Binary and Ternary Decay Dynamics of Nuclear Systems Within Fragmentation Approach

Abstract

The nuclei belonging to the heavy mass region are highly radioactive and may undergo different binary decay modes such as α decay, cluster radioactivity (CR), heavy particle radioactivity (HPR) and the spontaneous fission (SF). A radioactive nucleus may opt any of these decay modes and the disintegration process depends on different factors such as the shell effect, shape, size, deformation etc. In addition of these decay modes there exist a probability of three fragment emission, and the process is termed as ternary decay. The primary objectives of this work is to make a comparative analysis of different binary decay modes (α-decay, CR, HPR and SF), identification of the binary fission fragments and their comparison with the ternary fission fragments. In the present work, the binary decay analysis is carried out using preformed cluster model (PCM) and three cluster model (TCM) is employed to study ternary decay mechanism. The comparison of the binary decay modes is made using the fragmentation potential, preformation probability (P0), penetration probability (P) and the assault frequency (ν). Similarly, the ternary fission analysis is carried out for the light and heavy third fragment accompanied fission modes. In view of the above, the present work is divided into eight chapters, and their brief description is given below: In Chapter 1, historical background of the radioactivity and its importance in the modern time is described. The general radioactive decay modes such as α-decay,β-decay and gamma radiation emission are explained. In addition to this, other observed ground state decay modes such as the cluster radioactivity (CR), heavy particle radioactivity (HPR) and spontaneous fission (SF) are illustrated. After the exploration of the binary decay mechanisms, types of ternary decay mechanism and corresponding experimental and theoretical background is outlined. At the end of the chapter, the motivation of the present work and organisation of thesis is outlined. In Chapter 2, the methodology adopted for the binary and ternary decay analysis is briefly discussed. First of all, the quantum mechanical fragmentation theory (QMFT) and related terms have been explained. After this, the QMFT based preformed cluster model (PCM) and three cluster model (TCM ) are explained. In PCM, the fragmentation potential is ii iii calculated, which in turn used for the identification of the most probable fission fragments. This potential is further used to calculate the preformation probability (P0) of the decaying fragments. In addition to this, the penetration probability (P) of the probable decay channel is calculated as three step process using W.K.B. approximation. The classical and quantum mechanical view of the assault frequency (ν) is explained. Using the preformation probability (P0), penetration probability (P) and the assault frequency (ν), the decay constant and decay half-lives are calculated. In TCM, two kind of geometrical arrangement (i.e. ECT and CCT) of three fragments are illustrated. The three body fragmentation potential is described where third fragment of the outgoing channel is kept fixed. The barrier penetration probability is calculated with respect to the surface separation of three fragments. The calculated penetrability further is uesed further to calculate the relative yield of the fragment combinations. In Chapter 3, a comprehensive study of the alpha particle emergence from 188−218Po isotopes is carried out within the framework of the preformed cluster model (PCM). The barrier characteristics are studied using two choices of radii ( with surface diffuseness (Ci) and without surface diffuseness(Ri)). The α decay is found to be most prominent decay mode in the chosen set of isotopes. The preformation and penetrative probability of the decay fragments is studied with respect to increase in the neutron number of the parent nucleus. The alpha decay half lives of Polonium are calculated using classical assault frequency (νc) and quantum mechanical assault frequency (νq) and a comparison is made with the experimental data. Further, the alpha decay half-lives of the 198−220 Rn isotopes are calculated using effective assault frequency (νe) parameter and a comparison is made with the available experimental data. In Chapter 4, the binary decay analysis of the 253Es radioactive nucleus is carried out using PCM. A comparative study of all probable radioactive decay modes such as α-decay, CR, HPR and SF etc. is carried outin terms of the fragmentation potential, preformation probability (P0) and penetration probability (P). The fragmentation structure is explored using two kind of nuclear potentials , i.e., Yukawa plus exponential and proximity potential. The structure of the fragmentation potential and the location of potential minima are found to be independent of the choice of nuclear potential. The decaying fragments and their complementary fragments are found to lie near the shell closure. The decay half-lives (T1/2) are calculated for all decay modes by the optimization of the neck length parameter iv ΔR. The calculated half-lives of α decay and spontaneous fission find nice agreement with the experimental data. Also, the half-lives are predicted for cluster and heavy particle radioactivity, for which experimental verification would be of further interest. In Chapter 5, the role of deformation and orientation in the spontaneous fission (SF) is explored using PCM. The SF analysis of even mass 242−260Fm isotopes is carried out and the mass distributions is explored. The deformation effects are included upto the quadrupole (β2 ) deformed nuclei with optimum orientations (θopt. i ) leading to hot-compact (side-to-side) and cold-elongated (tip-to-tip) configurations. The spherical and hot-compact deformed configurations of decay fragments result in the symmetric fragment mass distributions for Fm isotopes; however, the symmetric peak gets sharper with an increase in the neutron (N) number of the parent nucleus. In the case of cold orientations, a transition from two-peaked (asymmetric fission) to three-peaked (multimodal fission) mass distribution is observed with an increase in the mass number of Fm isotopes. The SF half-lives (TSF 1/2) are calculated using the neck-length parameter (ΔR) for 242−260Fm isotopes and compared with the experimental data. Besides this, the induced fission of Fm isotopes is also studied within the dynamical cluster-decay model. The energy dependence of fission fragment mass distributions and the isotopic dependence is analyzed at energy range E∗ = 5–42 MeV. In addition, the role of temperature-dependent deformations is also explored in the preview of the fission dynamics. In Chapter 6, Light charge particle accompanied fission (ternary fission ) mode of the 253Es nucleus is explored using three cluster model (TCM). The calculations are performed using two kinds of nuclear potentials i.e. Yukawa plus exponential and proximity potential. The fragmentation structure is used for the identification of most probable fission fragment. The decay probability of the probable fission fragments seems relatively higher in case of the proximity potential. A comparison of the barrier characteristics for both nuclear potentials is made. In addition to this, comparison of the binary and ternary fission fragments is also carried out. The mass distribution for both kind of decay modes (binary and ternary ) is studied and it is observed that the contribution of relative yield is higher in case of binary fission as compared to ternary fission. Further, the role of shell effects is analyzed in binary as well as ternary decay channel. In Chapter 7, the ternary fission analysis of two Fm isotopes nuclei having atomic mass AP = 242 and 258 is carried out using TCM. First, the choice of third fragment (A3) is fixed by minimizing the probable A3 fragments having different proton neutron configurations. v Further, the fission fragment combinations (A1 + A2 + A3) are identified for the fixed third fragments by selecting the channel of lower ternary fragmentation potential and higher relative fission yield. Two type of tripartition of radioactive nuclei is considered, equatorial cluster tripartition (ECT) and collinear cluster tripartition (CCT). A comparative analysis of ternary fragmentation potential and relative fission yield within ECT and CCT geometrical arrangement is carried out for different choices of third fragment, i.e., A3 = 1 to AP /3. The choice of most probable fragments suggest that the proton and neutron magic shell closures play essential role in the ternary mass division. Finally, a relative analysis of binary and ternary fragmentation is worked out for better insight of the dynamics involved. In the last chapter of the thesis Summary, the results related to the binary and ternary decay mechanisms are briefly summarized and the future scope of the present work is briefly outlined.