A Theoretical Study of Heavy Ion Collisions and Related Dynamical Aspects

Abstract

In this thesis, a comprehensive study is carried out to explore the formation and decay mechanism of various nuclear systems formed in heavy ion induced reactions. The Wong and `-summed Wong formula are applied to account for the formation process, whereas dynamical cluster decay model (DCM) employed to address the decay paths of hot and rotating compound nuclei. To investigate both the pro cesses, two sets of nuclear interaction potentials are employed i.e. one based on the proximity based pocket formula and other obtained from the Skyrme Energy Den sity Formalism (SEDF). It is relevant to mention here that, the angular momentum, temperature, deformations and orientation effects of the reacting partners and de caying products are aptly contained respectively in Wong and DCM model. The thesis is organized into seven chapters, a transient discussion of which is discussed below. Chapter 1 presents the brief account of heavy ion induced nuclear reactions governing the dynamics at low energy region. A literature survey on the formation and decay of compound nucleus is discussed in detail. To understand the mechanisms involved in fusion-fission processes and to extract related nuclear structure effects, a deep understanding of nucleus-nucleus interaction potentials is crucial, a brief interpretation of which is discussed in this chapter. Along with this, the deformation and orientation effects in view of dynamics governed via heavy ion reactions is discussed in brief. Chapter 2 describe the details of the methodology used to probe the formation and decay mechanism of heavy ion induced compound nucleus reactions. Wong for mula and its extended version i.e. `-summed Wong formula are used to study the formation phenomena. The dynamical cluster-decay model (DCM) which originate from the well known Quantum Mechanical Fragmentation Theory (QMFT) is ap plied to investigate the decay mechanism of excited compound nucleus, in the form 1 of evaporation residue, intermediate mass fragments, heavy mass fragments and fis sion fragments etc. DCM is a non-statistical model and provide an advantage to treat all compound nucleus decay processes on parallel footing. Within DCM, the fragmentation potential is obtained as a sum of Coulomb, nuclear proximity (based on Blocki et al. potential and Skyrme energy density formalism) and centrifugal potentials, a brief detail of all outlined in this chapter. In Chapter 3, Entrance channel effect is studied in the dynamics of 200Pb∗ formed in 16O+184W, 19F+181Ta and 30Si+170Er reactions over a wide range of exci tation energies using the dynamical cluster-decay model (DCM) and Wong model. The effect of deformations upto β2 along with optimum orientation is investigated in both the formalisms. Fusion cross-section is studied using Wong model, which overestimates experimental data for 19F+181Ta and 30Si+170Er reactions and under estimates the data at few energies for 16O+184W. However with the use of extended `-summed Wong model, the overestimation is taken care and the cross-sections are fitted nicely. The Wong based calculations suggest that there might be some non compound nucleus contribution at few energies for 16O+184W channel, as the under estimation of cross-section persists even after the inclusion of deformation effects. This lead us to conclude that the formation of 200Pb∗ compound nucleus depends on the choice of incoming channel. In addition to this, in this chapter, the decay path of 200Pb∗ is investigated using DCM. Although, the overall decay pattern of compound nucleus 200Pb∗ seems similar for all the chosen reactions, some signature of variation are observed in fission and intermediate mass fragment (IMF) region for the deformed fragmentation process. It is to be noted that with the inclusion of deformation, the decay pattern changes from symmetric to asymmetric, there by suggesting that the deformation and orientation of decaying fragment are equally important in formation as well as in the decay process of proton magic nuclear sys tem 200Pb∗. Prediction of ER and fission cross-sections at higher as well as at lower incident energies are also worked out. Beside this, the dynamics of neighboring 2 nuclei 192Pb∗ and 202Pb∗ is also analyzed. In Chapter 4, the reaction dynamics of various even-mass Zr isotopes is ex plored formed in 16O-induced reactions is explored using DCM based on collective clusterization approach. In reference to the measured fusion cross-section data, var ious decay modes contributing towards 86,88,90,92Zr∗ nuclei are investigated. Also, the role of deformations and orientation degree of freedom is