Petra Schmalbrock

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.

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.

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.

Proton-induced direct capture on 21Ne and 22Ne☆

Abstract The direct capture process in the reactions 21 Ne(p, γ) 22 Na and 22 Ne(p, γ) 23 Na has been investigated at E p = 0.3–1.6 MeV using neon gas enriched to 91 % in 21 Ne and to 99 % in 22 Ne, respectively. The gas was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For 22 Ne(p, γ) 23 Na, the direct capture process has been observed to several final states in 23 Na up to E x = 8.83 MeV excitation energy. The deduced spectroscopic factors C 2 S are in fair agreement with the corresponding values from stripping reactions. The capture transition into the 23 Na ground state exhibits broad structures, which resemble Ericson fluctuations. The data remove the previously reported discrepancies in C 2 S for the 23 Na ground state. The excitation functions for the 21 Ne(p, γ) 22 Na reaction are dominated by broad and intense resonances, which hampered the measurement of the direct capture process. The nuclear and astrophysical aspects of the results are discussed.

Search for low-energy resonances in 21Ne(p, γ)22Na and 22Ne(p, γ)23Na

Abstract The reactions 21 Ne(p, γ) 22 Na and 22 Ne(p, γ) 23 Na have been investigated at E p (lab) = 70–355keV. Neon gas enriched to 91% in 21 Ne and to 99% in 22 Ne was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For 21 Ne(p, γ) 22 Na, new resonances were found at E p = 126, 272, 291 and 352 keV. The 291 keV resonance corresponds to a new unbound state in 22 Na. Excitation energies, γ-ray decay schemes, resonance widths and strengths as well as J π assignments are reported for all the resonances. Information on low-lying states in 22 Na is also obtained. Of the 9 expected resonances in 22 Ne(p, γ) 23 Na none has been observed. Upper limits on their ωγy strengths are presented. The astrophysical as well as the nulcear structure aspects of the results are discussed.