J.W. Hammer

Astrophysical Reaction Rate of 12C(α, γ)16O

A new astrophysical reaction rate of 12C(α, γ)16O has been calculated based on our recent determination of the E1- and E2-capture cross sections. The R-matrix method has been applied to describe the SE1- and SE2-factor functions as well as the data of elastic α scattering and the β-delayed α decay of 16N from other experiments. The resulting reaction rate for stellar temperatures of T9 = 0.04-10 is presented in both tabular form and an analytic expression. A new temperature dependence of the reaction rate was obtained when compared with reported evaluations. The associated uncertainties were reduced considerably in comparison to previous determinations.

Reaction rates of the s-process neutron sources Ne-22(alpha, n)Mg-25 and C-13(alpha, n)O-16

The excitation function of the s-process neutron source reactions 22 Ne(α, n) 25 Mg and 13 C(α, n) 16 O has been measured using improved ion-beam, target, and detection techniques, reaching a sensitivity limit of 50 picobarns in the best cases. For 22 Ne(α, n) 25 Mg the new data cover the energy range from threshold (E α,lab =570 keV) up to E α,lab =2300 keV. From these experiments some of the parameters of the 22 Ne(α, n) 25 Mg resonances have been determined using unfolding techniques. A resonance at about E α,lab =620 keV could clearly be ascribed to the background reaction 11 B(α, n) 14 N

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.

Nuclear and astrophysical aspects of 18O(p, γ)19F☆

Abstract The capture reaction 18O(p, γ)19F has been investigated in the energy range Ep = 80–2200 keV. The seven known resonances have been studied in detail and twelve new resonances have been found. The resonances at ER = 680, 977 and 1670 keV correspond to new states in 19F. The known resonance at ER = 631 keV is observed to consist of a doublet (ΔEp = 7 keV). Information on resonance energies, total and partial widths, branching and mixing ratios and ωγ values is reported. Transition strength arguments as well as analyses of γ-ray angular distribution data combined with results from previous work resulted in Jπ assignments for some of the resonances and low-lying states in 19F. The assignment of several states in 19F as T = 3 2 analogue states of 19O is discussed. A direct capture process to several final states in 19F up to Ex = 8.8 MeV has been observed revealing information on the orbital momenta of the captured protons in the final states, their spectroscopic factors and Jπ assignments for interfering resonances. Special efforts were made to detect this process to states near the proton threshold, which are of importance to stellar hydrogen burning of 18O. The results are compared with corresponding information from other reactions. The investigated energy range of the 18O(p, γ)19F reaction corresponds to the important stellar temperature range of T = 0.01 to 5 × 109 K. The energy-averaged astrophysical reaction rates determined from the present data are compared with previous estimates for this reaction. The data permit reliable conclusions to be drawn concerning the final termination of the CNO tri-cycle.

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.

Alpha scattering and capture reactions in the A =7 system at low energies

Differential cross sections for [sup 3]He-[alpha] scattering were measured in the energy range up to 3 MeV. These data together with other available experimental results for [sup 3]He+[alpha] and [sup 3]H+[alpha] scattering were analyzed in the framework of the optical model using double-folded potentials. The optical potentials obtained were used to calculate the astrophysical [ital S] factors of the capture reactions [sup 3]He([alpha],[gamma])[sup 7]Be and [sup 3]H([alpha],[gamma])[sup 7]Li, and the branching ratios for the transitions into the two final [sup 7]Be and [sup 7]Li bound states, respectively. For [sup 3]He([alpha],[gamma])[sup 7]Be excellent agreement between calculated and experimental data is obtained. For [sup 3]H([alpha],[gamma])[sup 7]Li an [ital S](0) value has been found which is a factor of about 1.5 larger than the adopted value. For both capture reactions a similar branching ratio of [ital R]=[sigma]([gamma][sub 1])/[sigma]([gamma][sub 0])[approx]0.43 has been obtained.

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.

The

Several resonances in the

^{15}

^{15}

N(

N(