Nuclear fissions inr andr-cooling processes

Abstract

The effect of nuclear fissions occurring during ther-cooling stage is investigated. The initial conditions for ther-cooling process are derived from the final conditions of the dynamicalr-process. Neutron-excess nuclides in the region ofZ≧80 are all assumed to have finite probabilities for spontaneous fission, neutron-induced fission and β-decay-induced fission. For the evaluation of the rates for delayed neutron emission, β-decay-induced fission and β-decay, three β-decay theories—the Fermi theory, the Gross theory and the theory by Cameron, Delano and Truran—are used. It becomes clear that the β-fissions and the symmetric spontaneous fissions duringr-cooling make a broad abundance peak of fission products, centring aroundA=128, whose effect is hidden by ther-abundance peak atA≅130. Asymmetric spontaneous fissions can form an apparent abundance peak atA≅105, with the position of this peak just corresponding to the position of the anomalous hump observed in ther-abundance curve. To explain the abundance of this observed hump by spontaneous fissions, ther-process must take place under conditions such that the number of free neutrons available for one seed nucleus is large enough to make the cyclicr-process, and that the final neutron-capture-path goes through the region of (neutron binding energy in the nuclei on the capture-path) ≲ 2 MeV.