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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 Ne-22(p,gamma)Na-23 reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to a large number of predicted but hitherto unobserved resonances at low energy. Purpose: A new direct study of low-energy Ne-22(p,gamma)Na-23 resonances has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, Italy. Method: The proton capture on Ne-22 was investigated in direct kinematics, delivering an intense proton beam to a Ne-22 gas target. gamma rays were detected with two high-purity germanium detectors enclosed in a copper and lead shield suppressing environmental radioactivity. Results: Three resonances at 156.2 keV [omega gamma = (1.48 +/- 0.10) x 10(-7) eV], 189.5 keV [omega gamma = (1.87 +/- 0.06) x 10(-6) eV] and 259.7 keV [omega gamma = (6.89 +/- 0.16) x 10(-6) eV] proton beam energy, respectively, have been observed for the first time. For the levels at E-x = 8943.5, 8975.3, and 9042.4 keV excitation energy corresponding to the new resonances, the gamma-decay branching ratios have been precisely measured. Three additional, tentative resonances at 71, 105, and 215 keV proton beam energy, respectively, were not observed here. For the strengths of these resonances, experimental upper limits have been derived that are significantly more stringent than the upper limits reported in the literature. Conclusions: Based on the present experimental data and also previous literature data, an updated thermonuclear reaction rate is provided in tabular and parametric form. The new reaction rate is significantly higher than previous evaluations at temperatures of 0.08-0.3 GK.

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 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.

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.

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

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 electron screening in the Li-7(p, alpha)alpha reaction has been studied at E-p = 30 to 100 keV for different environments: Li2WO4 insulator, Li metal, and PdLi alloys. For the insulator a screening potential energy of U-e = 185 +/- 150 eV was observed, consistent with previous work and the atomic adiabatic limit. However, for the Li metal and the PdLi alloys we find large values of U-e = 1280 +/- 60 and 3790 +/- 330 eV, respectively: the values can be explained by the plasma model of Debye applied to the quasi-free metallic electrons in these samples. Similar results have been found for the Li-6(p, alpha)He-3 reaction supporting the hypothesis of the isotopic independence of the electron screening effect. The data together with previous studies of d(d, p)t and Be-9(p, alpha)Li-6 in metals verify the Debye model scaling U-e proportional to Z(t) (charge number of target)

Die Arbeit beschreibt die Durchführung einer Messung des absoluten Wirkungsquerschnitts der Reaktion \(^{3}\)He(\(\alpha\), \(\gamma\))\(^{7}\)Be im Energiebereich von E\(_{cm}\)=0.65 - 3.15 MeV. Die Messung fand in inverser Kinematik am Europäischen Rückstoß-Separator für Nukleare Astrophysik ERNA statt. Mit diesem Gerät werden die Beryllium-Rückstoßkerne direkt nachgewiesen. Das fensterloses, doppelseitig differentiell gepumpte, rezirkulierende \(^{3}\)He-Gastarget wurde charakterisiert. In weiteren Messungen wurde dann zusätzlich die direkte \(\gamma\)-Strahlung gemessen, sowie in einem weiteren Abschnitt des Experiments eine Aktivierungsmessung vorgenommen. Die Übereinstimmungen der zur Extrapolation verwendeten Theorien mit den experimentellen Wirkungsquerschnitts-Daten wurden untersucht. This work reports on the results of a new experimental approach, where the absolute cross section of the nuclear reaction \(^{3}\)He(\(\alpha\), \(\gamma\))\(^{7}\)Be is determined by the direct detection of the \(^{7}\)Be recoils using the European Recoil mass separator for Nuclear Astrophysics (ERNA) at the Dynamitron Tandem Laboratorium of the Ruhr-Universität Bochum. The modifications to the experimental setup to match the requirements of this reaction, including the characterization of the windowless differentially pumped recirculating \(^{3}\)He gas-target are presented. Determinations of the cross section through the detection of the \(\gamma\)-rays in coincidence with the recoils at selected energies as well with the \(\it {activation}\) method are made. The extrapolation of the S-factor to solar energies with the available theories is discussed in detail.

Die Arbeit beschreibt die Durchführung einer Messung des absoluten Wirkungsquerschnitts der Reaktion 3He(α, γ)7Be im Energiebereich von Ecm=0.65 - 3.15 MeV. Die Messung fand in inverser Kinematik am Europäischen Rückstoß-Separator für Nukleare Astrophysik ERNA statt. Mit diesem Gerät werden die Beryllium-Rückstoßkerne direkt nachgewiesen. Das fensterloses, doppelseitig differentiell gepumpte, rezirkulierende 3He-Gastarget wurde charakterisiert. In weiteren Messungen wurde dann zusätzlich die direkte γ-Strahlung gemessen, sowie in einem weiteren Abschnitt des Experiments eine Aktivierungsmessung vorgenommen. Die Übereinstimmungen der zur Extrapolation verwendeten Theorien mit den experimentellen Wirkungsquerschnitts-Daten wurden untersucht.

