AGB models are critically dependent upon the assumed mass-loss rates and treatment of convection such that the predictive power of the current AGB models is diminished. That there is still no satisfactory explanation for the star-to-star abundance variations seen in every well studied Galactic globular cluster would suggest that our understanding of globular cluster chemical evolution and stellar nucleosynthesis is incomplete.

Two neutron-capture elements, rubidium and lead, may offer further clues regarding the processes that gave rise to the star-to-star abundance variations and possibly the formation of globular clusters. Rb has two stable isotopes, $^{85}$Rb and $^{87}$Rb. While the solar abundance of Rb is due to 50% $s$-process and 50% $r$-process [@burris00], the abundance of Rb relative to nearby elements such as Sr, Y, and Zr offers an insight into the neutron density at the site of the $s$-process and therefore the mass of the AGB star due to the 10.7 yr half-life of $^{85}$Kr (e.g., @tomkin83 [@tomkin99; @lambert95; @busso99; @abia01]). Along the $s$-process path, Rb is preceded by Kr. The path enters at $^{80}$Kr and exits at either $^{85}$Kr or $^{87}$Kr with $^{85}$Kr providing the branching point. At low neutron densities ($N_n~\le~1 \times 10^8$ cm$^{-3}$), $^{85}$Kr $\beta$-decays to the 