The Relative J/Psi to Psi' Suppression in Proton-Nucleus and Nucleus-Nucleus Collisions

We calculate the nuclear suppression for $J/\Psi$ and $\Psi'$ production within a coupled channel approach in the subspace of the $J/\Psi$ and $\Psi'$ states. We are able to explain, why (i) the $J/\Psi$ and $\Psi'$ show the same suppression from $200\ GeV$ to $800\ GeV$ in proton-nucleus collisions and why (ii) the $\Psi'$ is absorbed more strongly than the $J/\Psi$ in nucleus-nucleus collisions at $200\ GeV$. The numerical result which includes only interactions with nucleons acconts for half of the observed suppression in sulphur-uranium collisions.