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Background: The competing Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reactions control the production of neutrons for the weak s process in massive and asymptotic giant branch (AGB) stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. A number of experimental studies have been performed over recent years which necessitate the reevaluation of the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates. Evaluations of the reaction rates following the collection of new nuclear data presently show differences of up to a factor of 500, resulting in considerable uncertainty in the resulting nucleosynthesis. Purpose: To reevaluate the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates using updated nuclear data from a number of sources including updating spin and parity assignments. Methods: With updated spin and parity assignments, the levels which can contribute to the reaction rates are identified. The reaction rates are computed using a Monte Carlo method which has been used for previous evaluations of the reaction rates in order to focus solely on the changes due to modified nuclear data. Results: The evaluated Ne22(α,γ)Mg26 reaction rate remains substantially similar to that of Longland et al. but, including recent results from Texas A&M, the Ne22(α,n)Mg25 reaction rate is lower at a range of astrophysically important temperatures. Stellar models computed with newton and mesa predict decreased production of the weak branch s process due to the decreased efficiency of Ne22 as a neutron source. Using the new reaction rates in the mesa model results in Zr96/Zr94 and Ba135/Ba136 ratios in much better agreement with the measured ratios from presolar SiC grains. Conclusion: The Ne22+α reaction rates Ne22(α,γ)Mg26 and Ne22(α.n)Mg25 have been recalculated based on more recent nuclear data. The Ne22(α,γ)Mg26 reaction rate remains substantially unchanged since the previous evaluation but the Ne22(α.n)Mg25 reaction rate is substantially decreased due to updated nuclear data. This results in significant changes to the nucleosynthesis in the weak branch of the s process.

International audience; Background: The competing Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reactions control the production of neutrons for the weak s process in massive and asymptotic giant branch (AGB) stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. A number of experimental studies have been performed over recent years which necessitate the reevaluation of the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates. Evaluations of the reaction rates following the collection of new nuclear data presently show differences of up to a factor of 500, resulting in considerable uncertainty in the resulting nucleosynthesis. Purpose: To reevaluate the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates using updated nuclear data from a number of sources including updating spin and parity assignments. Methods: With updated spin and parity assignments, the levels which can contribute to the reaction rates are identified. The reaction rates are computed using a Monte Carlo method which has been used for previous evaluations of the reaction rates in order to focus solely on the changes due to modified nuclear data. Results: The evaluated Ne22(α,γ)Mg26 reaction rate remains substantially similar to that of Longland et al. but, including recent results from Texas A&M, the Ne22(α,n)Mg25 reaction rate is lower at a range of astrophysically important temperatures. Stellar models computed with newton and mesa predict decreased production of the weak branch s process due to the decreased efficiency of Ne22 as a neutron source. Using the new reaction rates in the mesa model results in Zr96/Zr94 and Ba135/Ba136 ratios in much better agreement with the measured ratios from presolar SiC grains. Conclusion: The Ne22+α reaction rates Ne22(α,γ)Mg26 and Ne22(α.n)Mg25 have been recalculated based on more recent nuclear data. The Ne22(α,γ)Mg26 reaction rate remains substantially unchanged since the previous evaluation but the Ne22(α.n)Mg25 reaction rate is substantially decreased due to updated nuclear data. This results in significant changes to the nucleosynthesis in the weak branch of the s process.

Background: The origin of fluorine is a widely debated issue. Nevertheless, the 15N(α,γ)19F reaction is a common feature among the various production channels so far proposed. Its reaction rate at relevant temperatures is determined by a number of narrow resonances together with the direct capture and the tails of the two broad resonances at Ec.m.=1323 and 1487 keV. Purpose: The broad resonances widths, Γγ and Γα, have to be measured with adequate precision in order to better determine their contribution to the 15N(α,γ)19F stellar reaction rate. Methods: Measurement through the direct detection of the 19F recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a 15N beam impinging onto a 4He windowless gas target. The observed yield of the resonances at Ec.m.=1323 and 1487 keV is used to determine their widths in the α and γ channels. Results: We show that a direct measurement of the cross section of the 15N(α,γ)19F reaction can be successfully obtained with the recoil separator ERNA, and the widths Γγ and Γα of the two broad resonances have been determined. While a fair agreement is found with earlier determination of the widths of the 1487 keV resonance, a significant difference is found for the 1323 keV resonance Γα. Conclusions: The revision of the widths of the two more relevant broad resonances in the 15N(α,γ)19F reaction presented in this work is the first step toward a more firm determination of the reaction rate. At present, the residual uncertainty at the temperatures of the 19F stellar nucleosynthesis is dominated by the uncertainties affecting the direct capture component and the 364 keV narrow resonance, both so far investigated only through indirect experiments.

