s -Process Implications from Osmium Isotope Anomalies in Chondrites

Abstract

Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) extracted from primitive carbonaceous chondrites tightly constrain the σn(190Os)/σ(188Os)/ ratio to be 0.859 ± 0.042 (± 5%). A recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for 186, 187, 188OsOs lowered the σn(188Os) by 27% but did not measure σn(190Os). From the σn(190Os)/σn(188Os) ratio, we infer σn(190Os) = 249 ± 18 mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is applicable to other isotopic anomalies in r-process/s-process ratios derived from meteorites for nuclei that do not have branching points between them. Branching at 185W and 186Re makes the 186Os/188Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, n n ∼ 3 × 108 cm3. This low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the 22Ne(α, n) 25Mg neutron source relative to the 13C(α, n) 16O neutron source. © 2007. The American Astronomical Society. All rights reserved.