Evidence for different fission behavior of hot nuclei formed in central and peripheral collisions of Ar-40+Bi-209 reaction at 25 MeV/u

Correlated fission fragments from the reaction of 25 MeV/n Ar-40+Bi-209 and their further correlation with alpha particles have been studied for perip heral and central collisions simultaneously. The excitation energy at sciss ion deduced from postscission multiplicity is about 172.5 MeV. The fission timescale deduced from prescission multiplicity is about 4x10(-21) s. Syste matic analysis of the mass and energy distributions of fission fragments as a function of the initial temperature of hot fissioning nuclei reveals the existence of different fission behavior of hot nuclei formed in central an d peripheral collisions. Experimental data demonstrate the change of fissio n behavior at T similar to 4 MeV.