The multiplicities of postsaddle protons and α particles of the heavy systems 234 Cf, 240 Cf, 246 Cf, and 240 U as functions of the postsaddle dissipation strength are calculated in the framework of a dynamical Langevin model coupled with a statistical decay model. It is found that with increasing isospin of the Cf system, the sensitivity of the postsaddle proton and α-particle multiplicity to the dissipation strength decreases substantially, and it disappears for the 240 U system. We suggest that on the experimental side, to accurately probe the postsaddle dissipation strength by measuring the prescission proton and α-particle multiplicity, it is best to populate heavy compound systems with low isospin.

Based on a Langevin equation coupled with a statistical decay model, we have studied the evaporation residue spin distribution of the nuclei 194 Pb, 200 Pb, 200 Pb, and 200 Os, and extracted a presaddle nuclear dissipation strength of 5 × 10 21 s −1 by comparing our results with the measured spin distribution of 200 Pb produced in the 16 O + 184 W reaction. We find that with increasing isospin of the system, the sensitivity of the spin distribution to nuclear dissipation decreases substantially. Moreover, for 200 Os, this spin distribution is no longer sensitive to the nuclear dissipation. These results suggest that on the experimental side, to accurately obtain the information of presaddle dissipation strength by measuring the evaporation residue spin distribution, it is best to populate those systems with low isospin.The nature and magnitude of nuclear dissipation is one of the most interesting and challenging problems in nuclear physics. In particular, presaddle nuclear dissipation strength is the focus of current experimental [1-5] and theoretical [6][7][8] research on the fission of highly excited nuclei. Because prescission light particles [9] and giant dipole resonance (GDR) γ rays [10] arise from the contribution of both presaddle and saddle-to-scission emission, it is very difficult to determine the presaddle dissipation strength by merely using the particle multiplicity. Under this circumstance, it is necessary to search for new observables that only depend on the presaddle dissipation effects. Besides the evaporation residue cross section [11,12] and the width of the fissionfragment charge distribution [1,4], the spin distribution of the evaporation residue cross section, namely, the angular momentum distribution leading to evaporation residues, may be a new and an extremely sensitive probe of the presaddle dissipation strength, as suggested in Ref. [13].To extract a precise value of dissipation strength by comparing theoretical predictions with experiment, a Langevin model is certainly preferable to a statistical model. This is because the Langevin model considers the time evolution of the fission decay width and contains a number of dynamical features in the decay of the hot compound nuclei, e.g., the angular momentum dependence of presaddle and saddle-to-scission time, etc. These advantages are not considered in a simple statistical model analysis. In addition, the Langevin model has been employed [14-18] to successfully reproduce a great deal of experimental data on prescission particle multiplicity, evaporation evaporation cross sections, and kinetic distributions of fission fragments over a wide range of excitation energy, angular momentum, and fissility.The present work is devoted to the study of the favorable experimental condition through which presaddle dissipation effects can be better revealed with evaporation residue spin distributions. For this aim, we use the Langevin model to reproduce the measured evaporation residue spin distribution of 200 Pb populated in the 16 O + 184 W reaction...

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Using a one-dimensional Langevin model, we study the effects of deformation on the multiplicities of postsaddle neutrons, protons, α particles, and giant dipole resonance (GDR) γ rays of a heavy fissioning system 240 Cf as probes of postsaddle nuclear dissipation (β). It is shown that postsaddle dissipation effects on these light particles have a significant deformation dependence. Furthermore, we find that the role of deformation depends on the type of the particle. It reduces the sensitive influence of β on protons, α particles, and GDR γ rays substantially, whereas it enhances the sensitivity of neutrons to β. The results suggest that to accurately extract the postsaddle friction strength by comparing measured prescission particle multiplicities of heavy nuclei with calculations based on statistical models or stochastic equations like Langevin equations, it is important to take into account the deformation effects. The influence of model dimensionality is discussed.