A novel approach for the measurement of the 3He(α, γ)7Be reaction cross section uses the European Recoil separator for Nuclear Astrophysics to directly detect the 7Be ions produced in the reaction and, additionally, the coincident capture γ-rays. The experiment is presented.

In the original publication [...]

12C+12C reactions are crucial in the evolution of massive stars and explosive scenarios. The measurement of these reactions at astrophysical energies is very challenging due to their extremely small cross sections, and the presence of beam induced background originated by the natural 1,2H contaminants in the C targets. In addition, the many discrepancies between different data sets and the complicated resonant structure of the cross sections make the extrapolation to low energies very uncertain. Recently, we performed a direct measurement of the 12C+12C reactions at the CIRCE Laboratory in Italy. Results from a study on target contamination were used, allowing us to measure cross sections at Ec.m. =2.51 − 4.36 MeV with 10-25 keV energy steps. Two stage ΔE-Erest detectors were used for unambiguous particle identification. Branching ratios of individual particle groups were found to vary significantly with energy and angular distributions were also found to be anisotropic, which could be a potential explanation for the discrepancies observed among different data sets.

The cross section of 7Be(p,γ)8B represents one of the most important nuclear inputs for the prediction of the high energy component of solar neutrinos and it has also a direct impact on the 7Li abundance after the Big Bang Nucleosynthesis. The importance of this reaction triggered an intense experimental work over the last decades, where discrepancies were observed between the results of different measurements. In addition, a question remains about possible common systematic effects, considering that all measurements share the same experimental approach, i.e. an intense proton beam impinging on a 7Be radioactive target. Inverse kinematics, i.e. a 7Be ion beam and a hydrogen target, with the direct measurement of the total reaction cross section by means of the detection of the 8B recoils, can shed light on such systematic effects. Efforts attempted so far were limited by the low 7Be beam intensity. We present here the results of a new measurement at Ecm = 376 to 819 keV using a high intensity 7Be beam in combination with a windowless gas target and the recoil mass separator ERNA (European Recoil mass separator for Nuclear Astrophysics) at CIRCE (Center for Isotopic Research on Cultural and Environmental heritage), Caserta, Italy. Our results, including the systematic error, are compatible with previous measurements that yields lower value of S17(0) and are compatible with the currently accepted value from [1] only at a 2-σ level.

The Center for Isotopic Research on Cultural and Environmental heritage (CIRCE) of the University of Campania L. Vanvitelli was established in 2005 within the action POR CAMPANIA 2000/2006—MISURA 3...

12C(α, γ)16O cross section plays a key-role in the stellar evolution and nucleosynthesis of massive stars. Hence, it must be determined with the precision of about 10% at the relevant Gamow energy of 300 keV. The ERNA (European Recoil mass separator for Nuclear Astrophysics) collaboration measured, for the first time, the total cross section of 12C(α, γ)16O by means of the direct detection of the 16O ions produced in the reaction down to an energy of Ecm = 1.9 MeV. To extend the measurement at lower energy, it is necessary to limit the extension of the He gas target. This can be achieved using a supersonic jet, where the oblique shock waves and expansion fans formed at its boundaries confine the gas, which can be efficiently collected using a catcher. A test version of such a system has been designed, constructed and experimentally characterized as a bench mark for a full numerical simulation using FV (Finite Volume) methods. The results of the commissioning of the jet test version and the design of the new system that will be used in combination with ERNA are presented and discussed.

Context. Material processed by the CNO cycle in stellar interiors is enriched in 17O. When mixing processes from the stellar surface reach these layers, as occurs when stars become red giants and undergo the first dredge up, the abundance of 17O increases. Such an occurrence explains the drop of the 16O/17O observed in RGB stars with mass larger than 1.5 M_\solar. As a consequence, the interstellar medium is continuously polluted by the wind of evolved stars enriched in 17O . Aims. Recently, the Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration released an improved rate of the 17O(p,alpha)14N reaction. In this paper we discuss the impact that the revised rate has on the 16O/17O ratio at the stellar surface and on 17O stellar yields. Methods. We computed stellar models of initial mass between 1 and 20 M_\solar and compared the results obtained by adopting the revised rate of the 17O(p,alpha)14N to those obtained using previous rates. Results. The post-first dredge up 16O/17O ratios are about 20% larger than previously obtained. Negligible variations are found in the case of the second and the third dredge up. In spite of the larger 17O(p,alpha)14N rate, we confirm previous claims that an extra-mixing process on the red giant branch, commonly invoked to explain the low carbon isotopic ratio observed in bright low-mass giant stars, marginally affects the 16O/17O ratio. Possible effects on AGB extra-mixing episodes are also discussed. As a whole, a substantial reduction of 17O stellar yields is found. In particular, the net yield of stars with mass ranging between 2 and 20 M_\solar is 15 to 40% smaller than previously estimated. Conclusions. The revision of the 17O(p,alpha)14N rate has a major impact on the interpretation of the 16O/17O observed in evolved giants, in stardust grains and on the 17O stellar yields., Comment: Accepted by A&A

