A study of competing decay channels of nuclear systems formed in low energy heavy-ion reactions

Abstract

The present study is carried out to understand the theoretical aspects and signatures of various decay processes observed in reactions involving heavy, intermediate and light mass nuclei. The decay of hot (T6=0) and rotating (`6=0) nuclear systems formed in low energy heavy ion reactions is studied in form of evaporation residues (ERs; also called light particles LPs), intermediate mass fragments (IMFs) and symmetric, asymmetric fission fragments, by applying the dynamical cluster-decay model (DCM), which treats all these decay processes on equal footing. The DCM, applied to study the aforementioned decay processes for various nuclei formed via complete fusion (CF) and incomplete fusion (ICF) processes focusses primarily on the role of deformations, related optimum orientations, temperature and angular momentum effects. Besides this an attempt is also made to address the non-compound nucleus (nCN) mechanisms such as quasi-fission (QF) and deep inelastic collision (DIC) in framework of DCM. The thesis is organized into seven chapters and the outline of wok carried is given below. Chapter 1 starts with general introduction of nuclear physics, its applications, importance, relevance to mankind and converges to the understanding of nuclear reaction dynamics and related nuclear structure effects involved in low energy heavy ion collisions. Further, an extensive study of compound nucleus (CN) formed through CF, the ICF process and the nCN mechanism is carried out explaining the factors influencing the Chapter 2 gives the detail of methodology used, the DCM for the decay of hot and rotating nuclei. It is based on quantum mechanical fragmentation theory (QMFT) and the temperature, angular momentum, deformations and orientations effects are incorporated within DCM description. It is a two step process where the stationary state Schrodinger equation having fragmentation potential as an input, is solved to obtain the preformation probability of decaying fragments while, the WKB approximation is used to establish the penetrability of decaying fragments. The fragmentation potential is calculated as sum of binding energies, Coulomb interaction potential, proximity potential and angular momentum dependent potential. In DCM, the emission of LPs, 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 The application of DCM is extended further in view of ICF, QF and DIC etc. In Chapter 3, the decay of CN in form of ERs and fission fragments is studied using DCM, for the odd-mass 211−219Fr compound systems formed in 18O+197Au and 19F+192,194,196,198,200Pt reactions. The ER cross-sections of 215Fr are predicted using the systematics attained through ER cross-sections of 213,217Fr isotopes. In addition to this, the fission and ER cross sections of 213,217Fr isotopes are extended to higher energies. Further, to check for the consistency of earlier observations of 215Fr and absence of QF contribution, the fission fragment anisotropy is calculated for 213,217Fr isotopes. Additionally, the entrance channel effect for 215Fr formed through 18O+197Au and 19F+196Pt reaction is analyzed through the variation of fragmentation potential, preformation factor and decay barrier height. Also, the shell closure effects of decaying fragments are explored in context of decay pattern observed for odd mass Fr isotopes, ACN=211-219. Besides this, the contribution of QF is worked out for isotopes of superheavy 278112 and 286112 nuclei, through the orientation degree of freedom studied using DCM. For the use of cold (polar) elongated orientation, an overestimation of fission cross-section in the deep sub-barrier region is observed which may be associated with the QF decay channel. In agreement with experimental observations, the contribution of quasi-fission is more for the neutron-deficient 278112 isotope as compared to 286112 nucleus. In Chapter 4, the role of static and dynamic (temperature dependent) quadrupole deformations is studied in framework of DCM for the decay of intermediate mass 158Tb nucleus formed in 6Li+152Sm reaction. The ER decay cross-sections are calculated using spherical choice of fragmentation, and by considering static 2i(0) and dynamic 2i(T) quadrupole deformations within optimum orientations opt i approach. The barrier modification and angular momentum dependence is duly addressed for 158Tb nucleus. Also, the shell closure effect and the iso-spin dependence of decay fragments is studied for 150Tb nucleus formed in 6Li+144Sm reaction, and comparative analysis is carried out with 158Tb nucleus. Furthermore, the orientation effect is investigated by considering hot (equatorial) compact as well as cold (polar) elongated orientational features. Finally, the ER cross-sections corresponding to ICF observed due to break up of loosely bound 6Li projectile induced on the deformed target 152Sm is worked out in the framework of DCM by applying relevant energy correction as discussed in chapter 2.In Chapter 5, the role of higher order deformation effects, upto hexadecapole ( 2- 4) are studied for the decay of heavy mass 201Bi nucleus formed in 20Ne+181Ta reaction. The decay of 201Bi system in form of ER decay channel is studied for spherical choice and with inclusion of quadrupole ( 2) and hexadecapole (i.e 2+ 3+ 4) deformations. Further, the decay cross-sections observed through ICF of 20Ne projectile are also studied using DCM and results obtained are found to be consistent with Morgestern systematics. In addition to this, for the ER decay of light mass 96Tc system formed in 6Li induced reaction, the effect of angular momentum in reference to the sticking (IS) and non-sticking (INS) limit of moment of inertia is analyzed. The effect of either of the two approaches on the angular momentum and hence the rotational energy associated with it, is assessed through the fragment mass distribution, preformation factor and the barrier penetrability and it is observed that IS approach is more favorable to address fusion excitation functions. Besides this, the role of angular momentum in disentangling the CF and ICF contribution observed due to break-up of loosely bound 6Li projectile is exercised explicitly. It is observed that for both 20Ne and 6Li induced reactions, the cross-sections calculated using DCM are in agreement with experimental observations for complete fusion and incomplete fusion processes. In Chapter 6, a systematic decay study of light mass 66As nucleus formed through proton-halo 8B induced reaction is carried out in framework of DCM in reference to the ER, IMF and fission decay fragments. The calculations suggest that the fusion excitation function of 66As consists of LPs as the most dominant contributors, followed by IMFs and fission fragments. A description of deformation effects is also provided by comparative analysis of spherical and deformed choice of fragmentation by opting deformations upto quadrupole ( 2i) and hexadecapole ( 2i− 4i). The DCM based excitation functions agree well with the observed experimental cross sections for all three choices of fragmentation. Also the cross-sections are predicted using DCM at higher energies, which need experimental verification in near future. Further, decay of very light mass 40Ca and 39K nuclei formed in asymmetric channels 12C+28Si, 11B+28Si and 12C+27Al are investigated using spherical choice of fragmentation in framework of DCM. In reference to the experimentally measured charge particle cross sections, the fragment masses and their relative contribution towards the decay of 40Ca and 39K nuclei is identified. Also, the role of entrance channel is investigated by studying the decay of 39K nuclear system formed in two different reactions at same excitation energy. The behavior of fragmentation potential, preformation probability and penetrability is analyzed to figure out the favorable mass fragments, their relative emergence and the entrance channel effects etc. In addition to this, the cross sections for the LPs and heavier charge fragments are estimated for the CN decay. Besides this, one of the nCN process, DIC is also addressed in context to DCM approach. The cross sections obtained in framework of DCM for both CN and nCN processes are found to have nice agreement with the available experimental data. Finally, in chapter 7, conclusions and an outlook of the work is presented.