s-process Ba, Nd, and Sm in presolar SiC from the Murchison meteorite

Ion microprobe isotopic measurements of Ba, Nd, and Sm in the K-series SiC size separates on which noble gas measurements were made by Lewis et al. (1990) are reported. All elements show isotopic abundance patterns characteristic for the s-process. The Ba-134/Ba-136 ratios are distinct from solar, indicating that s-Ba in SiC is different from that in the solar system. Ba-138/Ba-136 ratios decrease with grain size; if interpreted as being due to different neutron exposures, this trend is opposite of that shown by the Kr-86/Kr-82 ratios. Although other isotropic ratios for Ba and those for Nd and Sm differ in detail from theoretical predictions, the disagreements probably being due to uncertainties in the n-capture cross sections and simplifications in the models, the general agreement of the data with models of s-process nucleosynthesis support an AGB star origin for the relatively fine-grain SiC under consideration. Ba-136 in SiC is up to 85 percent pure s-process.