REACTION-RATES OF THE S-PROCESS NEUTRON SOURCES NE-22(ALPHA,N)MG-25 AND C-13(ALPHA,N)O-16(1)

The excitation function of the s-process neutron source reactions Ne-22(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-22(alpha, n)Mg-25 the new data cover the energy range from threshold (E(alpha,lab) = 570 keV) up to E(alpha,lab) = 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 E(alpha,lab) = 620 keV could clearly be ascribed to the background reaction B-11(alpha, n)N-14. The excitation function of C-13(alpha, n)O-16 has been measured over the energy range E(alpha, lab) = 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, possibly due to a state in O-17 below the (C-13 + alpha) threshold leading to a higher reaction rate. 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.