The first evidence of the occurrence of nucleosynthesis in stars was provided in the
1950s by the detection of the unstable heavy element technetium in the atmospheres
of stars on the Asymptotic Giant Branch (AGB), a late evolutionary phase of stars of
low mass. Technetium can be produced by slow neutron captures (the s process) and
thus its detection requires that neutron source reactions are activated in these
stars, triggering the production of heavy element. The best candidates as sources
of neutrons have been identified in the C13(alpha,n)O16 and
Ne22(alpha,n)Mg25 reactions. The C13(alpha,n)O16 reaction appears to be responsible
for the production of most of the neutrons, while the
Ne22(alpha,n)Mg25 reaction is important for the activation of branching points on
the s-process path. Detailed evidence of the operation of branching points is shown
by recent measurements of the isotopic composition of heavy elements in meteoritic
silicon carbide (SiC) grains that originated from AGB stars. Interesting examples
that will be discussed are the Ba134/Ba136, Kr86/Kr82 and Zr96/Zr94 ratios. During
the s process, elements lighter than iron represent "neutron poisons", stealing
neutrons from the production of heavy elements. Nevertheless, neutron captures on
light elements are of much interest in triggering secondary nucleosynthesis path.
For example, the activation of the
N14(n,p)C14 reaction on one hand can strongly inhibit the production of heavy
elements by the C13(alpha,n) source, on the other hand it leads to the production of
fluorine, which is observed to be enhanced in AGB stars.
Neutron-capture reactions on the isotopes of Mg, Si and Ti are also of great
interest as they change the composition of these elements, which are also recorded
in meteoritic grains.