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 N-14(p,gamma)O-15 reaction rate; thus, the determination of the cross section at astrophysical temperatures is of great interest.Purpose: The total cross section of the N-14(p,gamma)O-15 reaction has large contributions from the transitions to the E-x = 6.79 MeV excited state and the ground state of O-15. The E-x = 6.79 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 MeV transitions at theta(lab) = 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 150 degrees. Both TiN and implanted-N-14 targets were utilized. 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 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 +/- 0.04(stat) +/- 0.09(syst) keV b and S-g.s.(0) = 0.42 +/- 0.04(stat) keV b. While significant improvement and consistency is found in modeling the E-x = 6.79 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 (+0.09)(-0.19) keVb is recommended for the ground-state transition.

Background: The slowest reaction in the first CNO cycle is N-14(p,gamma)O-15, therefore its rate determines the overall energy production efficiency of the entire cycle. The cross section presents several strong resonance contributions, especially for the ground-state transition. Some of the properties of the corresponding levels in the O-15 compound nucleus remain uncertain, which affects the uncertainty in extrapolating the capture cross section to the low energy range of astrophysical interest.Purpose: The N-14(p,gamma) O-15 cross section can be described by using the phenomenological R matrix. Over the energy range of interest, only the proton and gamma-ray channels are open. Since resonance capture makes significant contributions to the N-14(p,gamma) O-15 cross section, resonant proton scattering data can be used to provide additional constraints on the R-matrix fit of the capture data.Methods: A 4 MV KN Van de Graaff accelerator was used to bombard protons onto a windowless gas target containing enriched 14N gas over the proton energy range from E-p = 1.0 to 3.0 MeV. Scattered protons were detected at theta(lab) = 90 degrees, 120 degrees, 135 degrees, 150 degrees, and 160 degrees using ruggedized silicon detectors. In addition, a 10 MV FN Tandem Van de Graaff accelerator was used to accelerate protons onto a solid Adenine (C5H5N5) target, of natural isotopic abundance, evaporated onto a thin self-supporting carbon backing, over the energy range from E-p = 1.8 to 4.0 MeV. Scattered protons were detected at 28 angles between theta(lab) = 30.4 degrees and 167.7 degrees by using silicon photodiode detectors.Results: Relative cross sections were extracted from both measurements. While the relative cross sections do not provide as much constraint as absolute measurements, they greatly reduce the dependence of the data on otherwise significant systematic uncertainties, which are more difficult to quantify. The data are fit simultaneously using an R-matrix analysis and level energies and proton widths are extracted. Even with relative measurements, the statistics and large angular coverage of the measurements result in more confident values for the energies and proton widths of several levels; in particular, the broad resonance at E-c.m. = 2.21 MeV, which corresponds to the 3/2(+) level at E-x = 9.51 MeV in O-15. In particular, the s- and d-wave angular-momentum channels are separated.Conclusion: The relative cross sections provide a consistent set of data that can be used to better constrain a full multichannel R-matrix extrapolation of the capture data. It has been demonstrated how the scattering data reduce the uncertainty through a preliminary Monte Carlo uncertainty analysis, but several other issues remain that make large contributions to the uncertainty, which must be addressed by further capture and lifetime measurements.

Background: The ratio between the rates of the reactions O-17(alpha,n)Ne-20 and O-17(alpha,gamma)Ne-21 determines whether O-16 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by O-16(n,gamma) 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 (alpha,gamma) channel have made it necessary to measure the (alpha,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars.Method: The O-17(alpha, n((0+1))) reaction has been measured with a moderating neutron detector. In addition, the (alpha, n(1)) channel has been measured independently by observation of the characteristic 1633 keV gamma transition in Ne-20. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (alpha, n) and (alpha, gamma) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature.Result: The reaction channels O-17(alpha, n(0))Ne-20 and O-17(alpha, n(1)gamma)Ne-20 were measured in the energy range E-alpha = 800 keV to 2300 keV. A new O-17(alpha, 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 (alpha, n) rate with a previously measured (alpha,gamma) 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 (alpha,gamma) 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 (alpha,gamma) 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. DOI: 10.1103/PhysRevC.87.045805

