High-Energy Fission of Heavy Elements. Nuclear Charge Dependence

Radiochemical studies of the fission of heavy elements (${}_{67}{}^{}{}_{}{}^{}\mathrm{Ho}_{}^{165}{}_{}{}^{}$ to ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}_{}^{232}{}_{}{}^{}$) with 450-Mev protons have been performed. Radioactive nuclides varying in mass number from 59 to 115 were isolated from the various targets and, from their measured cross sections, the cross section vs mass number dependence was determined on the assumption of a yield vs charge distribution curve constant with mass number of the fission product and atomic number of the target nucleus. The integrated fission cross sections, in barns, were calculated to be 0.67, 0.21, 0.061, 0.019, 0.0050, and ∼0.002 for thorium, bismuth, gold, rhenium, tantalum, and holmium, respectively. Anomalies in the cross sections and most probable charge values for the heavier nuclides isolated from holmium suggest the existence of another competing process, labelled "fragmentation," along with spallation and fission. The importance of this process in heavy element bombardment is discussed.