Study for fusion-fission and quasifission in reactions using 238U target nucleus

Abstract

Fragment mass distributions for fission after full momentum transfer (FMT) were measured for the 30Si,31P,34,36S,40Ar + 238U reactions at bombarding energies around the Coulomb barrier. The experiment was carried out at the JAEA tandem accelerator facility. We observed strong variation of the mass distribution on beam energy and projectile nucleus. In the reaction of 36S +238U we observed a transition from symmetry to asymmetry mass distributions when the beam energies were decreased from the above-barrier to sub-barrier values. The mass asymmetry was AL /AH = 74/200, which corresponds to the fission valley leading to the nuclei close to the doubly closed-shell nuclei 78Ni /208Pb. The fission channel is populated by quasifission, which is the disintegration without forming the compound nucleus. The incident-energy dependence is strongly correlated with the prolate deformation of 238U and the orientation at the initial impact. The results indicate that the reaction starting at the polar collisions on 238U has larger quasifission probability. The mass distributions are nicely reproduced by a model calculation using Langevin equation. The 40Ar + 238U reaction also has the similar mass asymmetry (AL/AH ≈ 78/200) in quasifission. In the reactions of 31P + 238U and 30Si + 238U, mass asymmetries are AL/AH 81/188 and 90/178, respectively. The results suggest that the system produced by the reaction using lighter projectile approaches closer to the shape of the compound nucleus even when quasifission occurs. In the reactions of 30Si +238U and 34S+ 238U, we also measured the evaporation residue (ER) cross sections to obtain information on the fusion probability. From the ER cross sections for the 34S+238U reaction it was suggested that the symmetric fission is also dominated by quasifission.