F. Strieder

Electron screening effect in the reactions 3 He(d, p) 4 He and d( 3 He, p) 4 He ☆ ☆ Supported in part by INFN, BMBF (06BO812), DFG (436UNG113-146) and OTKA (T025465).

The cross section of the reactions 3He(d, p)4He and d(3He, p)4He has been measured at the center-of-mass energies E=5 to 60 keV and 10 to 40 keV, respectively. The experiments were performed to determine the magnitude of the electron screening effect leading to the respective electron-screening potential energy Ue=219±7 and 109±9 eV, which are both significantly higher than the respective values from atomic physics models, Ue=120 and 65 eV.

First direct measurement of the total cross section of 12C(alpha,gamma)16O

Abstract.The total cross-section of 12C(α,γ)16O was measured for the first time by a direct and ungated detection of the 16O recoils. This measurement in inverse kinematics using the recoil mass separator ERNA in combination with a windowless He gas target allowed to collect data with high precision in the energy range E = 1.9 to 4.9 MeV. The data represent new information for the determination of the astrophysical S(E) factor.The total cross-section of 12C(α,γ)16O was measured for the first time by a direct and ungated detection of the 16O recoils. This measurement in inverse kinematics using the recoil mass separator ERNA in combination with a windowless He gas target allowed to collect data with high precision in the energy range E = 1.9 to 4.9 MeV. The data represent new information for the determination of the astrophysical S(E) factor.

Precision study of ground state capture in the N-14(p, gamma)O-15 reaction

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) 400kV 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 centerof-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 Sfactor.

Improved Direct Measurement of the 64.5 keV Resonance Strength in the O 17 (p,α) N 14 Reaction at LUNA

The ^{17}O(p,α)^{14}N reaction plays a key role in various astrophysical scenarios, from asymptotic giant branch stars to classical novae. It affects the synthesis of rare isotopes such as ^{17}O and ^{18}F, which can provide constraints on astrophysical models. A new direct determination of the E_{R}=64.5u2009u2009keV resonance strength performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) accelerator has led to the most accurate value to date ωγ=10.0±1.4_{stat}±0.7_{syst}u2009u2009neV, thanks to a significant background reduction underground and generally improved experimental conditions. The (bare) proton partial width of the corresponding state at E_{x}=5672u2009u2009keV in ^{18}F is Γ_{p}=35±5_{stat}±3_{syst}u2009u2009neV. This width is about a factor of 2 higher than previously estimated, thus leading to a factor of 2 increase in the ^{17}O(p,u2009α)^{14}N reaction rate at astrophysical temperatures relevant to shell hydrogen burning in red giant and asymptotic giant branch stars. The new rate implies lower ^{17}O/^{16}O ratios, with important implications on the interpretation of astrophysical observables from these stars.

Lifetime measurement of the 6792-keV state in O-15, important for the astrophysical S factor extrapolation in N-14(p,gamma)O-15

We report on a new lifetime measurement of the E{sub x}=6792 keV state in {sup 15}O via the Doppler-shift attenuation method at the E=259 keV resonance in the reaction {sup 14}N(p,{gamma}){sup 15}O. This subthreshold state is of particular importance for the determination of the ground state astrophysical S factor of {sup 14}N(p,{gamma}){sup 15}O at stellar energies. The measurement technique has been significantly improved over that used in previous work. The conclusion of a finite lifetime drawn there cannot be confirmed with the present data. In addition, the lifetimes of the two states at E{sub x}=5181 and 6172 keV have been measured with the same technique in order to verify the experimental method. We observe an attenuation factor F({tau})>0.98 for the E{sub x}=6172 and 6792 keV states, respectively, corresponding to {tau}<0.77 fs. The attenuation factor for the E{sub x}=5181 keV state results in F({tau})=0.78{+-}0.02 corresponding to {tau}=8.4{+-}1.0 fs, which is in excellent agreement with literature.

Recoil separator ERNA: improved measurements of the astrophysical key reaction 12C(α,γ)16O

The recoil separator ERNA (European Recoil Separator for Nuclear Astrophysics) for improved measurements of the astrophysical key reaction ^(12)C(α,γ)^(16)O is being developed at the 4 MV Dynamitron tandem accelerator in Bochum to detect directly the ^(16)O recoils. The total detection efficiency of about 50 % allows direct measurements in a much wider energy range than previously accessible. In addition this technique allows improved nmeasurements not hampered by cosmic background problems when compared to y-ray detection.

The He-3(alpha,gamma)Be-7 S-factor at solar energies: The Prompt gamma experiment at LUNA

The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement of the 3He(alpha,gamma)7Be 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.

Astrophysical S-factor of the 3He(alpha,gamma)7Be reaction measured at low energy via prompt and delayed gamma detection

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 gamma 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 Ecm = 170, 106 and 93 keV, the latter being the lowest interaction energy ever reached. nThe S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated S(0)= 0.560+/-0.017 keV barn is obtained. Moreover, branching ratios between the two prompt gamma-transitions have been measured with 5-8% accuracy.

Astrophysical S-factor of the He-3(alpha,gamma) Be-7 reaction measured at low energy via prompt and delayed gamma detection

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 gamma 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 Ecm = 170, 106 and 93 keV, the latter being the lowest interaction energy ever reached. nThe S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated S(0)= 0.560+/-0.017 keV barn is obtained. Moreover, branching ratios between the two prompt gamma-transitions have been measured with 5-8% accuracy.

The impact of the revised 17 O(p, α)14 N reaction rate on 17 O stellar abundances and yields

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.