Dynamics of nuclear reactions in superheavy mass region

Abstract

The aim of present work is to carry out an extensive theoretical investigation of the decay properties, and patterns of a variety of superheavy nuclear systems formed in heavy ion reactions. This investigation has been performed within the framework of Preformed cluster model (PCM) and Dynamical cluster decay model (DCM). The PCM is applied to understand the ground state decays such as -emission and heavy particle radioactivity (HPR), whereas DCM is applied to account for the decay of hot (E6=0) and rotating (`6=0) nuclei formed in low energy heavy ion reactions. The deformation and orientation effects of nuclei are explicitly included in these formalisms. The thesis is organized into seven chapters and a brief outline of the work is given below. Chapter 1 gives the general introduction related to present work, which includes the broad outline of experimental and theoretical developments related to the dynamics of hot and rotating compound nuclei in the superheavy mass region. A detailed discussion is made about the formation and decay processes of superheavy nuclei. The cold and hot fusion processes are described, which follows by various decay channels such as neutron evaporation, fusion-fission and quasi-fission. In addition to this, the ground state decays such as -decay and heavy particle radioactivity (HPR) are also explained for overall understanding of nuclear dynamics in superheavy region. Besides this, the role of excitation energy, deformations and orientations are discussed in view of reaction dynamics governed 1 2 via heavy ion collisions. Chapter 2 gives the details of the Preformed cluster model (PCM) and Dynamical cluster decay model (DCM) which are used to address the ground state and excited state decay patterns of nuclei. DCM is formulated from PCM (applied for ground state decays) by employing the temperature effects in its various interaction terms. Both models are based on the Quantum Mechanical Fragmentation Theory (QMFT). In this approach, the Schr¨odinger equation is solved in the mass asymmetry [ =(A1-A2)/(A1+A2)] co-ordinates to account for the probability of formation of decaying fragments at compound nucleus stage. Then the penetration probability of these preformed fragments is estimated using WKB approximation. The fragmentation potential which goes as input to solve the Schr¨odinger equation, is calculated as sum of binding energies, Coulomb interaction potential, proximity potential and angular momentum dependent potential. It may be noted that all the above terms in DCM are temperature dependent, whereas temperature and angular momentum effects are silent in the PCM. Besides this, the deformation and orientation effects are incorporated well within both formalisms. In DCM, the emission of light particles (LPs), intermediate mass fragments (IMFs) and fission fragments upto symmetric division of the compound nucleus, are treated on equal footings as the dynamical collective mass motions of preformed clusters or fragments through the barrier, in contrast to statistical models which follow different approaches for different processes. The ground state dynamics with PCM is also governed via collective clusterization process. In Chapter 3, the decay of 266Rf and 296116 superheavy nuclear systems formed in 18O+248Cm and 48Ca+248Cm reactions are studied using the Dynamical cluster decay model (DCM) to investigate the neutron proton magicity in superheavy mass region. The 3 calculations are carried out by using Z=114, 120, 126 and N=184 shell closures and the neutron evaporation data is addressed. The lower magnitude of the fragmentation potential and consequently higher values of preformation probability for Z=126 and N=184 magic pair, seem to suggest that Z=126 and N=184 are the best suited magic candidates to address the neutron evaporation residues. Using these magic shells, further in this chapter the evaporation residues of 297117 nucleus formed in 48Ca+249Bk are studied in the DCM framework. The 2n, 3n and 4n cross-sections of 297117 are explored using the quadrupole ( 2i) deformations, as the spherical fragmentation approach could not address the data nicely. Even after inclusion of deformation effects, the 4n channel cross-sections are underestimated. Finally 4He-decay contribution is added to 4n-channel cross-sections which fits the data nicely. In addition to this, the fusion-fission component in the decay of 297117 compound nucleus is also predicted. In Chapter 4, the decay of another odd mass superheavy nucleus 291115 formed in 48Ca+243Am reaction is studied over a wide range of compound nucleus energies using the Dynamical cluster decay model (DCM). The 2n, 3n and 4n decay of 291115 superheavy nucleus is investigated and it is observed that the neutron cross-sections find nice agreement with an experimental data with the inclusion of quadrupole ( 2i) deformations and optimum orientations ( opt i ) of decaying fragments. The comparative analysis of spherical, 2i-static and dynamic fragmentations is also carried out. Furthermore, the 2n, 3n and 4n cross-sections are predicted at the bass barrier. In addition to the neutron evaporation, the -decay and heavy particle radioactivity (HPR) are also discussed in this chapter using the Preformed cluster model (PCM). Both mechanisms are investigated within the hot optimum orientation approach. The - decay chains of 289115, 288115 and 287115 superheavy nuclei are studied by modifying the 4 penetration probability of the decaying alpha particle. On the other hand, the role of different proximity potentials such as Prox-77 and Prox-00 is studied to understand the heavy particle radioactivity of 278113, 287−289115 and 293,294117 superheavy nuclei. In Chapter 5, the role of orientation degree of freedom is investigated in the dynamics of 268Sg nucleus formed in 30Si induced reaction over a wide range center of mass energies across the Coulomb barrier. Hot equatorial configuration is taken into account to address the fission cross-sections at above barrier energies. Nice fitting of data is obtained within this approach with the emergence of symmetric mass fragmentation. On contrary to this, at below/sub barrier energies asymmetric fragmentation is observed for the cold polar configuration. The asymmetric mass distribution leads to demonstrate the fact that quasi-fission may compete fusion-fission at sub-barrier energies. Also the inbuilt property of barrier lowering effect of DCM is estimated for both oriented configurations, and its magnitude is higher for the use of cold polar configuration as compared to the hot compact. In addition to the explicit role of orientations, at the highest center of mass energy, the comparative analysis of spherical, 2i-static and dynamic deformations is carried out to understand the effect of deformations on the decay path of the 268Sg nucleus. In Chapter 6, the Dynamical cluster decay model (DCM) is applied to understand the effect of higher multipole deformations in reference to data on 40,48Ca+238U reactions at energies lying across the Coulomb barrier. The angle at which compact system is achieved, gets modified slightly with the inclusion hexadecapole deformation for 278,286112 nuclei. Also the effects of 4i deformations along with 2i and spherical considerations are investigated on the decay path of 278,286112 compound nuclei. In addition to this, isotopic analysis of 278,286112 nuclei is carried out by comparing the preformation probability of 5 the fission fragments at Ec.m.=180 and 230 MeV, using 2i-deformations, for hot equatorial and cold elongated configurations of nuclei. Suppression in the preformation factor is observed while going from hot equatorial to cold polar configuration, and is observed more for 278112 compound nucleus as compared to 286112 . The role of isospin is also observed for Z=114 isotopes by comparing the fragmentation and preformation profile of the 288,290,292,294114 nuclei at T 1.25 MeV. Different isotopes of Z=115 are also studied within the DCM framework by comparing the magnitudes of preformation probability, penetration probability and the neutron evaporation cross-sections of 289,291,293115 compound nuclei. Finally, in Chapter 7, conclusions and an outlook of the work is presented