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Background: The ³He(ɑ,ɣ)³Be reaction is important for the neutrino production in the sun's core and the production of 7Li during big bang nucleosynthesis. The reaction mechanism is characterized by a strong direct capture component and nearby broad unbound resonance levels. Purpose: Recent experiments have opened up a new energy window into the reaction mechanism and it becomes more and more evident that, in order to understand the shape of the S factor, theoretical calculations need to take into account possible resonance contributions from higher energies as well. Method: In the present work, a relatively wide energy window was investigated, Ec m. = 300-1460 keV, by detecting the prompt ɣ rays from the reaction. An extensive R-matrix analysis was performed, utilizing all modern literature capture data, as well as elastic scattering data, which are important in constraining some R-matrix parameters. Results: The new experimental data agree very well with the modern literature data. The final result from the R-matrix fit gives a zero-energy S factor of S(0) = 0.554(20) keV b. A table with the newly calculated reaction rate is given. Conclusions: The simultaneous R-matrix analysis of the ³He(ɑ,ɣ)³Be and ³He(ɑ,ɑ)³He channels yielded a reliable fit, consistent with all the included experimental data sets. In order to further constrain the reaction rate within the R-matrix framework, additional high-energy capture data, ɣ-ray angular distributions, and the inclusion of other relevant reaction channels are necessary. [ABSTRACT FROM AUTHOR]

The cross section of the 23Na(n,γ)24Na reaction was measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at kT=5.1 and 25 keV produced via the 18O(p,n)18F and 7Li(p,n)7Be reactions, respectively. The derived capture cross sections 〈σ〉kT=5keV=9.1±0.3 mb and 〈σ〉kT=25keV=2.03±0.05 mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first 23Na resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of s-process nucleosynthesis are discussed. [ABSTRACT FROM AUTHOR]

The stellar (η,γ) cross sections of the Ne isotopes are important for a number of astrophysical quests, i.e., for the interpretation of abundance patterns in presolar material or with respect to the s-process neutron balance in red giant stars. This paper presents resonance studies of experimental data in the keV range, which had not been fully analyzed before. The analyses were carried out with the R-matrix code sammy. With these results for the resonant part and by adding the components due to direct radiative capture, improved Maxwellian-averaged cross sections (MACS) could be determined. At kT=30keV thermal energy we obtain MACS values of 240±29,1263±160, and 53.2±2.7 µbarn for 20Ne, 21Ne, and 22Ne, respectively. In earlier work the stellar rates of 20Ne and 21Ne had been grossly overestimated. 22Ne and 20Ne are significant neutron poisons for the s process in stars because their very small MACS values are compensated by their large abundances. [ABSTRACT FROM AUTHOR]

Background: The slowest reaction in the first CNO cycle is N14(p,?)15O, 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 O15 compound nucleus remain uncertain, which affects the uncertainty in extrapolating the capture cross section to the low energy range of astrophysical interest. Purpose: The N14(p,?)15O cross section can be described by using the phenomenological R matrix. Over the energy range of interest, only the proton and ?-ray channels are open. Since resonance capture makes significant contributions to the N14(p,?)15O 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 N14 gas over the proton energy range from Ep=1.0 to 3.0 MeV. Scattered protons were detected at θlab=90, 120°, 135°, 150°, and 160° 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 Ep=1.8 to 4.0 MeV. Scattered protons were detected at 28 angles between θlab=30.4° and 167.7° 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 Ec.m.=2.21 MeV, which corresponds to the 3/2+ level at Ex=9.51 MeV in 15O. 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 fl-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. [ABSTRACT FROM AUTHOR]

The 12C(α,γ)16O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade y transitions through the excited states of 160 are contributing to the uncertainty. We constrained the contribution of the 0+ (6.05 MeV) and 3-(6.13 MeV) cascade transitions by measuring the asymptotic normalization coefficients for these states using the a-transfer reaction 6Li(12C, d)160 at sub-Coulomb energy. The contribution of the 0+ and 3- cascade transitions at 300 keV is found to be 1.96 ± 0.3 and 0.12 ± 0.04 keVb for destructive interference of the direct and resonance capture and 4.36 ± 0.45 and 1.44 ± 0.12 keVb for constructive interference, respectively. The combined contribution of the 0+ and 3- cascade transitions to the 12C(α,γ)16O reaction cross section at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced. [ABSTRACT FROM AUTHOR]

