Influence of fission dynamics on formation of fragment charge distribution

The results of calculations of the isobaric charge distribution of fission fragments are reviewed. The calculations were made in the framework of dynamical approaches based on the Fokker-Planck equation for the distribution function of the collective variables and a Schroedinger equation with friction for a harmonic oscillator with time-dependent coefficients. Three collective coordinates were taken into account: the main fission coordinate, i.e., the distance between the centers of mass of the nascent fragments (or elongation parameter), the neck parameter, and a collective coordinate that determines the charges of the future fragments (charge coordinate). The use of the dynamical model of the formation of the charge distribution shows that statistical equilibrium with respect to the charge mode can be established at each time during almost the entire descent from the saddle point to scission, except for the very final stage directly preceding the breaking of the neck. This explains the successful use of the statistical model for calculations of the fission-fragment charge distribution. The deviation of the variance of the charge distribution from its equilibrium value in the final stage of the process before fission is due to the sharp growth of the inertia and friction coefficients of the charge mode. This growth open-quotes freezesclose quotes the variance of the charge distribution. The study of the variances of the fragment charge distribution as a function of the excitation energy indicates that the fluctuations of the charge mode in fission are mainly of a quantum nature. The possibility of estimating the magnitude and mechanism of nuclear viscosity in fission from the observed even-odd effect in the yields of the fragment charges is discussed. 107 refs., 16 figs., 2 tabs