Thermodynamical features of multifragmentation in peripheral Au plus Au collisions at 35 A MeV

The distribution of fragments produced in events involving the multifragmen tation of excited sources is studied for peripheral Au + Au reactions at 35 A MeV. The Quasi-Projectile has been reconstructed from its de-excitation products . An isotropic emission in its rest frame has been observed, indicating tha t an equilibrated system has been formed. The excitation energy of the Quas i-Projectile has been determined via calorimetry. A new event by event effective thermometer is proposed based on the energy balance. A peak in the energy fluctuations is observed related to the heat capacity suggesting that the system undergoes a liquid-gas type phase trans ition at an excitation energy similar to 5 A MeV and a temperature 4-6 MeV, dependent on the freeze-out hypothesis. By analyzing different regions of the Campiplot, the events associated with the liquid and gas phases as well as the critical region are thermodynamically characterized. The critical exponents, tau, beta, gamma, extracted from the high moments o f the charge distribution are consistent with a liquid-gas type phase trans ition. (C) 1999 Elsevier Science B.V.