Fission time scale and viscosity for compound nucleus ${}^{220}\mathrm{Th}$ formed in reaction ${}^{40}{\mathrm{A}\mathrm{r}+}^{180}\mathrm{Hf}$ at $E_{\mathrm{lab}}=180,$ 190, 216, and 249 MeV

Double differential spectra of neutrons emitted from the reaction ${}^{40}{\mathrm{A}\mathrm{r}+}^{180}\mathrm{Hf}$ at $E_{\mathrm{lab}}=180,$ 190, 216, and 249 MeV were measured in coincidence with fission fragments using a multidetector array. Averaged prescission and postscission neutron multiplicites were extracted from the spectra together with temperature parameters. These values were analyzed in the framework of a modified version of the statistical model with inclusion of dynamical hindrance of fission, which depends on nuclear dissipation. From comparison of the experimental and calculated neutron multiplicities and temperatures it was deduced that nuclear viscosity is strong at excitation energies higher than $\sim 40\mathrm{MeV}$ and decreases with the decrease of excitation energy during evaporation cascade. The extracted fission time scale changes from $40\times {10}^{-20}\mathrm{s}$ to $17\times {10}^{-20}\mathrm{s}$ in the investigated projectile energy interval.