Neutron physics of the Re/Os clock. I. Measurement of the (n,gamma ) cross sections of 186,187,188Os at the CERN n TOF facility

The precise determination of the neutron capture cross sections of 186Os and 187Os is important to define the s-process abundance of 187Os at the formation of the solar system. This quantity can be used to evaluate the radiogenic component of the abundance of 187Os due to the decay of the unstable 187Re (t1/2 = 41.2 Gyr) and from this to infer the time duration of the nucleosynthesis in our galaxy (Re/Os cosmochronometer). The neutron capture cross sections of 186Os, 187Os, and 188Os have been measured at the CERN n TOF facility from 1 eV to 1 MeV, covering the entire energy range of astrophysical interest. The measurement has been performed by time-of-flight technique using isotopically enriched samples and two C6D6 scintillation detectors for recording the prompt γ rays emitted in the capture events.Maxwellian averaged capture cross sections have been determined for thermal energies between kT = 5 and 100 keV corresponding to all possible s-process scenarios. The estimated uncertainties for the values at 30 keV are 4.1, 3.3, and 4.7% for 186Os, 187Os, and 188Os, respectively