The ^ { 13 } C ( \alpha , n ) ^ { 16 } O 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 , ^ { 13 } C burns radiatively in a thin pocket which forms in the top layers of the He-intershell , by proton capture on the abundant ^ { 12 } C. Therefore , a mixing of a few protons from the H-rich envelope into the He-rich region is requested .
However , the origin and the efficiency of this mixing episode are still matter of debate and , consequently , the formation of the ^ { 13 } C-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 ^ { 13 } C-pocket forms for an AGB model with M = 2 M _ { \odot } and [ Fe/H ] = - 2.3 .
In particular , we concentrate on three isotopes ( ^ { 89 } Y , ^ { 139 } La and ^ { 208 } Pb ) , chosen as representative of the three s -process peaks .