Theoretical Analysis of Multicluster Emission of Heavy Nuclei and Pertinent Nuclear Effects

Abstract

The nuclei in the actinide region exhibit fission properties, where the atomic nucleus splits into two or more distinct fragments. This multifaceted fission behavior is of immense interest, as the diverse inputs associated with such dynamical processes are of vital importance in the field of nuclear physics. Actinide nuclei may decay through several competing modes, with the binary fission providing the dominant contribution in the exit channel. Evidently, nuclear fission and related processes play a significant role in various industrial and medical applications. Therefore, it is of extreme interest to acquire an appropriate knowledge of the decay dynamics of heavy nuclei. In view of this, the binary, ternary and quaternary decay processes are investigated within the framework of the Quantum Mechanical Fragmentation Theory (QMFT). The decay dynamics are analyzed using the Preformed Cluster Model (PCM) for binary fragmentation and the Three-Cluster Model (TCM) for three-fragment emission, enabling a consistent description of barrier penetrabilities and relative yield. Furthermore, the model is extended to explore four-body fragmentation as well. In addition, the role of nuclear deformation and orientation effects is studied in the decay dynamics of binary and ternary fission processes. In view of above, the present thesis is divided into seven chapters, which are briefly described as: Chapter 1 begins with the understanding related to the evolution of nuclear decay processes, with a particular emphasis on ground-state decay modes such as α-decay, CR, SF followed by heavy ion induced fission dynamics. Their physical significance and relevance in understanding the nuclear stability and reaction dynamics is highlighted. The chapter provides an overview of the various decay channels involved in binary decay. The discussion is further extended to ternary and quaternary fission dynamics and the related theoretical description is briefed. In Chapter 2, the formalism of the quantum mechanical fragmentation theory (QMFT) is presented. Within this framework, the preformed cluster model (PCM) and the three cluster model (TCM) are formulated. The formalism is further extended to quaternary fission, wherein the parent nucleus undergoes simultaneous emission of four fragments. Furthermore, the potential energy surfaces, mass distributions and relative yield are discussed to establish a comprehensive theoretical basis for the subsequent fission processes. In addition to PCM and TCM, the dynamical cluster decay model (DCM) has been employed to investigate the dynamics of heavy-ion induced reactions. In Chapter 3, the section I includes an analysis of binary decay (α, CF, HPR, and SF) for the nuclei ranging Z=89-102. Firstly, the α-decay logT1/2 are calculated for two sets of chosen nuclei by optimizing the neck parameter (∆R). The optimized ∆R is used to calculate Q-value dependent first turning point i.e. Ra(Q). The Ra(Q) relations are employed to calculate α decay half-lives of another set of nuclei within a chosen range and the calculated results found to be in good agreement with the experimental data. After the alpha decay analysis, the Ra(Q) relations are obtained for CR and HPR decay modes. Further, SF is studied using fragmentation potential and preformation distribution for extreme choice of nuclei (230Th and 256No) in the range Z=89-102. Whereas, section II presents a unified theoretical analysis of α, 2α and cluster emission from Ac and Th isotopes in the mass ranges 207−229Ac and 210−230Th. The study aims to understand the isotopic evolution of decay modes, particularly the transition from dominant α decay to the emergence of simultaneous 2α and the heavier cluster emissions. I have worked out the Q-values, barrier penetrabilities, and cluster preformation probabilities to systematically assess the viability of various decay channels. Through this approach, the aim is to understand the structural/dynamical conditions that prefer the simultaneous 2α emission, and identify the specific isotopes where such processes become increasingly probable. Chapter 4, extends to investigate binary and ternary fission modes of the 252Cf, incorporating deformation and orientation effects. The β2 deformations, with corresponding optimized orientations, are incorporated in this analysis. For the compact configurations, the decay pattern exhibits a clear predominance of symmetric and near-symmetric fragmentation, which is primarily governed by the stabilizing influence of spherical magic shell closures. In contrast, the elongated orientations reveal a distinctly asymmetric yield distribution, wherein the dominance of deformed magic shell closures becomes evident, leading to enhanced probabilities for mass-asymmetric decay modes. Further, the study examines the influence of equatorial cluster tripartition (ECT) and collinear cluster tripartition (CCT) configurations in the ternary decay process. The corresponding fragmentation potential and barrier penetrabilities are calculated and the corresponding relative yield compared with the experimental data. This provides a deeper insight into the role of deformation in binary and ternary fission dynamics. In Chapter 5, a comprehensive investigation of binary, ternary, and quaternary fission of the 252Cf nucleus is carried out in terms of fragmentation potential and the barrier characteristics. Various decay channels (4He, 46Ar, 82Ge and 116Pd) associated with the binary fission are systematically explored. Subsequently, the investigations extend to the feasibility of ternary fission of the same nucleus by keeping 4He, 46Ar, and 82Ge as fixed third fragment. Further, the quaternary fission is studied with particular emphasis on scenarios involving two fixed fragments, while investigating different possible collinear configurations. In addition to the geometrical arrangement, the energetically favourable fragment combinations are identified, providing valuable insights into the competing decay modes operating in 252Cf nucleus. To understand the influence of the different combinations of the middle fragments on the fragmentation potential, the various combinations of light fragments such as 1H, 2H, 3H, 4He, 6Li, and 8Be are considered. The study further explores heavier clusters such as 10Be and 14C, reinforcing the significance of shell effects and energy minimization principles in the dynamics of multicluster fission. Finally, the analyses of ternary and quaternary fission are performed with a particular emphasis on surface separation effects and on the different fragment arrangements in various geometrical configurations. For ternary fission, calculations show that the most probable fragment combinations 132Sn (Z = 50, N = 82) + 4He (Z = 2, N = 2) + 116Pd(Z = 46, N = 70) are independent of surface separation, although there is significant difference in the magnitude of fragmentation potential, particularly at higher separation distance. In addition, the quaternary fission analysis is performed for both equatorial and collinear configurations, considering various light charged particles as middle fragments. In Chapter 6, the spontaneous binary and ternary fission of 252Cf is studied. Firstly, the binary fragmentation potential of 252Cf is calculated by employing the two types of nuclear potentials. One with the proximity and the other with the Skyrme energy density formalism (SEDF) based potential. The different potentials significantly modify the structure and magnitude of the fragmentation potential. Notably, calculations using the SEDF approach (SKT3 force) demonstrate a reasonable agreement with experimental half-lives, validating the choice of nuclear potential. Furthermore, the SKT3 force is employed to investigate the ternary fission process in the decay of the 252Cf nucleus. The three-body fragmentation potential is calculated by considering spherical and deformed choices of the decaying fragments. The relative yield for the ternary fission of the 252Cf nucleus is calculated for the deformed case and compared with the available data. In addition to the ground state decay process, the decay dynamics of the compound nuclear system, i.e. 228U∗, formed via 31Al+197Au reaction is analyzed within the framework of dynamical cluster decay model (DCM). The most probable fission fragments of 228U∗ CN are studied over a wide range of center-of-mass energies, Ec.m.. The comparative analysis has also been worked out for another choice of projectile-target combination (i.e. 19F + 209Bi), forming the same CN (228U∗). The calculations are carried out by adopting SEDF based nuclear potential. At the end, Chapter 7 gives a brief note on the work done in reference to the objectives of the thesis, along with the future extension possibilities that arise from the established results.