What we do and do not know about the s-process
  AGB stars are the source for the main component of the $s$-process. Here we discuss both the properties which are reasonably well known and those which still suffer from substantial uncertainties. In the former case, we are fairly sure that the $s$-process contribution from AGB stars comes from masses between about 1 and 3 $\msun$, and the dominant neutron source is the $^{13}$C$(\alpha$,n)$^{16}$O reaction. In the latter category remains the formation mechanism for the $^{13}$C-pocket. Attempts at including rotation seem to inhibit neutron capture reactions. Explaining the observations seems to require a spread in the size of the $^{13}$C-pocket so some stochastic process, such as rotation, must be involved. 
J. C. Lattanzio and M. A. Lugaro