Background: The reaction O-18(alpha, n)Ne-21 is a part of the reaction chains leading to the production of F-19 and Ne-22 during He burning in low-mass and massive AGB stars, respectively. Additionally, it has been observed as a strong background source in the measurement of other (alpha, n) reactions.Purpose: Previously low-energy O-18(alpha, n)Ne-21 cross section data have only been available in a non-peer-reviewed form. An improved measurement of this reaction has been done to both clarify its astrophysical influence as well as to provide background yield data for future (alpha, n) experiments.Method: The O-18(alpha, n((0+1))) reaction has been measured with a moderating neutron detector. In addition the (alpha, n(1)gamma) channel has been measured independently by observation of the characteristic 350.7 keV gamma transition in Ne-21. The reaction cross section at energies above E-alpha = 1100 keV was determined by a simultaneous R-matrix fit to both channels. The strengths of the two lowest-energy resonances at E-alpha = 959 keV and E-alpha = 1066 keV were analyzed separately using individual Breit-Wigner fits.Results: The cross section of both reaction channels, O-18(alpha, n(0))Ne-21 and O-18(alpha, n(1)gamma)Ne-21, was determined from the threshold energies at 851 keV and 1280 keV, respectively, to 2300 keV. A new reaction rate has been deduced for the temperature range of 0.1 GK to 10 GK. A previously reported resonance at E-alpha = 888 keV is explained as background from the contaminant reaction O-17(alpha, n)Ne-20.Conclusions: In general, our reaction rate is slightly lower than the reaction rates in recent compilations. At temperatures below 0.2 GK the present rate is significantly lower because it could be shown that the lowest reported resonance is background from the reaction O-17(alpha, n)Ne-20 that has been wrongly assigned to O-18(alpha, n)Ne-21.

The ratio of the reaction rates of the competing channels O-17(alpha, gamma)Ne-21 and O-17(alpha, n)Ne-20 determines the efficiency of O-16 as a neutron poison in the weak s process. We present results of the first measurement of the reaction O-17(alpha, gamma)Ne-21 and an improved study of the reaction O-17(alpha, n)Ne-20, including an independent measurement of the O-17(alpha, n(1))Ne-20 channel and a simultaneous R-Matrix fit to both the n(o) and the n(1) channels. The impact of the new data on the weak s process is discussed.

Production of proton-rich nuclei beyond iron in stars proceeds via the p process, i.e., a sequence of photodisintegration reactions, (gamma,n), (gamma,p), and (gamma,alpha) on heavy nuclei at temperatures of 2-3 x 10(9) K. The involved reaction rates are typically calculated with the statistical Hauser-Feshbach (HF) model. However, the HF model performs poorly in calculating the critical (gamma,alpha) rates due to the uncertainty of the alpha optical potentials applied. To test the reliability of the HF calculations and provide a systematic understanding of the alpha optical potential at energies of astrophysical interest, a series of precision alpha scattering measurements were carried out at the Notre Dame FN Tandem Accelerator. Specifically, Cd-106, Sn-118, and Te-120,Te-124,Te-126,Te-128,Te-130 were studied at energies both below and above the Coulomb barrier. A new parametrization of the a optical potential was derived of the elastic scattering cross section data. The derived potential was applied for calculating the alpha-induced reaction cross sections on these nuclei using the HF approach. The results were compared to the corresponding experimental values obtained from previous activation measurements on Cd, Sn, and Te isotopes.

Background: Resonances observed through elastic scattering of protons on N-15 can provide information about the partial widths, spin parities, and energies of excited states in O-16 near the proton separation energy. This is the same energy region important for the nuclear astrophysics reactions N-15(p,gamma)O-16 and N-15(p,alpha)C-12. While previous measurements have been made, they are limited in scope, especially in their angular coverage. Purpose: Obtain additional N-15(p,p)N-15 reaction data which can be used in a global multiple-channel R-matrix analysis of the O-16 compound nucleus in order to better constrain the level parameters of states which contribute to the reaction N-15(p,gamma)O-16. Methods: Measure the excitation functions of N-15(p,p)N-15 over an energy range from E-p = 0.6 to 1.8 MeV at laboratory angles of 90 degrees, 105 degrees, 135 degrees, 150 degrees, and 165 degrees. The reaction N-15(p,alpha(0))C-12 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 O-16. 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 O-16 over a similar energy region.

