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

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The excitation function of the s-process neutron source reactions Ne-22N(alpha, n)Mg-25 and C-13(alpha, n)O-16 has been measured using improved ion-beam, target, and detection techniques, reaching a sensitivity limit of 50 picobarns in the best cases. For Ne-22Ne(alpha, n)Mg-25 the new data cover the energy range from threshold up to 2300 keV. From these experiments some of the parameters of the Ne-22(alpha, n)Mg-25 resonances have been determined using unfolding techniques. A resonance at about 620 keV could clearly be ascribed to the background reaction B-11(alpha, n)N-l4. The excitation function of C-13(alpha, n)O-16 has been measured over the energy range 350-1400 keV, where the cross section varies over eight orders of magnitude. The S-factor of this reaction indicates an increase toward lower energies. On the basis of the experimental data the reaction rates at helium-burning temperatures were determined for these two reactions: both rates are increased compared with previous evaluations.