In order to study the processes creating intermediate and heavy nuclei in massive stars it is necessary to provide neutron capture cross sections and reaction rates close to stability and for moderately unstable neutron-rich nuclei. Furthermore, one has to know the efficiency of neutron-releasing reactions in the main evolutionary phases of a massive star. We present simulations of the nucleosynthesis in a 15 and 25 solar mass star, for the first time followed completely from main sequence hydrogen burning until the type II supernova explosion including all nuclides up to Bi. Theoretical reaction rates were calculated with the NON-SMOKER code, providing a complete library of Hauser-Feshbach cross sections and rates for nuclear and astrophysical applications. Experimental rates at stability were taken from different sources. The impact of uncertainties in the rates on nucleosynthesis are illustrated by two examples, the reactions 62Ni(n,gamma)63Ni and 22Ne(alpha,n)25Mg.
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