Fission and quasifission of the composite system Z=114 formed in heavy-ion reactions at energies near the Coulomb barrier

Background: Study of competition between compound nucleus fission and quasifission in heavy-ion-induced reactions and its dependence on the reaction entrance channel are important for picking up the right target- projectile combination for the synthesis of superheavy elements. Purpose: We investigate the decrease of fusion probability in the Cr-52 + Th-232 reaction in comparison with that in the reactions Ca-48 + Pu-244 and Ti-48 + U-238. All reactions lead to the formation of composite systems with Z = 114. Methods: Mass-energy distributions of binary fragments formed in the reaction Cr-52 + Th-232 leading to the (284)Fl composite system at energies of 265, 288, 302, and 320 MeV have been measured using the double-arm time-of-flight spectrometer CORSET. To study the properties and entrance channel dependence of quasifission in more detail, the mass and energy distributions of fragments formed in the Kr-86 + Pt-198 reaction, leading to the same composite system (284)Fl, have also been measured. Results: The contribution of symmetric fragments in all fissionlike events is nearly the same for both the reactions T-48 + U-238 and Cr-52 + Th-232 at energies above the barrier, while it is greater for Ca-48 + Pu-244. The analysis of total kinetic energy distribution of symmetric fragments formed in the reaction Cr-52 + Th-232 shows that at an energy 15% over the Bass barrier the contribution of compound nucleus fission in the capture cross section is less than 0.4%. Conclusions: The fusion probability drops down by about a factor of 4 at the transition from the Ca-48 + Pu-244 reaction to Ti-48 + U-218 and about a factor of 25 at the transition to Cr-52 + Th-232 at energies above the barrier. It agrees with the trend of fusion probability to decrease exponentially with increasing the mean fissility parameter of the composite system.