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 responsibl e for recreation of Psi' from J/Psi in ultrarelativistic nucleus-nucle us collisions. This causes the charmonium yields to freeze out at temp eratures close to the chiral symmetry restoration temperature T-c. As a result the ratio Psi'/Psi may serve as a thermometer for T-c itself. Results in a detailed reaction model support the conjecture. They sho w good agreement with recent data of NA38 and NA50 for J/Psi and Psi' production in S on U and Pb on Pb collisions.