Background Themain energy productionmechanism for massive stars during hydrogen burning is the CNO cycle. The reactions N-15(p,gamma)O-16 and N-15(p,alpha(0))C-12 form a branch point in this cycle. The ratio of the corresponding reaction rates determines the CNO abundances evolving during this early stage of the star's life which affects the subsequent nucleosynthesis in later phases of stellar evolution. Determining the cross sections for these reactions at stellar energies is often very difficult. Measurements of other reactions that populate the same compound nucleus can often be used to indirectly determine the cross section of interest. Purpose The nuclear level properties of broad resonances in O-16 which characterize the cross section of the reactions N-15(p,gamma)O-16 and N-15(p,alpha(0))C-12 must be well known in order to accurately extrapolate the measured cross sections to the stellar energy range. The R-matrix formalism is a powerful technique for interpreting these cross sections and is greatly enhanced by additional data in other reaction channels. In a previous publication, measurements were reported for the cross section of the reaction N-15(p,gamma)O-16 for the ground state transition only. Concurrently, gamma-ray measurements were recorded for the cascade transitions to the E-x = 6.050, 6.130, and 7.117 MeV bound states of O-16 as well as from the reaction N-15(p,alpha(1)gamma)C-12. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the N-15(p,alpha(1)gamma)C-12 reaction found in the literature may suffer from substantial errors due to target contamination. Methods Angle integrated cross sections are measured over the proton energy range from E-p = 0.14 to 1.80 MeV for the gamma-ray cascade transitions and for the reaction N-15(p,alpha(1)gamma)C-12. Results De-excitations associated with several compound nucleus states in O-16 are observed in both the gamma-ray and alpha(1) channels. An R-matrix analysis is performed and partial decay widths are deduced for several previously unobserved decay branchings from these states. Conclusion For the first time, excitation curves for the cascade transitions to the O-16 bound states at E-x = 6.050, 6.130, and 7.117 MeV are reported over the energy range from E-p = 0.14 to 1.80 MeV. In addition, an improved measurement of the N-15(p,alpha(1)gamma)C-12 excitation curve has been made over a similar energy range.

Background The main energy production mechanism for massive stars during hydrogen burning is the CNO cycle. The reactions ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O and ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{0})}^{12}$C form a branch point in this cycle. The ratio of the corresponding reaction rates determines the CNO abundances evolving during this early stage of the star's life which affects the subsequent nucleosynthesis in later phases of stellar evolution. Determining the cross sections for these reactions at stellar energies is often very difficult. Measurements of other reactions that populate the same compound nucleus can often be used to indirectly determine the cross section of interest.Purpose The nuclear level properties of broad resonances in ${}^{16}$O which characterize the cross section of the reactions ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O and ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{0})}^{12}$C must be well known in order to accurately extrapolate the measured cross sections to the stellar energy range. The $R$-matrix formalism is a powerful technique for interpreting these cross sections and is greatly enhanced by additional data in other reaction channels. In a previous publication, measurements were reported for the cross section of the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma})}^{16}$O for the ground state transition only. Concurrently, $\ensuremath{\gamma}$-ray measurements were recorded for the cascade transitions to the ${E}_{x}$ $=$ 6.050, 6.130, and 7.117 MeV bound states of ${}^{16}$O as well as from the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C reaction found in the literature may suffer from substantial errors due to target contamination.Methods Angle integrated cross sections are measured over the proton energy range from ${E}_{p}$ $=$ 0.14 to 1.80 MeV for the $\ensuremath{\gamma}$-ray cascade transitions and for the reaction ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C.Results De-excitations associated with several compound nucleus states in ${}^{16}$O are observed in both the $\ensuremath{\gamma}$-ray and ${\ensuremath{\alpha}}_{1}$ channels. An $R$-matrix analysis is performed and partial decay widths are deduced for several previously unobserved decay branchings from these states.Conclusion For the first time, excitation curves for the cascade transitions to the ${}^{16}$O bound states at ${E}_{x}$ $=$ 6.050, 6.130, and 7.117 MeV are reported over the energy range from ${E}_{p}$ $=$ 0.14 to 1.80 MeV. In addition, an improved measurement of the ${}^{15}$N${(p,\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\alpha}}_{1}\ensuremath{\gamma})}^{12}$C excitation curve has been made over a similar energy range.

