more details). IM-AGBs of solar metallicity are expected to have a high neutron density with $^{22}$Ne($\alpha$,n)$^{25}$Mg providing the neutron source. (Low-mass AGBs, whose neutron source is $^{13}$C($\alpha$,n)$^{16}$O, provide a lower neutron density.) For metal-poor or zero metallicity IM-AGBs, @busso01 suggest that such stars do run the $s$-process though the details are model dependent. Nevertheless, the Rb abundance relative to Sr, Y, and Zr (which are not affected by the $^{85}$Kr branch) is a potential diagnostic of the s-process site and may offer an additional insight into the role of IM-AGBs in the chemical evolution of globular clusters.

The isotopes of Pb, along with bismuth, comprise the last stable nuclei along the $s$-process path. In low-mass AGB stars, the neutron source is provided by $^{13}$C whereas in IM-AGBs, $^{22}$Ne provides the neutron source with the division occurring at roughly 4 M$_\odot$. In low-mass AGBs and IM-AGBs of low metallicity, overabundances of Pb and Bi may be expected if the neutron supply per seed exceeds a certain value (e.g., @goriely00 [@travaglio01; @busso01]). @goriely01 suggest that for AGB stars with $Z < 0.001$, the available neutrons per seed nuclei is greater than the number required to produce Pb and Bi. @travaglio01 suggest that 