Theory of mean primary charge distribution in low energy fission of even-even nuclei

A molecular model of fission is introduced in which the compound nucleus before scission is described. The ground state wave function is assumed to be a BCS wave function. The single particle wave functions are expanded in terms of eigenfunctions of the unperturbed spherical fragments. The BCS wave function is determined from the minimum condition for the total energy. The fragment masses, the centers of mass, and the total proton and neutron numbers are kept constant. The resulting BCS and Hartree-Bogoliubov equations are solved approximately within the framework of an extended Nilsson model. The numerical results for the charge distribution in low energy fission of²³⁶U are in agreement with experiments. The heavier (lighter) fragment has on the average 0.5 protons less (more) than expected on the basis of the socalled unchanged charge distribution (UCD). At magic configurations the charge distribution shows characteristic deviations from the average value due to the shell structure of the fragments. The charge distribution seems to result mainly from three competing effects:

1. a)
   from the level density of the unperturbed fragments in the vicinity of the Fermi energy.