Spallation-Fission Competition in Astatine Compound Nuclei Formed by Heavy-Ion Bombardment

Abstract

Cross sections for neutron-evaporation reactions from compound nuclei produced by bombardment of gold with carbon ions and of platinum with nitrogen ions have been determined. The magnitudes of the cross sections are considerably lower than would be predicted on the assumption that neutron emission is the only important mode of decay of the intermediate nuclei. This observation is explained on the basis of fission competition with neutron emission. To a much lesser extent, charged-particle evaporation is also a competing mode of decay. The arguments presented indicate that fission occurs either with comparable magnitudes in several nuclei in the neutron-evaporation chain, or preferentially in one or two nuclei near the end of the chain, rather than predominantly in the initial compound nucleus. Problems arising from the possible existence of isomers in the odd-odd astatine nuclides are discussed.