Brownian Shape Evolution in Nuclear Fission
- Jorgen Randrup, Lawrence Berkeley National Laboratory (LBNL)
Wednesday, January 23, 4:00 PM - Nuclear Science Seminar
NSCL Lecture Hall
Ever since fission was discovered, it was recognized that the phenomenon can be considered as an evolution of the nuclear shape and ever more advanced transport treatments have been developed within this conceptual framework. While significant progress has been made with regard to the calculation of the shape dependence of the potential energy, the associated inertia and the dissipation are less well understood and, consequently, it has not been possible to obtain quantitatively useful results for even such basic quantities as the distribution of the fission fragment masses. The problem simplifies significantly if the dissipation is strong, because then the rate of shape change and the associated accelerations are small and the inertial forces play only a minor role. The shape evolution is then akin to Brownian motion, which can readily be simulated numerically, starting from the excited compound nucleus and stopping when the shape approaches scission and no further change of the mass asymmetry is likely to occur. The resulting fission-fragment mass distribution depends only weakly on the details of the dissipation tensor and particular simplicity emerges if this tensor is isotropic, because then the shape evolution reduces to a simple Metropolis walk on the potential-energy lattice. This simple treatment has yielded mass distributions that are in remarkably good agreement with the experimental data. Because the approach is essentially free of adjustable parameters, it can readily be applied to any nucleus for which suitable energy surfaces are available, thus offering unprecedented predictive power.