility was unfortunately disregarded in the previous works [23,34]. Three data sets are available for neutrons in the low MeV energy region.…”

“…Two versions of N A σv are listed in [23]. The so-called experimental rate N A σv exp is calculated from the experimental resonance strengths (excluding contributions from resonances outside the experimental energy range from 0.7 to 1.9 MeV).…”

“…However, as the Best recommended N A σv rec is based on well-chosen average properties of several unobserved low-lying resonances, the resulting N A σv rec should not deviate by orders of magnitude from a statistical model calculation which is also based on average properties. For completeness it can be 20 Ne reaction from the NACRE compilation, from the experimental resonance properties by Best et al [23], and from the statistical model calculations using TALYS. The upper part (a) shows the rates NA σv ; the lower part (b) shows the rates normalized to the NACRE fit, including horizontal arrows which indicate the approximate validity of the different rates.…”