  The 13C(a, n)16O reaction is the major neutron source in low mass asymptotic
giant branch (AGB) stars, where the main and the strong s process components
are synthesised. After a third dredge-up (TDU) episode, 13C burns radiatively
in a thin pocket which forms in the top layers of the He-intershell, by proton
capture on the abundant 12C. Therefore, a mixing of a few protons from the
H-rich envelope into the He-rich region is requested. However, the origin and
the effciency of this mixing episode are still matter of debate and,
consequently, the formation of the 13C-pocket represents a significative source
of uncertainty affecting AGB models. We analyse the effects on the
nucleosynthesis of the s-elements caused by the variation of the hydrogen
profile in the region where the 13C-pocket forms for an AGB model with M = 2
Msun and [Fe/H] = -2.3. In particular, we concentrate on three isotopes (89Y,
139La and 208Pb), chosen as representative of the three s-process peaks.
