EVOLUTION OF THE DECAY OF HIGHLY EXCITED NUCLEI

The two-fragment angle correlation functions of mass-symmetric ternary fragment events from highly excited nuclei produced in Ar-40+ Bi-209/ Au-197/Tb-159/In-115 reactions at 25 MeV/nucleon are used to extract t he mean time interval in cascade emissions (tau) of intermediate mass and heavy mass fragments (mass number greater than 20). It is shown th at the mean time interval decreases from the characteristic time tau = 2000-1000 fm/c for the Ar+Bi and Ar+Au reactions to tau= 750 fm/c for the Ar+Tb reaction and to tau= 50 fm/c for the Ar+In reaction. The de cay mode of hot nuclei evolves from cascade fission to simultaneous mu ltifragmentation as the excitation energy increases and the mass of ho t nuclei decreases.