Evidence for quasifission in the sub-barrier reaction of 30Si+238U

Fragment mass distributions for fission after full momentum transfer were measured for the 30Si+238U reaction at bombarding energies around the Coulomb barrier. At energies above the Bass barrier, the mass distributions were Gaussian with mass symmetry. An asymmetric fission channel with mass AL/AH ≅ 90/178 emerged at the sub-barrier energies, where competition between fusion and quasifission was suggested from the evaporation residue (ER) cross section produced in the fusion 30Si+238U. We thus conclude the asymmetric channel is attributed to quasifission. It was supported by a model calculation using the Langevin equation to give the fragment mass distribution, where fusion-fission and quasifission were separated. The observed mass asymmetry for quasifission as well as the calculation is significantly smaller than those observed in actinide targets bombarded with heavier projectiles, which suggests that the system 30Si+238U approaches the shape of the compound nucleus before disintegrating as quasifission.