Background . The R-Matrix formalism is a crucial tool in the study of nuclear astrophysics reactions, and many codes have been written to implement the relevant mathematics. One such code makes use of Visual Basic macros. A further open-source code, AZURE, written in the FORTRAN programming language is available from the JINA collaboration and a C++ version, AZURE2, has recently become available. Purpose The detailed mathematics and extensive programming required to implement broadly applicable R-Matrix codes make comparisons between different codes highly desirable in order to check for errors. This paper presents a comparison of the three codes based around data and recent results of the astrophysically important 18 F(p, α ) 15 O reaction. Methods Using the same analysis techniques as in the work of Mountford et al. parameters are extracted from the two JINA codes, and the resulting cross-sections are compared. This includes both refitting data with each code and making low-energy extrapolations. Results All extracted parameters are shown to be broadly consistent between the three codes and the resulting calculations are in good agreement barring a known low-energy problem in the original AZURE code. Conclusion The three codes are shown to be broadly consistent with each other and equally valid in the study of astrophysical reactions, although one must be careful when considering low lying, narrow resonances which can be problematic when integrating. [ABSTRACT FROM AUTHOR]

Background: The ³He(α,γ)7Be reaction is of critical importance in determining the flux of solar neutrinos through the pp-II and pp-III chains. For this reason and others, the description of the cross section and its extrapolation towards low-energy has always been a matter of intense debate. While large systematic differences have been present in the past, several recent measurements are all in excellent statistical agreement. Purpose: The convergence of the recent individual experimental measurements of the ³He(α,γ)7Be reaction prompts a global analysis of the reaction data. From the combined data, a more precise estimate of the low-energy cross section can be determined. Results: A global R-matrix fit is used to describe the ³He(α,γ)7Be data as well as scattering data over a similar energy range. The R-matrix fit is then subjected to a Monte Carlo analysis to extract the uncertainties on the cross section and corresponding reaction rate. Conclusion: By combining several recent measurements of the ³He(α,γ)7Be reaction, the combined data yield a zero energy S factor of S(0) = 0.542 ± 0.011(MC fit) ± 0.006(model)-0.011+0.019 (phase shifts) keV b. This gives a total uncertainty in S(0) of +0.023/-0.017 keV b. [ABSTRACT FROM AUTHOR]

Background: The reaction 180(α,n)21Ne is a part of the reaction chains leading to the production of 19F and 22Ne 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 (α,n) reactions. Purpose: Previously low-energy 180(α,n)21Ne 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 (α,n) experiments. Method: The 180(α,n(0+1)) reaction has been measured with a moderating neutron detector. In addition the (α,n1γ) channel has been measured independently by observation of the characteristic 350.7 keV γ transition in 21Ne. The reaction cross section at energies above Eα = 1100 keV was determined by a simultaneous R-matrix fit to both channels. The strengths of the two lowest-energy resonances at Eα = 959 keV and Eα = 1066 keV were analyzed separately using individual Breit-Wigner fits. Results: The cross section of both reaction channels, 180(α; n0)21Ne and 180(α,n1γ)21Ne, 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 Eα = 888 keV is explained as background from the contaminant.reaction 17O(α,n)20Ne. 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 17O(α,n)20Ne that has been wrongly assigned to 18O(α,n)21Ne. [ABSTRACT FROM AUTHOR]

Background: The ratio between the rates of the reactions 17O(α,n)20Ne and 17O(α,γ)21Ne determines whether 160 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by 160(n,γ) 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 (α,γ) channel have made it necessary to measure the (α,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. Method: The 170(α,n(0+1)) reaction has been measured with a moderating neutron detector. In addition, the (α,n1) channel has been measured independently by observation of the characteristic 1633 keV γ transition in 20Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (α,n) and (α,γ) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. Result: The reaction channels 170(α,n0)20Ne and 170(α,n1γ)20Ne were measured in the energy range Eα = 800 keV to 2300 keV. A new 170(α,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 (α,n) rate with a previously measured (α,γ) 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 (α,γ) 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 (α,γ) 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. [ABSTRACT FROM AUTHOR]

