A measurement of the 13C(α,α) differential cross section and its application to the 13C(α,n) reaction.

The C-13(alpha ,n) reaction is considered as the main neutron source for the s process in AGE-stars of 1-3 solar masses. The reaction takes place at temperatures of about 10(8) K which corresponds to a Gamov peak at 190 keV. At this energy a direct measurement of the C-13(alpha ,n) cross section is extremely difficult and therefore one has to extrapolate the measured data to lower energies. Since the cross section is expected to be influenced by a subthreshold resonance and a resonance just above threshold, R-matrix analyses are especially suited for this extrapolation. An exact measurement of the excitation function and the angular distribution of the C-13(alpha ,n) reaction, which represents another decay channel of the O-17 compound nucleus, can provide additional constraints to improve the quality and reliability of the R-matrix extrapolation. New results for the C-13(alpha,alpha) differential cross section in the energy range 2.6 - 6.2 MeV are presented. The experiment was performed with the 10 MV tandem accelerator at the University of Notre Dame. Angular distributions for 29 angles between 44 degrees and 165 degrees could be obtained for 375 energies. Preliminary results for the reaction rate at kT = 8 keV are presented.