Negative heat capacity in the critical region of nuclear fragmentation: anexperimental evidence of the liquid-gas phase transition

An experimental indication of negative heat capacity in excited nuclear sys tems is inferred from the event by event study of energy fluctuations in Au quasi-projectile sources formed in Au + Au collisions at 35 A.MeV. Equilib rated events are selected and the excited source configuration is reconstru cted through a calorimetric analysis of its de-excitation products. Fragmen t partitions show signs of a critical behavior at about 4.5 A.MeV excitatio n energy. Around this value the heat capacity shows a negative branch provi ding a direct evidence of a first order Liquid gas phase transition. (C) 20 00 Published by Elsevier Science B.V. All rights reserved.