Psi'/Psi ratio in Nucleus-Nucleus Collisions : a Measure for the Chiral Symmetry Restoration Temperature ?

We argue that a decrease of the chiral scalar meson mass is responsible for re-creation of $\Psi'$ from $J/\Psi$ in ultrarelativistic nucleus-nucleus collisions. This causes the charmonium yields to freeze out at temperatures close to the chiral symmetry restoration temperature $T_c$. As a result $\Psi'/\Psi$ may serve as a thermometer for $T_c$ itself. Results in a detailed reaction model support the conjecture. They show good agreement with recent data of NA38 and NA50 for $J/\Psi$ and $\Psi'$ production in S on U and Pb on Pb collisions.