H. P. Trautvetter

Astrophysical S(E) factor of 3He(3He, 2p)4He at solar energies

Abstract The 3 He( 3 He, 2p) 4 He reaction has been investigated in the energy range E c.m. = 17.9 to 342.5 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems. At low energies the studies required the measurement of coincidences between the two protons in the exit channel as well as precautions for contributions from cosmic background and electronic noise. The data extend into the thermal energy region of the sun, e.g., σ = 7 ± 2 pb at E c.m. = 24.5 keV, and upper limits for the reaction yield have been obtained down to E c.m. ∼- 17.9 keV. No evidence for a suggested low-energy resonance has been found. The astrophysical S ( E ) factor at zero energy is S (0) = 5.57 ± 0.32 MeV · b. The implications of the data for the solar-neutrino problem are discussed.

The3He(α,γ)7Be reaction and the solar neutrino problem

The capture reaction3He(α,γ)7Be has been investigated in the energy range ofEc.m. =107 to 1,266 keV. The4He or3He beams of up to 300 μA particle current were incident on3He or4He gas targets, respectively. The gas target systems were all of the windowless and recirculating type. Excitation functions have been obtained with the use of an extended-static gas target, while the measurements ofγ-ray angular distributions involved a quasi-point supersonic jet system. The determination of absolute cross sections has been carried out with both types of gas target systems. Theγ-ray yields in the3He(α,γ)7Be reaction were detected using 80 cm3 Ge(Li) detectors. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)=0.30±0.03 keV-b. This result reduces the calculated solar neutrino rate by a factor of 1.76.

The4He(12C, γ)16O reaction at stellar energies

The capture reaction4He(12C, γ)16O (Ec.m.= 1.34–3.38 MeV) as well as the elastic scattering process4He(12C,12C)4He (Ec.m.=1.44–3.38 MeV) have been investigated with the use of an intense12C beam and a windowless and4He recirculating gas target system. The measurements involved two large NaI(T1) crystals in close geometry to an extended gas target, whereby angle-integrated γ-ray yields were obtained. A large area plastic detector was used for the suppression of time-independent background. A search for cascade γ-ray transitions was carried out by coincidence techniques. The measurement of absolute cross sections is also reported. Theoretical fits of the excitation function for the groundstate γ-ray transition requireE1 as well asE2 capture amplitudes, which are of equal importance at stellar energies. This result increases significantly the stellar burning rate of4He(12C, γ)16O and leads to16O as the dominant product at the end of helium burning in massive stars. The observed capture yield to the 6.92 MeV state is dominated by the direct capture mechanism and plays a small role at stellar energies.

The 12C(α, γ)16O cross section at stellar energies☆

Abstract The capture reaction 12 C(α, γ) 16 O has been investigated at E = 0.94 to 2.84 MeV with the use of an intense α beam and implanted 12 C targets of high isotopic purity. The studies involved NaI(Tl) crystals and, for the first time, germanium detectors. The measurement of absolute cross sections, γ-ray angular distributions and excitation functions is reported. A cross section of 48 pb is found at E = 0.94 MeV. The data provide information on the E1 and E2 capture amplitudes involved in the transition to the ground state as well as to excited states. The S -factor at stellar energies has been determined by means of theoretical fits. The results verify the previous report of a substantial higher S -value compared to the value recommended in 1975. The present uncertainty in the S -value as well as possible improvements are discussed. This S -value is of crucial importance to nuclear astrophysics.

The scattering of alpha particles from 12C and the 12C(α, γ)16O stellar reaction rate

Abstract The elastic scattering of alpha particles from 12 C has been investigated for 35 angles in the range θ lab = 22° to 163° and for 51 energies at E α = 1.0 to 6.6 MeV. The extracted phase shifts for l = 0 to 6 partial waves have been parametrized in terms of the multilevel R -matrix formalism. Information on the deduced parameters of states in 16 O is reported. The data reveal reduced α-particle widths for the 6.92 and 7.12 MeV subthreshold states consistent with recent work. The implications for the stellar reaction rate of 12 C (α, γ) 16 O are discussed.

The 18O(p, α)15N reaction at stellar energies

Abstract The 18O(p, α)15N reaction has been investigated in the energy range Ep = 72–935 keV. The three known resonances above Ep = 620 keV have been confirmed and four new resonances have been found below Ep = 340 keV. All observed resonances correspond to known compound states in 19F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 109 K. The energy averaged astrophysical reaction rates are compared with predictions.

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.

Low-energy resonances in 25Mg(p, γ)26Al, 26Mg(p, γ)27Al and 27Al(p, γ)28Si☆

Abstract Gamma-ray decay schemes have been measured with bare and Compton-suppressed Ge detectors at low-energy resonances ( E P 25 Mg, 26 Mg and 27 Al. Altogether 58 new decay branches have been observed and a new 26 Mg(p, γ) 27 Al resonance has been found at E P = 154.5 ± 1.0 keV. The new branchings lead to J π ; T determinations (or limitations) for two states in 26 Al and four states in 28 Si. The absolute strengths of the 25 Mg(p, γ) 26 Al and 26 Mg(p, γ) 27 Al resonances have also been obtained, and the uncertainties of the stellar rates, deduced from the available data for both reactions, are significantly reduced. Some astrophysical consequences are discussed.

Cross section and astrophysical S-factor for the 13C(p, γ)14N reaction☆

Abstract We have studied the 13C(p, γ)14N reaction in the energy range Ep = 120–950 keV and obtained cross section curves for capture into the first six states of 14N. The 1−, T = 1 state in 14 N was observed at laboratory energy ER = 557.6 ± 0.5 keV (Ex = 8068.1 ± 0.5 keV) with width ΓR = 40 ± 1 keV, and the 2−, T = 0 state at ER = 450.4 ± 0.5 keV (Ex = 7968.6 ± 0.5 keV). Spectroscopic factors have been obtained from observation of direct capture to all states; they are in fair agreement with results from stripping reaction studies. The S-factor curve for the transition to the ground state is in excellent agreement with previous results; S-factor curves for the other transitions are reported for the first time. The total S-factor extrapolated to 25 keV is 7.7 ± 1.0 keV·b, somewhat higher than previous values.

Stellar reaction rate of 20Ne(α, γ)24Mg☆

Abstract The reaction 20Ne(α, γ)24Mg has been investigated at Eα(lab) = 0.55–3.20 MeV. Neon gas enriched to 99.95% in 20Ne was recirculated in differentially pumped gas target systems of the extended and quasipoint jet types. New resonances were found at Eα(lab) = 958, 1226, 1260, 1704 and 2277 keV, which correspond to known states in 24Mg. Excitation energies, γ-ray decay schemes, γ-ray angular distributions, resonance widths and strengths as well as Jπ and T-assignments are reported for all the resonances. Information on low-lying states in 24Mg is also obtained. The nuclear and astrophysical aspects of the results are discussed.