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

The C-13(alpha ,n) reaction is considered as the main neutron source for th e s process in AGE-stars of 1-3 solar masses. The reaction takes place at t emperatures 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 ana lyses 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 nucl eus, can provide additional constraints to improve the quality and reliabil ity of the R-matrix extrapolation. New results for the C-13(alpha,alpha) differential cross section in the ene rgy range 2.6 - 6.2 MeV are presented. The experiment was performed with th e 10 MV tandem accelerator at the University of Notre Dame. Angular distrib utions for 29 angles between 44 degrees and 165 degrees could be obtained f or 375 energies. Preliminary results for the reaction rate at kT = 8 keV ar e presented.