  High resolution (R > 40 000) and high S/N spectra have been acquired with
UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for several main
sequence turnoff stars (V ~ 17 mag) and subgiants at the base of the Red Giant
Branch (V ~ 16 mag) in three globular clusters (NGC 6397, NGC 6752 and 47
Tuc/NGC 104) at different metallicities (respectively [Fe/H] = -2.0; -1.5;
-0.7$). Spectra for a sample of 25 field halo subdwarves have also been taken
with equal resolution, but higher S/N. These data have been used to determine
the abundances of several neutron-capture elements in these three clusters:
strontium, yttrium, barium and europium. This is the first abundance
determination of these heavy elements for such unevolved stars in these three
globular clusters. These values, together with the [Ba/Eu] and [Sr/Ba]
abundance ratios, have been used to test the self-enrichment scenario. A
comparison is done with field halo stars and other well known Galactic globular
clusters in which heavy elements have already been measured in the past, at
least in bright giants (V > 11-12 mag). Our results show clearly that globular
clusters have been uniformly enriched by r- and s-process syntheses, and that
most of them seem to follow exactly the same abundance patterns as field halo
stars, which discards the ``classical'' self-enrichment scenario for the origin
of metallicities and heavy elements in globular clusters.
