  Globular clusters display significant variations in their light-element
content, pointing to the existence of a second stellar generation formed from
the ejecta of an earlier generation. The nature of these internal polluters is
still a matter of debate: the two most popular scenarios indicate
intermediate-mass AGB stars and fast rotating massive stars. Abundances
determination for some key elements can help distinguish between these
competitor candidates. We present in this paper Y abundances for a sample of
103 red giant branch stars in NGC 6121. Within measurement errors, we find that
the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a
subsample of six stars we also find [Rb/Fe] to be constant, consistent with
previous studies showing no variation in other s-process elements. We also
present a new set of stellar yields for intermediate-mass AGB stellar models of
5 and 6 solar masses, including heavy element s-process abundances. The
uncertainties on the mass-loss rate, the mixing-length parameter, and the
nuclear reaction rates have a major impact on the stellar abundances. Within
the IM-AGB pollution scenario, the constant abundance of heavy elements inside
the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a
constrain could still be satisfied by the present models assuming a stronger
mass-loss rate. The uncertainties mentioned above are limiting the predictive
power of intermediate-mass AGB models. For these reasons, at the moment we are
not able to clearly rule out their role as main polluters of the second
population stars in globular clusters.
