FISSION TIME-SCALE AND VISCOSITY FOR COMPOUND NUCLEUS TH-220 FORMED IN REACTION AR-40-180 AT E-LAB = 180, 190, 216, AND 249 MEV(HF)

Double differential spectra of neutrons emitted from the reaction Ar-4 0+Hf-180 at E-lab=180, 190, 216, and 249 MeV were measured in coincide nce with fission fragments using a multidetector array. Averaged presc ission and postscission neutron multiplicites were extracted from the spectra together with temperature parameters. These values were analyz ed in the framework of a modified version of the statistical model wit h inclusion of dynamical hindrance of fission, which depends on nuclea r dissipation. From comparison of the experimental and calculated neut ron multiplicities and temperatures it was deduced that nuclear viscos ity is strong at excitation energies higher than similar to 40 MeV and decreases with the decrease of excitation energy during evaporation c ascade. The extracted fission time scale changes from 40x10(-20) s to 17x10(-20) s in the investigated projectile energy interval.