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.

The Ne-22(p,gamma)Na-23 reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle affects the synthesis of the elements between Ne-20 and Al-27 in asymptotic giant branch stars and novae. The Ne-22(p,gamma)Na-23 reaction rate is very uncertain because of a large number of unobserved resonances lying in the Gamow window. At proton energies below 400 keV, only upper limits exist in the literature for the resonance strengths. Previous reaction rate evaluations differ by large factors. In the present work, the first direct observations of the Ne-22(p,gamma)Na-23 resonances at 156.2, 189.5, and 259.7 keV are reported. Their resonance strengths are derived with 2%-7% uncertainty. In addition, upper limits for three other resonances are greatly reduced. Data are taken using a windowless Ne-22 gas target and high-purity germanium detectors at the Laboratory for Underground Nuclear Astrophysics in the Gran Sasso laboratory of the National Institute for Nuclear Physics, Italy, taking advantage of the ultralow background observed deep underground. The new reaction rate is a factor of 20 higher than the recent evaluation at a temperature of 0.1 GK, relevant to nucleosynthesis in asymptotic giant branch stars.

We report on measurements of resonance strengths and energies for the $ E_{p} = 151$ and 193 keV resonances in the 18O(p, $\alpha$ )15N and 17O(p, $\alpha$ )14N reactions, respectively, obtained during commissioning of a new setup for alpha-particle detection studies at the LUNA underground laboratory. Our values, $\omega\gamma (151)=164.2\pm 0.9_{stat} {}^{+12.1}_{-11.7} {}_{syst}$ meV and $\omega\gamma (193)=1.68\pm 0.03_{stat} \pm 0.12_{syst}$ meV, are in excellent agreement with those reported in the literature. New values of resonance energies are $ E_{p}=151.2 \pm 0.3$ keV and $ E_{p}=194.8 \pm 0.3$ keV, respectively, this latter with the highest precision to date. Comparative background measurements in silicon detectors overground and underground were also carried out, yielding up to a factor of 15 in background suppression at LUNA at energies around 200keV. This clearly demonstrates the usefulness of underground measurements in charged-particles experiments, especially at low detection energies.

Background: The O17(p,γ)F18 reaction affects the production of key isotopes (e.g., F18 and O18) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at Ec.m.=183keV and by the combined contributions of direct capture and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae. Purpose: This study aims at the measurement of the O17(p,γ)F18 reaction cross section in order to determine its reaction rate in the temperature region appropriate to explosive hydrogen burning in novae. Method: The O17(p,γ)F18 reaction cross section was measured using both the prompt detection of the emitted γ rays and an activation technique. Measurements were carried out at the Laboratory for Underground Nuclear Astrophysics (Gran Sasso, Italy) where the strongly reduced cosmic-ray-induced background allows for improved sensitivity compared to previous studies. Results: The O17(p,γ)F18 reaction cross section was measured in the range Ec.m.=160 to 370keV. The strength of the Ec.m.=183keV resonance, ωγ=1. 67±0.12μeV, was determined with unprecedented precision. The total S factor was obtained through a combined fit of prompt γ-ray and activation results. An overall global fit including other existing data sets was also carried out and a recommended astrophysical reaction rate is presented. Conclusions: The reaction rate uncertainty attained in this work is now below the required precision for nova models. We verified, following a full set of hydrodynamic nova models, that the abundances of oxygen and fluorine isotopes obtained with the present reaction rate are determined with 10% precision and put firmer constraints on observational signatures of novae events.

Background: The He3(α,γ)Be7 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 He3(α,γ)Be7 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 He3(α,γ)Be7 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 He3(α,γ)Be7 reaction, the combined data yield a zero energy S factor of S(0)=0.542±0.011(MCfit)±0.006(model)+0.019−0.011(phaseshifts) keV b. This gives a total uncertainty in S(0) of +0.023/−0.017 keV b.

