Proton Capture on ^{17}O and its astrophysical implications

The reaction $^{17}$O$(p,\gamma)^{18}$F influences hydrogen-burning nucleosynthesis in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars and classical novae. In the relevant temperature range for these environments ($T_{9}=0.01-0.4), the main contributions to the rate of this reaction are the direct capture process, two low lying narrow resonances ($E_{r}=65.1$and 183 keV) and the low-energy tails of two broad resonances ($E_{r}=557$and 677 keV). Previous measurements and calculations give contradictory results for the direct capture contribution which in turn increases the uncertainty of the reaction rate. In addition, very few published cross section data exist for the high energy region that might affect the interpretation of the direct capture and the contributions of the broad resonances in the lower energy range. This work aims to address these issues. The reaction cross section was measured in a wide proton energy range ($E_{c.m.}=345$- 1700 keV) and at several angles ($\theta_{lab}=0^{\circ},45^{\circ},90^{\circ},135^{\circ}$). The observed primary$\gamma$-transitions were used as input in an$R$-matrix code in order to obtain the contribution of the direct capture and the two broad resonances to the low-energy region. The extrapolated S-factor from the present data is in good agreement with the existing literature data in the low-energy region. A new reaction rate was calculated from the combined results of this work and literature S-factor determinations. Resonance strengths and branchings are reported for several$^{18}$F states. We were able to extrapolate the astrophysical S-factor of the reaction$^{17}$O$(p,\gamma)^{18}$F at low energies from cross section data taken at higher energies. No significant changes in the nucleosynthesis are expected from the newly calculated reaction rate.Comment: Accepted in Physical Review

Cross section measurement of N 14 ( p , γ ) O 15 in the CNO cycle

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}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{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}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O} reaction has large contributions from the transitions to the ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ excited state and the ground state of $^{15}\mathrm{O}$. The ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ 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. Furthermore, 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 ${E}_{p}=0.7$ to 3.6 MeV for both the ground state and the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transitions at {\ensuremath{\theta}}_{\text{lab}}={0}^{\ensuremath{\circ}}, {45}^{\ensuremath{\circ}}, {90}^{\ensuremath{\circ}}, {135}^{\ensuremath{\circ}}, and {150}^{\ensuremath{\circ}}. Both TiN and implanted-$^{14}\mathrm{N}$ targets were utilized. \ensuremath{\gamma} rays were detected by using an array of high-purity germanium detectors.Results: The excitation function as well as angular distributions of the two transitions were measured. A multichannel $R$-matrix 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.Conclusion: The astrophysical $S$ factors of the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ 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 {S}_{6.79}(0)=1.29\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.09(\mathrm{syst})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b} and {S}_{\text{g.s.}}(0)=0.42\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b}. While significant improvement and consistency is found in modeling the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transition, large inconsistencies in both the $R$-matrix fitting and the low-energy data are reaffirmed for the ground-state transition. Reflecting this, a systematic uncertainty of {}_{\ensuremath{-}0.19}^{+0.09}\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b} is recommended for the ground-state transition

Mg And Al production in intermediate-mass asymptotic giant branch stars

We study how the production of the magnesium and aluminum isotopes in intermediate mass Asymptotic Giant Branch stars is affected by uncertainties in the 22Ne + α-capture reaction rates. We use new estimates of these rates with smaller uncertainties com

Elastic and Inelastic Scattering of Negative Pions on Carbon between 120 and 280 MeV

info:eu-repo/semantics/publishe

R-matrix analysis of ^{16}O compound nucleus reactions

Background: Over the past 60 years, a large amount of experimental nuclear data have been obtained for reactions which probe the 16O compound nucleus near the α 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 15N(p,γ0)16O reaction data. Methods: Reaction data for all available reaction channels were fit simultaneously 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 16O below Ex=13.5 MeV. The resulting fit was used to extract an improved determination of the low-energy S factor for the reactions 15N(p, γ)16O and 15N(p, α)12C. Conclusion: The feasibility and advantages of a complete multiple entrance/exit channel R-matrix description for the broad level structure of 16O has been achieved. A future publication will investigate the possible effects of the multiple-channel analysis on the reaction 12C(α, γ)16O

Reaction Rate Uncertainties and the Production of 19F in Asymptotic Giant Branch Stars

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(α, γ)180 and 19F(α, p) 22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(α, γ) 18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass ∼3 M⊙ and of about a factor of 7 in stellar models of mass ∼5 M⊙. This larger effect at high masses is due to the high uncertainties of the 19F(α, p) 22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(α, γ) 180 reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged

A ³He neutron detector for the measurement of (α, n) reactions

(alpha,n) reactions are possible neutron sources for nucleosynthesis processes in stars. At the Nuclear Science Laboratory (NSL) of the University of Notre Dame a neutron detector based on He-3 counters embedded in a polyethylene matrix was built to experimentally investigate this type of reactions. Monte Carlo simulations were used to guide the design of the detector used in the experiments. The efficiency of the final detector was experimentally determined and compared to simulations. The detector will be used in future measurements in a next-generation underground laboratory for nuclear astrophysics. (C) 2012 Elsevier B.V. All rights reserved