Brownian shape motion: Fission fragment mass distributions

Although nuclear fission can be understood qualitatively as an evolution of the nuclear shape, a quantitative description has proven to be very elusive. In particular, until now, there exists no model with demonstrated predictive power for the fission fragment mass yields. Exploiting the expected strongly damped character of nuclear dynamics, we treat the nuclear shape evolution in analogy with Brownian motion and perform random walks on five-dimensional fission potential-energy surfaces which were calculated previously and are the most comprehensive available. Test applications give good reproduction of highly variable experimental mass yields. This novel general approach requires only a single new global parameter, namely the critical neck size at which the mass partition is frozen in, and the results are remarkably insensitive to its specific value. In order to achieve a better understanding of these results, we have embarked on more involved calculations using the friction tensor implied by the one-body wall dissipation. Generally, an anisotropic friction deflects the multi-dimensional shape trajectory away from the directions of the the driving force provided by the deformation energy (and thus changes the shape evolution), but there are reasons to expect that this complication will have only a minor bearing on the mass partition. The results should be available at the time of the conference and, if so, they will be discussed as well. Relative to previously employed methods, the present approach represents a significant advance with regard to predictive power. Thus, it can be readily employed in regions of the nuclear chart that are of special astrophysical interest and it may, for example, help to clarify the importance of fission recycling for the r-process. Taking explicit account of the equilibration process, our treatment extends in a natural way the compound nucleus concept and it builds directly on the general picture of low-energy nuclear dynamics as being dissipation dominated