Neutron-capture Element Trends in the Halo

Most isotopes of elements with atomic numbers Z > 30 are synthesized via neutron capture reactions. These “n-capture” elements are the majority of the periodic table. In the so-called s-process, neutron fluxes are small enough to allow β-decays to occur between successive neutron captures, and element buildup proceeds along the valley of β-stability. In the r-process, huge but short-lived neutron fluxes overwhelm β-decays, creating very neutron-rich isotopes out to the neutron drip line. Then multiple β-decays drive the nuclei back to the valley of β-stability. The final isotopic mixes will be very different in rand s-process synthesis episodes, as will the elemental abundances summed over the isotopes. The s-process mainly occurs in the helium shell burning phases of low-intermediate mass AGB stars, while the r-process is probably associated with the explosive deaths of high mass stars. Thus, rand s-process elemental abundance variations with metallicity should trace the contributions of different mass ranges of stars over the Galaxy’s history. Detailed comparisons of solar system meteoritic abundances of n-capture isotopes (Cameron, Käppeler et al.) have yielded accurate breakdowns into rand s-process parts of each isotope. But stellar spectroscopy generally cannot resolve the finely split isotopic absorption components of atomic tran-