Some Studies of the Products of the High-Energy Fission Process

The energy dependence of the formation cross section of certain nuclides formed in fission of uranium and thorium has been studied. Excitation functions in the hundred-Mev region are given for Ni⁶⁶, Zr⁹⁷, Ag¹¹¹, Ba¹³¹, and Ba¹³⁹. Protons, deuterons, and alpha particles from the Berkeley 184-inch frequency-modulated synchrocyclotron were used to induce the activities.It was found that the yields of neutron-deficient fission products increase with energy; those with an excess of neutrons decrease in yield. Helium ions impart greater nuclear excitation than do protons of the same energy. Neutron-deficient fission products have nearly identical cross sections for formation in uranium and thorium; those with an excess of neutrons are formed in higher yield in uranium. The results indicate that some of the fissioning events may be preceded by the evaporation of about twenty-five nucleons.