s-PROCESS IMPLICATIONS FROM OSMIUM ISOTOPE ANOMALIES IN CHONDRITES

Correlated isotopic anomalies in osmium (Os, Os, Os, measured with respect to Os) extracted from primitive carbonaceous chondrites tightly constrain the ratio to be ( 5%). A 190 188 j ( Os)/j ( Os) 0.859 0.042 n n recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for 186, 187, Os lowered the jn( Os) by 27% but did not measure jn( Os). From the jn( Os)/jn( Os) ratio, we infer 190 j ( Os) p n mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is 249 18 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 W and Re makes the Os/Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, cm . This 8 n ∼ 3 # 10 n low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the Ne(a, n)Mg neutron source relative to the C(a, n)O neutron source. Subject headings: nuclear reactions, nucleosynthesis, abundances — solar system: general