The ratio of the reaction rates of the competing channels 17O(αγ)21Ne and 17O(α,n)20Ne determines the efficiency of 16O as a neutron poison in the s process in low metallicity rotating stars. It has a large impact on the element production, either producing elements to the mass range of A=90 in case of a significant poisoning effect or extending the mass range up to the region of A=150 if the γ channel is of negligible strength. We present an improved study of the reaction 17O(α,n)20Ne, including an independent measurement of the 17O(α,n1)20Ne channel. A simultaneous R-Matrix fit to both the n0 and the n1 channels has been performed. New reaction rates, including recent data on the 17O(α,γ)21Ne reaction, have been calculated and used as input for stellar network calculations and their impact on the s process in rotating massive stars is discussed.

Background: Resonances observed through elastic scattering of protons on ${}^{15}$N can provide information about the partial widths, spin parities, and energies of excited states in ${}^{16}$O near the proton separation energy. This is the same energy region important for the nuclear astrophysics reactions ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O and ${}^{15}$N${(p,\ensuremath{\alpha})}^{12}$C. While previous measurements have been made, they are limited in scope, especially in their angular coverage. Purpose: Obtain additional ${}^{15}$N${(p,p)}^{15}$N reaction data which can be used in a global multiple-channel $R$-matrix analysis of the ${}^{16}$O compound nucleus in order to better constrain the level parameters of states which contribute to the reaction ${}^{15}$N${(p,\ensuremath{\gamma})}^{16}$O. Methods: Measure the excitation functions of ${}^{15}$N${(p,p)}^{15}$N over an energy range from ${E}_{p}$ $=$ 0.6 to 1.8 MeV at laboratory angles of 90${}^{\ensuremath{\circ}}$, 105${}^{\ensuremath{\circ}}$, 135${}^{\ensuremath{\circ}}$, 150${}^{\ensuremath{\circ}}$, and 165${}^{\ensuremath{\circ}}$. The reaction ${}^{15}$N${(p,{\ensuremath{\alpha}}_{0})}^{12}$C 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 ${}^{16}$O. 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 ${}^{16}$O over a similar energy region.

The ratio of the reaction rates of the competing channels O-17(alpha,gamma)Ne-21 and O-17(alpha,n)Ne-20 determines the efficiency of O-16 as a neutron poison in the s process in low metallicity rotating stars. It has a large impact on the element production, either producing elements to the mass range of A=90 in case of a significant poisoning effect or extending the mass range up to the region of A=150 if the gamma channel is of negligible strength. We present an improved study of the reaction O-17(alpha, n)Ne-20, including an independent measurement of the O-17(alpha, n(1))Ne-20 channel. A simultaneous R-Matrix fit to both the n(0) and the n(1) channels has been performed. New reaction rates, including recent data on the O-17(alpha,gamma)Ne-21 reaction, have been calculated and used as input for stellar network calculations and their impact on the s process in rotating massive stars is discussed.

The ratio of the reaction rates of the competing channels {sup 17}O({alpha}{gamma}){sup 21}Ne and {sup 17}O({alpha},n){sup 20}Ne determines the efficiency of {sup 16}O as a neutron poison in the s process in low metallicity rotating stars. It has a large impact on the element production, either producing elements to the mass range of A=90 in case of a significant poisoning effect or extending the mass range up to the region of A=150 if the {gamma} channel is of negligible strength. We present an improved study of the reaction {sup 17}O({alpha},n){sup 20}Ne, including an independent measurement of the {sup 17}O({alpha},n{sub 1}){sup 20}Ne channel. A simultaneous R-Matrix fit to both the n{sub 0} and the n{sub 1} channels has been performed. New reaction rates, including recent data on the {sup 17}O({alpha},{gamma}){sup 21}Ne reaction, have been calculated and used as input for stellar network calculations and their impact on the s process in rotating massive stars is discussed.