  We follow the chemical evolution of the Galaxy for elements from Ba to Eu,
using an evolutionary model suitable to reproduce a large set of Galactic
(local and non local) and extragalactic constraints. Input stellar yields for
neutron-rich nuclei have been separated into their s-process and r-process
components. The production of s-process elements in thermally pulsing
asymptotic giant branch stars of low mass proceeds from the combined operation
of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases
neutrons in radiative conditions during the interpulse phase, and the reaction
22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The
resulting s-process distribution is strongly dependent on the stellar
metallicity. For the standard model discussed in this paper, it shows a sharp
production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process
yields, we assume that the production of r-nuclei is a primary process
occurring in stars near the lowest mass limit for Type II supernova
progenitors. The r-contribution to each nucleus is computed as the difference
between its solar abundance and its s-contribution given by the Galactic
chemical evolution model at the epoch of the solar system formation. We compare
our results with spectroscopic abundances of elements from Ba to Eu at various
metallicities (mainly from F and G stars) showing that the observed trends can
be understood in the light of the present knowledge of neutron capture
nucleosynthesis. Finally, we discuss a number of emerging features that deserve
further scrutiny.
