Decay of Hot and Rotating Nuclei Formed in Heavy Ion Reactions at Low Energies

Abstract

The aim of the present work is to carry out an extensive theoretical investigation of the decay behavior of a variety of nuclear systems formed in heavy ion reactions. This investigation has been performed within the framework of dynamical cluster decay model (DCM), which is a non-statistical model used to account for the decay of hot (E6=0) and rotating (`6=0) nuclei formed in low energy heavy ion reactions. The deformation, orientation, temperature and angular momentum effects of decaying fragments are explicitly included in this model. The thesis is organized into eight 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 theoretical developments related to the dynamics of hot and rotating compound nuclei formed in heavy ion reactions. A brief discussion is made in reference to competing non-compound nucleus contribution. Beside this, the role of angular momentum, anisotropies, deformations and orientations etc. is discussed in brief. Chapter 2 gives the details of the methodology used, the dynamical cluster decay model (DCM) for the decay of hot and rotating nuclei. DCM is formulated from preformed cluster decay model (PCM) (for ground state decays) by considering the temperature effects in its various interaction terms. It is based on quantum mechanical fragmentation 1 2 theory (QMFT). In this formalism, the Schrodinger equation is solved which includes fragmentation potential as an input. The fragmentation potential 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 are temperature dependent. Besides this, the deformations and orientations are incorporated well within DCM. Here 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 formalisms for different processes. In Chapter 3, the decay of actinide nuclear system 254Fm formed in 11B+243Am reaction is studied using the dynamical cluster decay model (DCM), with choices of spherical, quadrupole deformation 2 alone and higher multipole deformations 2- 4. For 2 deformations, the optimum orientations opt i are used whereas for higher multipole deformations the compact orientations c i of decaying fragments are taken into account. Besides 2-static, the effects of dynamical- 2 deformations is also explored. The calculated crosssections find excellent agreement with the available experimental data with spherical as well as deformed choices of fragmentations, enabling us to account for the role of important nuclear deformation effects in the 11B induced nuclear reaction. Spontaneous decay of 254Fm with cold elongated configuration and optimum orientation is also worked out. The mass distributions of excited fermium isotopes in the neighborhood of 254Fm is also explored. In addition, the role of temperature, angular momentum and fission fragment anisotropies is investigated in the context of chosen reaction. 3 In Chapter 4, the decay of 220Ra nucleus formed in two different entrance channels 12C+208Pb and 13C+207Pb is investigated over a wide range of incident energies using the dynamical cluster decay model (DCM). The excitation functions are calculated by considering quadrupole ( 2) deformations with optimum orientations ( opt i ) of decaying fragments. The DCM based cross sections for evaporation residue (ER), fission, xn and neutron decay processes find nice agreement with the reported experimental data over wide range of incident energies. The cross sections corresponding to different decay mechanism are worked out within DCM by fitting neck length parameter (4R). The entrance channel and angular momentum effects are investigated in reference to the above mentioned reaction channels. In addition to this, the fragment mass distribution is worked out by colliding 13C weakly bound stable projectile with a variety of target nuclei resulting in 13C+159Tb, 13C+181Ta and 13C+207Pb reactions. At comparable projectile energies, the increase in target mass is shown to favor asymmetric fragmentation in the fissioning region. Beside this, the incomplete fusion (ICF) contribution is worked out for 12C and 13C channels by applying necessary energy corrections in the framework of DCM. In Chapter 5, the role of deformations, orientations, angular momentum dependence along with possible presence of non compound nucleus (nCN) component is investigated in the decay of pre-actinide nucleus 204Po formed in 16O and 28Si induced reactions over a wide range of projectile energies having comparable Ec.m./Vc values for two channels. An experiment was performed to extract fusion-fission and evaporation residue cross-sections for 16O + 188O!204Po and 28Si +176Yb!204Po reactions over a wide range of energies (Elab = 84-155 MeV). Within the Dynamical cluster decay model (DCM), the evaporation residue and the fission cross-sections are calculated in reference to the available data at various incident energies by simultaneously fitting the only parameter, neck-length ( R) 4 for evaporation residue and fission. The choice of different neck-length parameter( R) values for ER and fission indicate that the two decay processes do not occur simultaneously i.e they occur in different time scales and evolve subject to the nature of dynamics of compound nucleus formed. The calculated evaporation residue cross-sections and fission cross-sections show excellent agreement with the reported data at all incident center of mass energies, except at one highest energy for the channel 28Si +176Yb!204Po for fission. The disagreement between DCM calculations and reported data at highest incident center of mass energy for the 28Si +176Yb entrance channel may be associated with the presence of small amount of nCN effects which is in line with the predictions of the Preequilibrium model. Also the inbuilt property of barrier lowering effect of DCM seems to be operating in context of these reactions. The fission fragment anisotropies are also calculated using DCM based parameters for the non sticking moment of inertia, and it find reasonable comparison with experimental data. Finally, the isotopic effect is worked out by studying the decay of 202Po and 204Po nuclei formed in 16O induced reactions at comparable Ec.m./Vc value. In Chapter 6, the dynamical cluster decay model (DCM) is applied in reference to data on 78,82Kr+40Ca reactions at bombarding energy of 5.5 MeV/nucleon. For the nuclear systems 118,122Ba , the experimental data for complete charge spectrum is also available along with evaporation residue (ER) and fission cross-sections. Within the DCM approach, the total fission and evaporation residue cross-sections are fitted nicely for spherical choice of nuclei by simultaneously fitting of the neck length parameter. Effect of different level density parameter is also studied. Results of DCM calculations are compared with BUSCO, GEMINI and DNS based calculations. All the models use the maximum angular momentum `max as the fitting parameter, which in this work is fixed 5 via neck-length parameter(4R) for the penetrability P!1. Also the role of non zero pairing strength ( >0) is seen, using (T) in VLDM as a fitting parameter, say to Li data. The effect of different proximity potentials is also studied. Finally some non-compound nucleus contribution is shown to be operating in context of reactions under study. The N/Z dependence of decay fragments is also studied for Ba isotopes with A = 114-126. In Chapter 7, the decay of 112Xe compound system formed in massive heavy ion reaction 58Ni+54Fe is studied using DCM at both below and above barrier energies with the deformations and orientation degrees of freedom of the nuclei included in it. DCM calculations give nice description of the experimental fusion excitation function, ER as a function of center of mass energy, within one parameter fitting, the neck length parameter ( R) whose value remains within the range of proximity interaction. The barrier height corresponding to the neck length parameter gives barrier modification in a straight forward way, which helps to address the fusion- evaporation cross sections particularly in below barrier region. The role of deformations, orientations, angular momentum and diffuseness parameter is investigated to look for the structure effect of decaying fragments. Finally the N/Z dependence of fragmentation structure of different compound systems formed via 58Ni beam is explored. Finally, in chapter 8, conclusions and an outlook of the work is presented.