BIMODAL NATURE OF LOW-ENERGY FISSION OF LIGHT ACTINIDES

Studies of mass division phenomena in low energy proton-induced fissio n of light actinides are discussed. From experimental data of excitati on functions and angular distributions of fission products, it has bee n found that there are two kinds of fission threshold energies (fissio n barrier heights); symmetric and asymmetric mass divisions require di fferent threshold energies. Two kinds of scission configurations for c ertain mass division have been verified by accurate measurements of fr agment velocities: compact and elongated scission configurations. A co rrelation between the two types of threshold energies and the two kind s of scission configurations has been revealed by observing the incide nt energy dependence of the yields corresponding to each compact and e longated scission configuration. It indicates that there are two indep endent deformation paths in the proton-induced fission of light actini des; one type of the fission process has a lower threshold energy and leads to a compact scission configuration with reflection asymmetry, w hile the other type has a higher threshold energy and leads to a symme tric elongated scission configuration.