Bimodal Nature of Low Energy Fission of Light Actinides

Abstract

Studies of mass division phenomena in low energy proton-induced fission of light actinides are discussed. From experimental data of excitation functions and angular distributions of fission products, it has been found that there are two kinds of fission threshold energies (fission barrier heights); symmetric and asymmetric mass divisions require different threshold energies. Two kinds of scission configurations for certain mass division have been verified by accurate measurements of fragment velocities : compact and elongated scission configurations. A correlation between the two types of threshold energies and the two kinds of scission configurations has been revealed by observing the incident energy dependence of the yields corresponding to each compact and elongated scission configuration. It indicates that there are two independent deformation paths in the protoninduced fission of light actinides; one type of the fission process has a lower threshold energy and leads to a compact scission configuration with reflection asymmetry, while the other type has a higher threshold energy and leads to a symmetric elongated scission configuration.

Two-mode fission – experimental verification and characterization of two fission-modes

“…The figure indicates that the overall mass yield curve can be smoothly decomposed into the two components in the mass region A ∼ 130. The correlation between the scission configurations and the mass-yield distributions reveals that the elongated scission-configuration is associated with the mass-symmetric fission process while the more compact scission-configuration is with the mass-asymmetric one in the fission of the actinides [7,[28][29][30].…”

“…The correlation between scission configurations and mass distributions has ascertained that elongated scission configurations are associated with the symmetric mass distribution and more compact scission configurations with the asymmetric mass distribution [26,27]. The presence of two independent deformation-paths in low energy fission of actinides was demonstrated by studying the correlation among saddle-point configurations, scission-point configurations and mass-yield distributions [7,[28][29][30]. This is essentially experimental verification and further extension of the working hypothesis of the two-mode fission by Turkevich and Niday [11].…”

“…The difference (B [15]. Adapted from [29]. Closed and open circles show the ratios of σ( 143 Ce)/σ sym and σ( 105 Rh)/σ sym , respectively.…”

“…The resulting D is plotted in Fig. 6a as a function of the heavier fragment mass number in the fission of 232 Th with 13 MeV protons [28]. The open and solid squares correspond to the distances obtained from the twocomponent analysis.…”

“…Double velocity TOF experiments for various incident energies were carried out and excitation functions of two kinds of the TKE components were constructed [28]. A typical comparison of the decomposed mass yields at different proton energies is shown in Fig.…”