At temperatures below 0.1 GK, the 19F(p, γ) 20Ne reaction is the only breakout path out of the CNO cycle. Experimental studies of this reaction are challenging from a technical perspective due to ...copious γ -ray background from the far stronger 19F(p, α) 16O reaction channel. Here, we present the first inverse kinematics study of the 19F(p, γ) 20Ne reaction, in which we measure the strength of the 323-keV resonance. We find a strength value of ωγ = $3.3$$^{+1.1}_{–0.9}$ meV, which is a factor of two larger than the most recent previous study. Here, the discrepancy is likely the result of a direct to ground state transition which previous studies were not sensitive to. We also observe the transition to the first 2– state, which has not been observed for this resonance in previous studies. A new thermonuclear reaction rate is calculated and compared with the literature.
The efficiency of the slow neutron-capture process in massive stars is strongly influenced by neutron-capture reactions on light elements. At low metallicity, 16O is an important neutron absorber, ...but the effectiveness of 16O as a light-element neutron poison is modified by competition between subsequent 17O(α,n)20Ne and 17O(α,γ)21Ne reactions. The strengths of key 17O(α,γ)21Ne resonances within the Gamow window for core helium burning in massive stars are not well constrained by experiment. This work presents more precise measurements of resonances in the energy range Ec.m. = 612–1319 keV. We extract resonance strengths of ωγ638 = 4.85 ± 0.79 μeV, ωγ721 =13.1$^{+3.2}_{-2.4}$ μeV, ωγ814 = 7.72 ± 0.55 meV, and ωγ1318 = 136 ±13 meV, for resonances at Ec.m. = 638, 721, 814, and 1318 keV, respectively. We also report an upper limit for the 612 keV resonance of ωγ < 140 neV (95% c.l.), which effectively rules out any significant contribution from this resonance to the reaction rate. From this work, a new 17O(α,γ)21Ne thermonuclear reaction rate is calculated and compared to the literature. The effect of present uncertainties in the 17O(α,γ)21Ne reaction rate on weak s-process yields are then explored using postprocessing calculations based on a rotating 20M⊙ low-metallicity massive star. The resulting 17O(α,γ)21Ne reaction rate is lower with respect to the preexisting literature and found to enhance weak s-process yields in rotating massive star models.
Context.
By changing the internal composition of stars, nuclear reactions play a key role in their evolution and spur their contribution to the chemical evolution of galaxies. The STELLA ...collaboration recently carried out new direct measurements of the
12
C +
12
C fusion cross section – one of the key reactions occurring in C-burning regions in massive stars. Using a coincidence technique, accurate measurements were obtained for many different energies, with the lowest energy explored according to the Gamow window for massive stars.
Aims.
This work presents new
12
C +
12
C reaction rates in the form of numerical tables with associated uncertainty estimations, as well as analytical formulae that can be directly implemented into stellar evolution codes. We also describe the impact of these new rates on C-burning in stars.
Methods.
We determined reaction rates for two cross section extrapolation models: one based on the fusion-hindrance phenomenon and the other on fusion-hindrance plus a resonance. We then compared our results to prior data. Using the GENEC stellar evolution code, we study how these new rates impact the C-burning phases in two sets of stellar models for stars with 12
M
⊙
and 25
M
⊙
initial masses, which were chosen to be highly representative of the diversity of massive stars.
Results.
The effective temperatures of C-burning in both sets of stellar models are entirely covered by the sensitivity of the present experimental data and no extrapolation of the rates is required. Although the rates may differ by more than an order of magnitude for temperatures typical of C-burning, the impacts on the stellar structures during that phase remain modest. This is a consequence of the readjustment of the stellar structure to a change of nuclear reaction rate for reactions that are shown to be important for energy production. For the hindrance case, the C-burning phase is found to occur at central temperatures that are 10% higher than with the hindrance plus resonance rate. Its C-burning lifetime is reduced by a factor of two. This model, nevertheless, loses more entropy than the other one; thus, it enters into the degeneracy regime earlier, which will impact the last stages of the evolution at the pre-core collapse time. The hindrance model produces up to 60% more neon. The impact of the different rates on the
s
-process occurring during the C-burning phase is modest, affecting the final abundances of
s
-processed elements by at most 20% (cobalt).
Among reactions with strong impact on classical novae model predictions, 30P(p,γ)31S is one of the few remained that are worthy to be measured accurately, because of their rate uncertainty, as like ...as 18F(p,α)15O and 25Al(pγ)26Si. To reduce the nuclear uncertainties associated to this reaction, we performed an experiment at ALTO facility of Orsay using the 31P(3He,t)31S reaction to populate 31S excited states of astrophysical interest and detect in coincidence the protons coming from the decay of the populated states in order to extract the proton branching ratios. After a presentation of the astrophysical context of this work, the current situation of the 30P(p,γ)31S reaction rate will be discussed. Then the experiment set-up of this work and the analysis of the single events will be presented.
The 12C+12C fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle ...coincidence technique to measure the 12C+12C S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below Ec.m. = 3 MeV.
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