We investigate the impact of the new LUNA rate for the nuclear reaction $^{22}$Ne$(p,\gamma)^{23}$Na on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim we use the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range $3.0\,M_{\odot} - 6.0\,M_{\odot}$, and metallicities $Z_{\rm i}=0.0005$, $Z_{\rm i}=0.006$, and $Z_{\rm i} = 0.014$. We find that the new LUNA measures have much reduced the nuclear uncertainties of the $^{22}$Ne and $^{23}$Na AGB ejecta, which drop from factors of $\simeq 10$ to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of $^{23}$Na, the uncertainties that still affect the $^{22}$Ne and $^{23}$Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available., Comment: 21 pages, 14 figures, accepted for publication in MNRAS

Radiochimica acta, 101 (8), ISSN:0033-8230, ISSN:2193-3405

The nucleosynthesis of light elements, from helium up to silicon, mainly occurs in Red Giant and Asymptotic Giant Branch stars and Novae. The relative abundances of the synthesized nuclides critically depend on the rates of the nuclear processes involved, often through non-trivial reaction chains, combined with complex mixing mechanisms. In this review, we summarize the contributions made by LUNA experiments in furthering our understanding of nuclear reaction rates necessary for modeling nucleosynthesis in AGB stars and Novae explosions., 23 pages, 12 figures

Accurate knowledge of thermonuclear reaction rates is important in understanding the generation of energy, the luminosity of neutrinos, and the synthesis of elements in stars. Cross-section measurements for quiescent stellar H-burning are mainly hampered by extremely low counting rate and cosmic background. The LUNA Collaboration has shown that, by going underground and by using the typical techniques of low background physics, it is possible to measure nuclear cross-sections down to the energy of the nucleosynthesis inside stars. This paper reports an overview of the experimental techniques adopted in underground nuclear astrophysics through a summary of the main recent results and achievements. The future developments of the LUNA experiment are also given. © 2013 Elsevier B.V. All rights reserved.

A pure 7 Be beam with an energy E = 1–8 MeV is available for nuclear and applied physics at the 3 MV Pelletron tandem accelerator CIRCE in Caserta. The beam is produced using an offline technique. Typical analyzed beam intensities are about 2 ppA, using cathodes with an activity of the order of 200 MBq. The 7 Be implantation has been used for both fundamental nuclear physics and applied physics. In particular, different metals have been implanted with 7 Be in order to study the influence of the chemical composition and of the number of quasi-free electrons of the host material on the 7 Be half-life. In the field of applied physics, the 7 Be implantation turns out to be very interesting for wear measurement. In fact, in this case 7 Be is used as a depth-sensitive tracer. The continuous detection of the sample activity during the wear allows a high sensitivity measurement of wearing speed. The 7 Be beam production at CIRCE, the implantation procedure and the results obtained from the 7 Be half-life measurements and the wear characterization of implanted steel samples are described.

The electron screening in the d(d,p)t reaction has been studied for the deuterated metal Pt at a target temperature T = 20 degrees C to 340 degrees C, and for Co at T = 20 degrees C and 200 degrees C. The enhanced electron screening decreases with increasing temperature, where the data agree with the plasma model of Debye applied to the quasi-free metallic electrons. The data represent the first observation of a temperature dependence of a nuclear cross-section. We also measured the screening effect for the deuterated metal Ti (an element of group 4 of the periodic table) at T = -10 degrees C to 200 degrees C: above 50 degrees C the hydrogen solubility dropped to values far below unity and a large screening effect became observable. Similarly, all metals of groups 3 and 4 and the lanthanides showed a solubility of a few percent at T = 200 degrees C (compared to T = 20 degrees C) and a large screening became also observable. Within the Debye model the deduced number of valence electrons per metallic atom agrees with the corresponding number from the Hall coefficient, for all metals investigated. RI Fulop, Zsolt/B-2262-2008; Aliotta, Marialuisa /H-2567-2012

Non-radiative transitions in nuclear capture reactions between light nuclei play a relevant role in stellar nuclear astrophysics, where nuclear processes occur at typical energies from tens to hundreds of keV. At higher energies, instead, the E0 contributions may be shadowed by more intense transitions. The experimental study of E0 transitions requires a specific detection setup, able to uniquely identify events where an electron-positron pair is produced. A compact ΔE-E charged-particle spectrometer based on two silicon detectors has been designed to be installed in the jet gas target chamber of the recoil mass separator ERNA (European Recoil separator for Nuclear Astrophysics) at the CIRCE laboratory of Caserta, Italy. The detector design, its performances and the first foreseen applications are described.