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

Coulomb dissociation of N 20,21

Neutron-rich light nuclei and their reactions play an important role in the creation of chemical elements. Here, data from a Coulomb dissociation experiment on N20,21 are reported. Relativistic N20,21 ions impinged on a lead target and the Coulomb dissociation cross section was determined in a kinematically complete experiment. Using the detailed balance theorem, the N19(n,γ)N20 and N20(n,γ)N21 excitation functions and thermonuclear reaction rates have been determined. The N19(n,γ)N20 rate is up to a factor of 5 higher at

Statistical Model Calculations for (n,γ) Reactions

Hauser-Feshbach (HF) cross sections are of enormous importance for a wide range of applications, from waste transmutation and nuclear technologies, to medical applications, and nuclear astrophysics. It is a well-observed result that different nuclear input models sensitively affect HF cross section calculations. Less well known however are the effects on calculations originating from model-specific implementation details (such as level density parameter, matching energy, back-shift and giant dipole parameters), as well as effects from non-model aspects, such as experimental data truncation and transmission function energy binning. To investigate the effects or these various aspects, Maxwellian-averaged neutron capture cross sections have been calculated for approximately 340 nuclei. The relative effects of these model details will be discussed

Statistical Model Calculations for (n,

Hauser-Feshbach (HF) cross sections are of enormous importance for a wide range of applications, from waste transmutation and nuclear technologies, to medical applications, and nuclear astrophysics. It is a well-observed result that different nuclear input models sensitively affect HF cross section calculations. Less well known however are the effects on calculations originating from model-specific implementation details (such as level density parameter, matching energy, back-shift and giant dipole parameters), as well as effects from non-model aspects, such as experimental data truncation and transmission function energy binning. To investigate the effects or these various aspects, Maxwellian-averaged neutron capture cross sections have been calculated for approximately 340 nuclei. The relative effects of these model details will be discussed

Statistical Model Calculations for (n, γ

Hauser-Feshbach (HF) cross sections are of enormous importance for a wide range of applications, from waste transmutation and nuclear technologies, to medical applications, and nuclear astrophysics. It is a well-observed result that different nuclear input models sensitively affect HF cross section calculations. Less well known however are the effects on calculations originating from model-specific implementation details (such as level density parameter, matching energy, back-shift and giant dipole parameters), as well as effects from non-model aspects, such as experimental data truncation and transmission function energy binning. To investigate the effects or these various aspects, Maxwellian-averaged neutron capture cross sections have been calculated for approximately 340 nuclei. The relative effects of these model details will be discussed

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

Elastic scattering of protons from 15N

Background: Resonances observed through elastic scattering of protons on 15N can provide information about the partial widths, spin parities, and energies of excited states in 16O near the proton separation energy. This is the same energy region important for the nuclear astrophysics reactions 15N(p,γ)16O and 15N(p,α)12C. While previous measurements have been made, they are limited in scope, especially in their angular coverage. Purpose: Obtain additional 15N(p,p)15N reaction data which can be used in a global multiple-channel R-matrix analysis of the 16O compound nucleus in order to better constrain the level parameters of states which contribute to the reaction 15N(p,γ)16O. Methods: Measure the excitation functions of 15N(p,p)15N over an energy range from Ep = 0.6 to 1.8 MeV at laboratory angles of 90∘, 105∘, 135∘, 150∘, and 165∘. The reaction 15N(p,α0)12C was measured concurrently. Results: Ratios of the excitation functions were extracted from the yield data. Resonances were identified in the yield ratio data which correspond to previously reported levels in 16O. An R-matrix analysis, which fits the present data as well as previous measurements from the literature simultaneously, finds reasonable agreement between the current measurements and those in the literature. Conclusions: The additional data from this measurement will be combined with previous literature data in a comprehensive R-matrix analysis of reactions which populate 16O over a similar energy region