Systematics of fission-channel probabilities

Fissioning nuclei produce with increasing excitation varying yields of fragments. For compound nuclei ranging from Th-232 to Pu-242 endowed with excitation energies typically between 0 and 10 MeV, we analyze these variations in terms of a fission-channel model. Most of the variations can be attributed to changing channel probabilities. We present a systematics of channel probabilities with respect to compound nuclei and their excitation energies, and we relate the systematics to potential energy which the nuclei experience when they float to scission. The trend, which is most difficult to explain, is a shift in the energy sensitivity of the standard channels, as one compares light with heavy compound nuclei. According to their behavior, we divide nuclei into standard I increasers and decreasers and attribute the difference to the standard secondary barriers. As a basic concept, bifurcation ratios are introduced, and a novel expression for transmission coefficients is proposed. [S0556-2813(99)05402-3].