The key reactions in Stellar helium burning: 12C(α,γ)16O and 22Ne(α,n)25Mg

The excitation functions of 12C(α,γ)16O and 22Ne(α,n)25Mg have been determined with a yet unequaled experimental sensitivity of 10-11 b. From γ-angular distributions the SE1-and SE2-factors for 12C(α,γ)16O have been deduced and extrapolated into the range of helium burning temperatures using the R-matrix method. An improved reaction rate has been calculated: NA<σν>=(7.9±2.5)x10-15cm3(mols)-1 at T9 = 0.2. For a more precise extrapolation the excitation function has to be measured with better statistics. Therefore a new experiment on 12C(α,γ)16O with a better suited setup has been started recently. The accuracy of the 22Ne(α,n)25Mg reaction rate could be improved considerably compared to previous compilations: NA<σν> = (0.88-0.3+2.6) x 10-16cm3(mols)-1 at T9 = 0.2