We discuss the influence of the 12C(alpha,gamma)16O$ reaction rate on the central He burning of stars in the mass range 0.8-25 Msun, as well as its effects on the explosive yields of a 25 Msun star of solar chemical composition. We find that the central He burning is only marginally affected by a change in this cross section within the currently accepted uncertainty range. The only (important) quantity which varies significantly is the amount of C left by the He burning. Since the C(alpha,gamma)16O is efficient in a convective core, we have also analyzed the influence of the convective mixing in determining the final C abundance left by the central He burning. Our main finding is that the adopted mixing scheme does not influence the final C abundance provided the outer border of the convective core remains essentially fixed (in mass) when the central He abundance drops below about 0.1 dex by mass fraction; vice versa, even a slight shift (in mass) of the border of the convective core during the last part of the central He burning could appreciably alter the final C abundance. Hence, we stress that it is wiser to discuss the advanced evolutionary phases as a function of the C abundance left by the He burning rather than as a function of the efficiency of the 12C(alpha,gamma)16O reaction rate. Only a better knowledge of this cross section and/or the physics of the convective motions could help in removing the degeneracy between these two components. We also prolonged the evolution of the two 25 M_sun stellar models up to the core collapse and computed the final explosive yields., Comment: 39 pages, Latex, accepted by ApJ

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.

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive ^{18}F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The ^{17}O(p,\gamma)^{18}F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range Ecm = 200 - 370 keV appropriate to hydrogen burning in classical novae. In addition, the E=183 keV resonance strength, \omega \gamma=1.67\pm0.12 \mueV, has been measured with the highest precision to date. The uncertainty on the ^{17}O(p,\gamma)^{18}F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis., Comment: accepted by Phys. Rev. Lett

Background The main energy production mechanism for massive stars during hydrogen burning is the CNO cycle. The reactions 15N(p, γ)16O and 15N(p, α0)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, γ)16O and 15N(p, α0)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, γ)16O for the ground state transition only. Concurrently, γ-ray measurements were recorded for the cascade transitions to the Ex = 6.050, 6.130, and 7.117 MeV bound states of 16O as well as from the reaction 15N(p, α1γ)12C. Excitation curves for the cascade transitions have never been measured and the excitation curve data for the 15N(p, α1γ)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 γ-ray cascade transitions and for the reaction 15N(p, α1γ)12C. Results De-excitations associated with several compound nucleus states in 16O are observed in both the γ-ray and α1 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 16O 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, α1γ)12C excitation curve has been made over a similar energy range.

The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is controlled by the 14N(p,γ )15O reaction. The reaction proceeds by capture to the ground states and several excited states in 15O. In order to obtain a reliable extrapolation of the excitation curve to astrophysical energy, fits in the R-matrix framework are needed. In an energy range that sensitively tests such fits, new cross-section data are reported here for the four major transitions in the 14N(p,γ )15O reaction. The experiment has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400-kV accelerator placed deep underground in the Gran Sasso facility in Italy. Using a composite germanium detector, summing corrections have been considerably reduced with respect to previous studies. The cross sections for capture to the ground state and to the 5181, 6172, and 6792 keV excited states in 15O have been determined at 359, 380, and 399 keV beam energy. In addition, the branching ratios for the decay of the 278-keV resonance have been remeasured.

The radiative capture reaction View the MathML source has been investigated in the energy range E=3.3 to 4.5 MeV. This experiment focused in particular on the cascade transition to the 0+ state at Ex=6.05 MeV in 16O and was performed by detecting the capture γ-rays with a NaI detector array at the windowless 4He gas target of the recoil mass separator ERNA in coincidence with the 16O ejectiles. The 6.05 MeV transition has been considered recently as a component accounting for up to 15% of the View the MathML source total cross section at astrophysical energies. The arrangement of the detector array yielded additional information on the γ-ray multipolarity, i.e. the ratio σE2/σE1, and it was found that the 6.05 MeV transition is entirely E2 in the studied energy range. The results for this transition are analyzed in an R-matrix formalism and extrapolated to the relevant Gamow energy of stellar helium burning, E0≃300 keV. In contrast to a previous analysis, the present extrapolation suggests a negligible contribution from this amplitude, View the MathML source. Additional data for cascade transitions to excited states at Ex=6.13, 6.92, and 7.12 MeV, respectively, as well as to the ground state were obtained and the corresponding S factors in the studied energy range are given.

