STUDY OF FUSION AND FISSION DYNAMICS OF NEUTRON-DEFICIENT SUB-LEAD NUCLEI

Abstract

The advancement in the accelerator facilities and the upgraded experimental techniques unlocked several doors to explore various nuclear processes, which emerged in the heavy-ion (HI)-induced reactions. The HI-induced reactions of two massive nuclei serve as a route to processes like- complete fusion (CF), incomplete fusion (ICF), pre-equilibrium (PEQ), fusion-fission, etc. Depending on its mass and fissility, the compound nucleus (CN) formed in the CF process can undergo deexcitation through two possible routes: fusion-evaporation and fusion-fission. The CN residing in the sub-lead region has registered traces of competition between evaporation and fission during deexcitation. Moreover, unexpected multimodal fissions were observed in the nuclei residing in this region. The possibility of getting several exciting results for the nuclei in the sub-lead region developed a curiosity towards its inspection. In general, the scarcity of the experimental data declared the sub-lead region worth exploring in the context of understanding the fusion-fission dynamics at energies around and above the Coulomb barrier. Following this, we have focused on understanding different reaction processes, e.g., CF, ICF, PEQ, fusion-fission, as well as the fragment properties derived through the fission fragment mass distributions, known as essential fission observable. Consequently, the co-existence of symmetric and asymmetric fission modes has been registered. The deformed shells have also been spotted to influence the asymmetric fission modes. Chapter 1 sheds light on the general information regarding nuclear physics. Different kinds of reactions classified based on impact parameter have been elaborated. Majorly, the discussion focuses on the reactions induced by HIs and the associated phenomena, like – CF, ICF, PEQ, fusion-fission, etc., along with a brief description of the fission in the preactinide region. Chapter 2 describes several theoretical models that have been opted in the present work to check the reliability of the experimental measurements. Specifically, the compound nuclear models, the Weisskopf-Ewing (WE) and Hauser-Feshbach (HF), with various nuclear level density models, are illustrated. Moreover, the pre-equilibrium models, like the Exciton model (EM) and Hybrid model (HM), are explained briefly. This chapter discusses the one-dimensional barrier penetration model (1D-BPM) and the coupled channel model. Moreover, the fission reaction models, specifically the rotating liquid drop model (RLDM) and semi-empirical GEF model, also have been covered. Chapter 3 demonstrates the utilized accelerator facilities and detectors, mainly the high-purity Germanium (HPGe) detector and multi-wire proportional counter (MWPC), along with their visual representations. It also discusses the adopted experimental techniques and efficient data acquisition systems. The data analysis process adopted to extract the fusion cross sections, fission fragment mass, and total kinetic energy (TKE) distributions has been described. Chapter 4 presents the excitation functions for the residues populated by evaporating particles like neutrons, protons, and alphas for 12C+181Ta and 12C+197Au reactions at energies within 3.7–6.1 MeV/nucleon. Comparison of the measured excitation functions with various nuclear reaction model codes like PACE4 and EMPIRE3.2.2 revealed PACE4 predictions showing better agreement in both the reactions. The model-dependent calculations gave evidence of the presence of ICF in both systems. Moreover, for both reactions, the residues having masses around half of the mass of CN have been obtained and identified as the fission fragments. Consequently, these fission fragments’ mass distributions are explored to understand the fusion-fission dynamics better. Chapter 5 compares the last few years’ mass distribution data for 19 neutrondeficient sub-lead nuclei and corresponding GEF (General description of fission obvii servables) model predictions. The 3-Gaussian fitting of all the distributions has been carried out in order to understand the reason behind the observed asymmetry. The quadrupole deformed shell gaps for Z = 34, 42, 44, and 46 were found to influence the fate of CN’s disintegration. The compatibility of the GEF outcomes with the experimental results has also been explored. We have discussed the fission dynamics of 186Pt and 192Hg compound nucleus, produced in 28Si + 158Gd and 32S + 160Gd reaction, covering the excitation energies within the range of 47–68 MeV in Chapter 6. The flat-topped mass distributions obtained at all the beam energies provided evidence of residing multiple fission modes. For both reactions, the disintegration of the neutron-deficient CN into the asymmetric fragments has been understood on the basis of deformed shell gaps. The experimental mass distributions and fragment properties are also compared with the GEF predictions in order to test this model code’s reliability in the sub-lead region. Chapter 7 includes the thesis summary covering all the work discussed above. Moreover, the future outlook of this work has been described.