Background: Over the past 60 years, a large amount of experimental nuclear data have been obtained for reactions which probe the I60 compound nucleus near the a and proton separation energies, the energy regimes most important for nuclear astrophysics. Difficulties and inconsistencies in -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 /{-matrix framework. Using a consistent set of data from multiple reaction channels, attain an improved fitting for the I5N(p, γo)I6O reaction data. Methods: Reaction data for all available reaction channels were fit simultaneously using a multichannel Ä-matrix code. Results: Over the wide range of experimental data considered, a high level of consistency was found, resulting in a single consistent Ä-matrix fit which described the broad level structure of 160 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)160 and 15N(p,α)12C. Conclusion: The feasibility and advantages of a complete multiple entrance/exit channel /-matrix description for the broad level structure of 160 has been achieved. A future publication will investigate the possible effects of the multiple-channel analysis on the reaction 12C(a,γ)160. [ABSTRACT FROM AUTHOR]

Background: The reaction 17O(p, γ)18F 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 (T9 = 0.01-0.4), the main contributions to the rate of this reaction are the direct capture process, two low-lying narrow resonances (Er = 65.1 and 183 keV) and the low-energy tails of two broad resonances (Er = 557 and 677 keV). Purpose: 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. Method: The reaction cross section was measured in a wide proton energy range (Ec.m. = 345-1700 keV) and at several angles (θlab = 0°, 45°, 90°, 135°). The observed primary γ 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. Results: 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 18F states. Conclusions: We were able to extrapolate the astrophysical S factor of the reaction 17O(p, γ)18F 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. [ABSTRACT FROM AUTHOR]

Background The main energy production mechanism for massive stars during hydrogen burning is the CNO cycle. The reactions 15N(p,&ggr;)160 and 15N(p,&agr;0)12C 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 16O which characterize the cross section of the reactions 15N(p,&ggr;)160 and 15N(p,&agr0)12C 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 15N(p,&ggr;)160 for the ground state transition only. Concurrently, &ggr;-ray measurements were recorded for the cascade transitions to the Ex = 6.050, 6.130, and 7.117 MeV bound states of 160 as well as from the reaction 15N(p,&agr;1&ggr;)12C. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the 15N(p,&agr;1&ggr;)12C 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 Ep = 0.14 to 1.80 MeV for the &ggr;-ray cascade transitions and for the reaction 15N(p,&agr;1&ggr;)12C. Results De-excitations associated with several compound nucleus states in 160 are observed in both the &ggr;-ray and &agr;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 160 bound states at Ex = 6.050, 6.130, and 7.117 MeV are reported over the energy range from Ep = 0.14 to 1.80 MeV. In addition, an improved measurement of the 15N(p,&agr;1&ggr;)12C excitation curve has been made over a similar energy range. [ABSTRACT FROM AUTHOR]

Background: Reactions that probe the compound nucleus structure of 160 near the a and proton separation energies have important implications for nuclear astrophysics. New, more sensitive measurements of the reaction 15N(p, γ)16O have motivated the need for improved compound-nucleus reaction data in other channels. A previous work [P. Tischhauser et al., Phys. Rev. C 79, 055803 (2009)] reported extensive a-scattering data over the energy region from Eα = 2.6 to Eα = 6.6 MeV. During this experiment data for higher energies, from Eα = 6.6 to Eα = 8.2 MeV, were also recorded but were not analyzed. The unpublished data cover the excitation energy range from the proton separation energy at Ex = 12.13 MeV up to 13.31 MeV, the same energy range important for the reaction 15N(p, γ)16O. Purpose: These previously unpublished data for 12C(α, α)12C as well as the reactions 12C(α, α1)12C and 12C(α, p)15N are analyzed in order to provide additional data for a comprehensive R-matrix analysis of compound-nucleus reactions populating 16O. Methods: Expanding on the previous publication's analysis, the ratio of the reaction yields was analyzed using a multiple-channel R-matrix calculation. Results: The excitation curves resolved resonances at Eα (Jπ) = 7.05 (l-), 7.75 (2+), 7.91 (1-), 7.97 (3-), and 8.13 (3-) MeV. The R-matrix analysis revealed excellent agreement between the measured yield ratios and cross-section data from the literature. The two 1- levels at Eα = 7.05 and 7.91 MeV are those that dominate the ISN(p, γ)16O cross section. Conclusions: In a future publication, the additional constraints provided by these data will be used in a comprehensive multiple-channel R-matrix analysis to investigate the low-energy cross section of the reaction 15N(p, γ)16O. [ABSTRACT FROM AUTHOR]

