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Register a new userConstraining the S factor of 15N(p,g)16O at Astrophysical Energies
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The 15N(p,g)16O reaction represents a break out reaction linking the first and second cycle of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to the ground state of 16O. Interference effects between these contributions in both the low energy region (Ep < 338 keV) and in between the two resonances (338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O reaction has been remeasured covering the energy range from Ep=1800 keV down to 130 keV. The results have been analyzed in the framework of a multi-level R-matrix theory and a S(0) value of 39.6 keV b has been found.
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The astrophysical S-factor of 14N(p,gamma)15O has been measured for effective center-of-mass energies between E_eff = 119 and 367 keV at the LUNA facility using TiN solid targets and Ge detectors. The data are in good agreement with previous and recent work at overlapping energies. R-matrix analysis reveals that due to the complex level structure of 15O the extrapolated S(0) value is model dependent and calls for additional experimental efforts to reduce the present uncertainty in S(0) to a level of a few percent as required by astrophysical calculations.
The astrophysical S-factor of the 4He-12C radiative capture is calculated in the potential model at the energy range 0.1-2.0 MeV. Radiative capture 12C(alpha,gamma)16O is extremely relevant for the fate of massive stars and determines if the remnant of a supernova explosion becomes a black hole or a neutron star. Because this reaction occurs at low-energies the experimental measurements is very difficult and perhaps impossible. In this paper, radiative capture of the 12C(alpha,gamma)16O reaction at very low-energies is taken as a case study. In comparison with other theoretical methods and available experimental data, excellent agreement is achieved for the astrophysical S-factor of this process.
In the present work we report on a new measurement of resonance strengths in the reaction 25Mg(p,gamma)26Al at E_cm= 92 and 189 keV. This study was performed at the LUNA facility in the Gran Sasso underground laboratory using a 4pi BGO summing crystal. For the first time the 92 keV resonance was directly observed and a resonance strength omega-gamma=(2.9+/-0.6)x10E-10 eV was determined. Additionally, the gamma-ray branchings and strength of the 189 keV resonance were studied with a high resolution HPGe detector yielding an omega-gamma value in agreement with the BGO measurement, but 20% larger compared to previous works.
The $^{12}$C+$^{12}$C fusion reaction plays a crucial role in stellar evolution and explosions. Its open reaction channels mainly include $alpha$, $p$, $n$, and ${}^{8}$Be. Despite more than a half century of efforts, large discrepancies remain among the experimental data measured using various techniques. In this work, we analyze the existing data using the statistical model. Our calculation shows: 1) the relative systematic uncertainties of the predicted branching ratios get smaller as the predicted ratios increase; 2) the total modified astrophysical S-factors (S$^*$ factors) of the $p$ and $alpha$ channels can each be obtained by summing the S$^*$ factors of their corresponding ground-state transitions and the characteristic $gamma$ rays while taking into account the contributions of the missing channels to the latter. After applying corrections based on branching ratios predicted by the statistical model, an agreement is achieved among the different data sets at ${E}_{cm}>$4 MeV, while some discrepancies remain at lower energies suggesting the need for better measurements in the near future. We find that the recent S$^*$ factor obtained from an indirect measurement is inconsistent with the direct measurement at energies below 2.6 MeV. We recommend upper and lower limits for the ${}^{12}$C+${}^{12}$C S$^*$ factor based on the existing models. A new $^{12}$C+$^{12}$C reaction rate is also recommended.
The $^{14}textrm{N(p,}gammatextrm{)}^{15}textrm{O}$ reaction is the slowest reaction of the carbon-nitrogen cycle of hydrogen burning and thus determines its rate. The precise knowledge of its rate is required to correctly model hydrogen burning in asymptotic giant branch stars. In addition, it is a necessary ingredient for a possible solution of the solar abundance problem by using the solar $^{13}$N and $^{15}$O neutrino fluxes as probes of the carbon and nitrogen abundances in the solar core. After the downward revision of its cross section due to a much lower contribution by one particular transition, capture to the ground state in $^{15}$O, the evaluated total uncertainty is still 8%, in part due to an unsatisfactory knowledge of the excitation function over a wide energy range. The present work reports precise S-factor data at twelve energies between 0.357-1.292~MeV for the strongest transition, capture to the 6.79~MeV excited state in $^{15}$O, and at ten energies between 0.479-1.202~MeV for the second strongest transition, capture to the ground state in $^{15}$O. An R-matrix fit is performed to estimate the impact of the new data on astrophysical energies. The recently suggested slight enhancement of the 6.79~MeV transition at low energy could not be confirmed. The present extrapolated zero-energy S-factors are $S_{6.79}(0)$~=~1.24$pm$0.11~keV~barn and $S_{rm GS}(0)$~=~0.19$pm$0.05~keV~barn.
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