The s-process efficiency in massive stars

The s-process is studied in the He burning core of a Mα=8M· He star using temperature and density profiles obtained in standard stellar evolution models and an updated nuclear network. We discuss in detail the s-process efficiency for metallicities z/z· ranging between 1 and 10-3 in the light of recent nuclear data. We show that 16O acts as a powerful neutron poison at low z although its effect on the s-process yields is still subject to large uncertainties due to our imperfect knowledge of some nuclear reaction rates