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 160 near the proton separation energy. This is the same energy region important for the nuclear astrophysics reactions 15N(p,γ)160 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 160 compound nucleus in order to better constrain the level parameters of states which contribute to the reaction 15N(p,γ)160. 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 160. 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 160 over a similar energy region. [ABSTRACT FROM AUTHOR]

The reaction 17O(α, γ) 21Ne has been measured by in-beam y spectroscopy for the first time in the energy range E~ = 750-1650 keV using highly enriched anodized Ta2(17O)5 targets. Resonances were found at Eα = 1002, 1386, and 1619 keV. Their strengths and primary γ-ray branchings are given. The new results exclude the low reaction rate of Descouvemont and support the rate of Caughlan and Fowler. Implications for the neutron poisoning efficiency of 16O in the weak s-process are discussed. [ABSTRACT FROM AUTHOR]

Abstract: An 60Fe target for studying the 60Fe(n, γ)61Fe cross-section at stellar energies was prepared using radio-chemical separation techniques. In total, 7.8×1015 60Fe atoms (777ng) were separated from a copper beam dump for the 590MeV proton beam of the high intensity accelerator at PSI. The final target was prepared by evaporating the iron-containing aqueous solution onto a graphite backing. With this sample the keV neutron capture cross-section of 60Fe has been measured at FZ Karlsruhe. The work is part of the ERAWAST-initiative (Exotic Radionuclides from Accelerator WAste for Science and Technology) which is aimed at extracting rare valuable radionuclides from accelerator waste by chemical means. [Copyright &y& Elsevier]

The TexAT (Texas Active Target) detector is a new active-target time projection chamber (TPC) that was built at the Cyclotron Institute Texas A&M University. The detector is designed to be of general use for nuclear structure and nuclear astrophysics experiments with rare isotope beams. TexAT combines a highly segmented Time Projection Chamber (TPC) with two layers of solid state detectors. It provides high efficiency and flexibility for experiments with low intensity exotic beams, allowing for the 3D track reconstruction of the incoming and outgoing particles involved in nuclear reactions and decays. [ABSTRACT FROM AUTHOR]

The 22Ne(α , γ)26Mg and 22Ne(α ,n)25Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial α -widths of resonances in 26Mg located in the Gamow window for the Ne 22 + α capture. These resonances were populated using 22Ne(6Li,d)26Mg and 22Ne(7Li,t)26Mg reactions at energies near the Coulomb barrier. At these low energies α -transfer reactions favor population of low spin states and the extracted partial α -widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the 22Ne(α , γ)26Mg and the 22Ne(α ,n)25Mg reactions are shown to be significantly different than the previously suggested values. [ABSTRACT FROM AUTHOR]

Background: Clustering plays an important role in the structure of 10Be. Exotic molecular-like configurations, such as α:2n:α, have been suggested at relatively low excitation energies. Purpose: To search for the high-spin states that may belong to the molecular-like α:2n:α configuration in 10Be. Method: Measuring excitation functions for 6He+α scattering, populating states in the excitation energy range from 4.5 MeV to 8 MeV in 10Be using a 6He rare-isotope beam and a thick helium gas target. Results: No new excited states in 10Be have been observed. Stringent limits on the possible degree of α-clustering of the hypothetical yrast 6+ state have been obtained. Conclusions: The high-spin members of the α:2n:α molecular-like rotational band configuration, that is considered to have a 0+ band head at 6.18 MeV, either do not exist or have small overlap with the 6He(g.s.)+α channel. [ABSTRACT FROM AUTHOR]

The total cross section of the 12C+16O fusion reaction has been measured at low energies to investigate the role of this reaction during late stellar evolution burning phases. A high-intensity oxygen beam, produced by the 5 MV pelletron accelerator at the University of Notre Dame, impinged on a thick, ultrapure graphite target. Protons and γ rays were simultaneously measured in the center-of-mass energy range from 3.64 to 5.01 MeV for singles and from 3.73 to 4.84 MeV for coincidence events, using silicon and Ge detectors. Statistical model calculations were employed to interpret the experimental results. The emergence of a new resonance-like broad structure and a decreasing trend in the S-factor data towards lower energies (opposite to previous data) are found for the 12C+16O fusion reaction. Based on these results the uncertainty range of the reaction rate within the temperature range of late stellar burning environments is discussed. [ABSTRACT FROM AUTHOR]