Fission dynamics of excited nuclei is studied in the framework of Langevin equation. The one body wall-and-window friction is used as the dissipative force in the Langevin equation. In addition to the usual wall formula friction, the chaos weighted wall formula developed earlier to account for nonintegrability of single-particle motion within the nuclear volume is also considered here. The fission rate calculated with the chaos weighted wall formula is found to be faster by about a factor of two than that obtained with the usual wall friction. The systematic dependence of fission width on temperature and spin of the fissioning nucleus is investigated and a simple parametric form of fission width is obtained.

A theoretical model of one-body nuclear friction which was developed earlier, namely the chaosweighted wall formula, is applied to a dynamical description of compound nuclear decay in the framework of the Langevin equation coupled with statistical evaporation of light particles and photons. We have used both the usual wall formula friction and its chaos-weighted version in the Langevin equation to calculate the fission probability and prescission neutron multiplicity for the compound nuclei 178 W, 188 Pt, 200 Pb, 213 Fr, 224 Th, and 251 Es. We have also obtained the contributions of the presaddle and postsaddle neutrons to the total prescission multiplicity. A detailed analysis of our results leads us to conclude that the chaos-weighted wall formula friction can adequately describe the fission dynamics in the presaddle region. This friction, however, turns out to be too weak to describe the postsaddle dynamics properly. This points to the need for a suitable explanation for the enhanced neutron emission in the postsaddle stage of nuclear fission. * Electronic address: gargi@veccal.ernet.in † Electronic address: santanu@veccal.ernet.in

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We present a modification of the wall formula for one body dissipation in order to include the effect of irregularity in the shape of the one body potential on the dissipation mechanism. We arrive at a dissipation rate which is a scaled version of the wall formula developed earlier by Blocki et al. We show that the scaling factor is determined by a measure of chaos in the single particle motion. As an illustration, we obtain this measure of chaos for particles in a multipole deformed cavity with a view to use the scaled wall formula to calculate the damping widths of multipole vibrations of the cavity wall. Considering the amplitudes of the vibrations typical of the giant resonances in nuclei, it is observed that the effect of the shape dependence is to strongly suppress the damping caused by the original wall formula. ͓S0556-2813͑96͒04809-1͔PACS number͑s͒: 24.60. Lz, 21.60.Ev, 05.45.ϩb, 24.30.Cz