Symmetric and asymmetric mass division in proton-induced fission of 232Th and 238U

The authors studied proton-induced fission of 232Th and 238U from three aspects; (1) Excitation functions, (2) Angular anisotropies as a function of fragment-mass ratios and (3) Coefficient of dissymmetry(CD). It was found a careful studies of exp.(1) that there existed the difference of threshold energies between the symmetric and the asymmetric modes. The authors could successfully unfold the observed cross sections into these two modes. A strong fragment-mass dependence of anistropy was observed from exp.(2) in the 232Th(p,xnf) at 15 MeV but not clear in the 232Th(p,xnf) at 13.5 MeV and 238U(p,xnf) at 14.5 and 16 MeV. The overall feature of observed anisotropy data could be well explained by an assumption of two kinds of threshold energies at saddle, one leading to the symmetric mass division, the other to the asymmetric one being about 2 MeV lower. A drastic change in the CD of exp.(3) was well explained by the mixing of these two different fission modes