The shell effect in sequential fission induced by 238U with energy just above coulomb barrier

The experimental results of sequential fission are presented for collision of 238U (5.4 MeV/u) on 48Ca, 45Sc and 238U (6.0 MeV/u) on 16O, 27Al, 48Ca, 45Sc, 48Ti, 58Fe, 64Ni and 89Y targets. The measurements of ternary events in exit channel have been performed by triple coincidence among 4 PPAC with area of 20 cm x 30 cm. The reconstruction of ternary events are performed by direct kinematics and the fission partner among three fragments is identified by relative velocities between them. The detection geometry efficiency is determined by a Monte-Carlo simulation. Before the sequential fission, U-like fragments are scattered near grazing angle. The most probable value of TKEL in the scattering process associated with S.F. changes from 25 MeV to 55 MeV for different reaction systems. The angular distributions of debris of S.F. in second C.M. are isotropic. In conclusion, because of lower fission threshold at uranium region, the sequential fission mainly follows quasielastic scattering, in S.F. cross-section σsub(S.F.), there is a strong shell effect. A simple relationship between σsub(S.F.) over σsub(R), the total reaction cross-section and Sommerfeld parameters S for nuclei with magic number are obtained