Data from J/psi and psi' production in p-A collisions are used to determine
the cross section for absorption of pre-resonance charmonium in nuclear matter.
The J/psi suppression in O-Cu, O-U and S-U collisions is fully reproduced by
the corresponding nuclear absorption, while Pb-Pb collisions show an additional
suppression increasing with centrality. We study the onset of this change in
terms of hadronic comover interactions and conclude that so far no conventional
hadronic description can consistently account for all data. Deconfinement,
starting at a critical point determined by central S-U collisions, is in accord
with the observed suppression pattern.Comment: 37 pages, 12 figures, uses epsfig style, LaTe
