Reaction dynamics of the 12C+181Ta system near the Coulomb barrier: Evidence of fusion-fission events

Background: Interaction of a heavy projectile with a heavy target produces a massive compound nucleus (CN), which deexcites through two possible modes: evaporation of light particles and fission. In order to develop a clear understanding of fusion-fission (FF) dynamics, it is important to explore the preactinide region at energies around and above the Coulomb barrier. Purpose: The objective is to study the dynamics involved in the fusion of 12C, a cluster structured projectile, with 181Ta target, at energies within 4.3-6.0 MeV/nucleon, by measuring the production cross sections of the radionuclides formed through complete/incomplete fusion (CF/ICF) and fission processes. In addition, the mass distribution of fission fragments, which is an important fission observable, is probed for a better understanding of dynamics. Method: Tantalum foil, backed by the aluminum catcher, arranged in a stack, was bombarded by 12C ions of 52-73 MeV energy. The off-beam gamma -ray spectrometry method was adopted to measure the activity of the residues produced in each Ta target after the end of bombardment (EOB), and cross sections were calculated. To analyze the experimental cross sections, PACE4 and EMPIRE3.2.2 reaction model codes were employed. Further, the mass distribution of fission fragments was explored to obtain mass variance information. Results: In general, measured excitation functions of the residues produced through the evaporation of particles in 12C + 181Ta reaction showed good agreement with the predictions of PACE4. ICF of 12C in 181Ta target with strength up to 7.7% has been observed and compared with the strength deduced from previously reported data for the same system. However, no signature of precompound processes has been observed in this study. A total of 12 fission fragments within 71 A 135 mass range have been identified. Further, mass variance is rising with increasing mass asymmetry, which hints at the importance of entrance-channel parameters' role in deciding the spread of the mass distribution. Conclusions: Good agreement of PACE4 with the excitation functions of xn channel evaporation residues confirms their production through the CF mechanism. Enhancement of cross sections in the alpha xn channel is attributed to the ICF process. Further, evidence of the production of fission fragments through a compound nuclear mechanism is indicated as a broad and symmetric mass distribution, observed through the Gaussian fitting. The trend of mass variance with rising excitation energy and mass asymmetry is found to be growing.