Two modes in three-fold fragmentation of hot nuclei

Splitting of a hot nucleus into three comparable pieces in the Ar-40 + Tb-159 reaction at 30MeV/u was measured. For the splitting there are two modes: one is a successive binary process, such as sequential fission or a cascade fission, another is a prompt threefold fragmentation (3F). The paper presents the identification between the two modes of the threefold fragmentation by the experimental results, such as the mass distribution of the fragments, the orientation of the fragmentation plane, etc.. In the two modes, the prompt one with collective deformation is emphasized. Furthermore fitting the mass distribution and finding a scaling law about the most-probable mass ratio in the 3F are given. In addition, the orientation of the fragmentation plane could also be used to measure the impact parameters, there for to distinguish 3F events associated with central collision from the sequential fission after a quasi-elastic scattering or deep-inelastic collision.