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-shell burning. In particular, the strengths of low-energy resonances with E

The 3He(α,γ)7Be reaction presently represents the largest nuclear uncertainty in the predicted solar neutrino flux and has important implications on the big bang nucleosynthesis, i.e., the production of primordial 7Li. We present here the results of an experiment using the recoil separator ERNA (European Recoil separator for Nuclear Astrophysics) to detect directly the 7Be ejectiles. In addition, off-beam activation and coincidence γ-ray measurements were performed at selected energies. At energies above 1 MeV a large discrepancy compared to previous results is observed both in the absolute value and in the energy dependence of the cross section. Based on the available data and models, a robust estimate of the cross section at the astrophysical relevant energies is proposed.

The rate of the hydrogen-burning carbon-nitrogen-oxygen (CNO) cycle is controlled by the slowest process, 14N(p,γ )15O, which proceeds by capture to the ground and several excited states in 15O. Previous extrapolations for the ground state contribution disagreed by a factor 2, corresponding to 15% uncertainty in the total astrophysical S factor. At the Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator placed deep underground in the Gran Sasso facility in Italy, a new experiment on ground state capture has been carried out at 317.8, 334.4, and 353.3 keV center-of-mass energy. Systematic corrections have been reduced considerably with respect to previous studies by using a Clover detector and by adopting a relative analysis. The previous discrepancy has been resolved, and ground state capture no longer dominates the uncertainty of the total S factor.

The He-3(alpha, gamma)Be-7 process is a key reaction in both Big-Bang nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement of the He-3(alpha, gamma)Be-7 cross section has been performed at the INFN Gran Sasso underground laboratory by both the activation and the prompt gamma detection methods. The present work reports full details of the prompt gamma detection experiment, focusing on the determination of the systematic uncertainty. The final data, including activation measurements at LUNA, are compared with the results of the last generation experiments and two different theoretical models are used to obtain the S-factor at solar energies.

The flux of Be-7 and B-8 neutrinos from the Sun and the production of Li-7 via primordial nucleosynthesis depend on the rate of the He-3(alpha,gamma)Be-7 reaction. In an extension of a previous study showing cross section data at 127-167 keV center-of-mass energy, the present work reports on a measurement of the He-3(alpha,gamma)Be-7 cross section at 106 keV performed at Italy's Gran Sasso underground laboratory by the activation method. This energy is closer to the solar Gamow energy than ever reached before. The result is sigma=0.567 +/- 0.029(stat)+/- 0.016(syst) nb. The data are compared with previous activation studies at high energy, and a recommended S(0) value for all He-3(alpha,gamma)Be-7 activation studies, including the present work, is given.

Solar neutrino fluxes depend both on astrophysical and on nuclear physics inputs, namely on the cross sections of the reactions responsible for neutrino production inside the Solar core. While the flux of solar 8B neutrinos has been recently measured at Superkamiokande with a 3.5% uncertainty and a precise measurement of 7Be neutrino flux is foreseen in the next future, the predicted fluxes are still affected by larger errors. The largest nuclear physics uncertainty to determine the fluxes of 8B and 7Be neutrinos comes from the 3He(alpha,gamma )7Be reaction. The uncertainty on its S-factor is due to an average discrepancy in results obtained using two different experimental approaches: the detection of the delayed γ rays from 7Be decay and the measurement of the prompt-gamma emission. Here we report on a new high precision experiment performed with both techniques at the same time. Thanks to the low background conditions of the Gran Sasso LUNA accelerator facility, the cross section has been measured at Ec.m. = 170, 106, and 93 keV, the latter being the lowest interaction energy ever reached. The S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated S(0) = 0.560 +/- 0.017 keVbarn is obtained. Moreover, branching ratios between the two prompt-gamma transitions have been measure with 5–8% accuracy.

The COMPTEL instrument performed the first mapping of the 1.809 MeV photons in the Galaxy, triggering considerable interest in determing the sources of interstellar 26Al. The predicted 26Al is too low compared to the observation, for a better understanding more accurate rates for the 25Mg(p; gamma)26Al reaction are required. The 25Mg(p;gamma)26Al reaction has been investigated at the resonances at Er= 745; 418; 374; 304 keV at Ruhr-Universitat-Bochum using a Tandem accelerator and a 4piNaI detector. In addition the resonance at Er = 189 keV has been measured deep underground laboratory at Laboratori Nazionali del Gran Sasso, exploiting the strong suppression of cosmic background. This low resonance has been studied with the 400 kV LUNA accelerator and a HPGe detector. The preliminary results of the resonance strengths will be reported., Comment: Accepted for publication in Journal of Physics G

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity, and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148, and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S factor to solar energies.