stable isotope $^{85}$Rb. At high neutron densities, $^{85}$Kr will capture neutrons to form $^{86}$Kr and then $^{87}$Kr which $\beta$-decays to $^{87}$Rb (effectively stable with a half-life of $4.7\times10^{10}$ yr). Clearly the isotopic mix of Rb depends upon the neutron density. Unfortunately, stellar Rb isotope ratios cannot be measured [@lambert76]. In the presence of a steady flow along the $s$-process path, the density of a nuclide satisfies the condition $\sigma_iN_i \simeq$ constant, where $\sigma_i$ and $N_i$ are the cross-section and abundance of nuclide $i$ respectively. The neutron-capture cross-sections differ by a factor of 10 between the two Rb isotopes ($\sigma_{87}~=~\sigma_{85}/10$). The $^{85}$Kr branch does not affect the Zr abundances since the low and high neutron density $s$-process paths converge at Sr. Therefore, in a high neutron density environment such as the helium intershell during a thermal pulse in IM-AGBs, the Rb abundance may increase by a factor of 10 relative to nearby $s$-process elements such as Sr, Y, and Zr. In reality, the situation is slightly more complex since neutron capture on $^{84}$Kr leads to the ground state of $^{85}$Kr as well as a short lived isomeric state that decays to either the $^{85}$Kr ground state or $^{85}$Rb (see @beer89 for 