of the 11 th Workshop on “ Nuclear Astrophysics ” Ringberg Castle , Tegernsee , Germany February 11 - 16 , 2002

The excitation functions of the key reactions of stellar nucleosynthesis, 12C(α, γ)16O and 22Ne(α, n)25Mg, have been determined with an experimental sensitivity of 10−11 barn. For 12C(α, γ)16O the SE1and SE2-factors deduced from the γ angular distributions have been extrapolated into the range of helium burning temperatures applying the R-matrix method, which yielded S300 E1 = (76± 20) keVb and S300 E2 = (85± 30) keVb. Based on the new data, improved reaction rates were calculated as a function of temperature. For 12C(α, γ)16O the reaction rate at T9 =0.2 is NA〈σ v〉= (7.9± 2.5)× 10−15 cm3 (mol s)−1 and for 22Ne(α, n)25Mg it is NA〈σ v〉= (0.88 +2.6 −0.3)× 10−16 cm3 (mol s)−1, improving the accuracy considerably compared to previous determinations.