analyzed by compar ing results with spherical choice of fragmentation. In addition to this, the entrance channel impact is explored for 92Zr∗ and 76Kr∗ nuclei formed in 16O and 18O induced reactions. Besides this the dynamics of relatively heavier Sn isotopes formed using using 16O and 18O beams is analysed. The DCM calculated decay cross-sections find a nice agrement with available experimental data. In Chapter 5 the dynamical cluster decay model (DCM) is employed to explore the relative effect of sticking (IS) and non-sticking (INS) limits of moment of iner tia (MOI) in the decay of hot and rotating 214,216Rn∗ compound nuclei, formed in 16,18O+198Pt reactions. Beside this, the nuclear deformation effects i.e. quadrupole β2 (static and dynamic) and higher order static deformation up to hexadecapole (β4) are duly incorporated within the framework of DCM. The influence of both IS and INS addressing rotational energy component is gauged through the barrier characteristics, preformation factor and barrier lowering effects via proper inclusion of deformation effects of decaying fragments. The experimentally given ER and fusion-fission (ff) data is addressed by optimizing the neck-length ∆R, that strongly depends on the limiting angular momentum, which in turn depends on the sticking or non-sticking limits of interaction. In addition to this, the influence of increase in energy and neutron number is probed in reference to ER survival probability of Rn compound nucleus. Finally, the ff cross-sections of 214,216Rn∗ nuclei are predicted within sticking limit of moment of inertia as the same seems to be more suitable heavy particle emission. In all previous chapters, the nuclear dynamics and related deformations effects 3 are investigated within Blocki based nuclear proximity potential. Such kind of an alytical form of nuclear interaction potential does not contain individual contri bution from spin-orbit density dependent part, though it affects the fusion-fission dynamics appreciably. Henceforth in Chapter 6, role of deformation and orien tation in nuclear dynamics is investigated in terms of spin-orbit density dependent part VJ of nuclear potential (VN=VP+VJ) obtained within semi-classical Thomas Fermi approach of Skyrme energy density formalism. Calculations are performed for 24−54Si+30Si reactions, with spherical target 30Si and projectiles 24−54Si having prolate and oblate shapes. The quadrupole deformation β2 varies within range of 0.023 ≤ β2≤ 0.531 for prolate and -0.242 ≤ β2≤ -0.592 for oblate projectiles. Si nuclei with β2 <0 have higher spin-orbit barrier (compact spin-orbit configuration) in comparison to systems with β2 >0. The possible role of spin-orbit potential on barrier characteristics such as barrier height, barrier curvature and on the fu sion pocket etc is probed. In reference to prolate and oblate systems, the angu lar dependence of spin-orbit potential is further studied on fusion cross-sections. Along with this, the effect of spin-orbit interaction potential is explored by consid ering a two nucleon transfer process by employing various entrance channels such as 23Na+49V,25Mg+47Ti,27Al+45Sc,29Si+43Ca and 31P+41K, all forming the same com pound system 72Se∗, using both spherical as well as β2-deformed choice of nuclei. For spherical nuclei, the spin-orbit density part VJ of nucleon-nucleon potential re main unaffected with the transfer of two nucleon from target to projectile, however show notable variation in magnitude after inclusion of deformation effects. Like wise, deformations play important role in spin-orbit density independent part VP, as the fusion pocket start appearing, which otherwise diminish for the spherical nu clei. Further, the effect of increase in N/Z ratio of Se is explored on VJ as well as VP and results are compared with transfer channels. In addition to this, the role of double spin-orbit parameter (W0 and W0 0) is explored in view of SkI2, SkI3 and SkI4 Skyrme forces. Also, the decay path of 72Se∗ nucleus formed in 27Al+45Sc reaction is 4 investigated within framework of dynamical cluster decay model (DCM), where the nuclear proximity potential is obtained by both Skyrme energy density formalism (SEDF) and proximity pocket formula. The fusion hindrance in 27Al+45Sc reaction is also addressed via barrier lowering parameter ∆VB. Finally, the contribution of spin-orbit density dependent interaction potential is estimated for 27Al+45Sc reac tion using single (W0) and double spin-orbit parameter (W0 and W0 0). Chapter 7, summarizes the work carried out in this thesis. A brief account of results obtained and conclusion drawn is discussed and possible extension of this work from future prospective is outlined.