The radioactive decay of 187Re 1870s (tl/2 = 43 Gyr) is suited for dating the onset of heavy-element nucleosynthesis. The radiogenic contribution to the 1870s abundance is the difference between the natural abundance and the corresponding s-process component. This component can be obtained via the well-established sigmaN systematics using the neighboring s-only isotope 186Os, provided the neutron-capture cross sections of both isotopes are known with sufficient accuracy. We report on a new set of experiments perforrned with a C6D6 detector array at the n_TOF neutron spallation facility of CERN. The capture cross sections of 1860s, 1870s, and 1880s have been measured in the neutron-energy range between1 eV and 1 MeV, and Maxwellian-averaged cross sections were deduced for the relevant thermal energies frolli kT=5 keV to 100kev.
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