Facets of heavy-ion reactions around the coulomb barrier

Abstract

Over the last many decades, heavy-ion (HI) induced reactions have captivated nuclear physicists across the globe, and prodigious theoretical and experimental e orts have been made to understand the underlying dynamics. Semi-classically, on the basis of impact parameter, the HI induced reactions are categorized into four di erent regions, viz., i) the Coulomb region: HIs interact at large impact parameter, and can be scattered elastically following Rutherford scattering or inelastically with Coulomb excitation, ii) the peripheral region: HIs brush past each other in a grazing collision resulting in direct reaction involving inelastic scattering or a few nucleon transfer or quasi-elastic scattering (QEL), iii) the deep inelastic region: HIs strongly interact with each other in close collisions resulting in massive transfer or incomplete fusion (ICF) reactions, and iv) the fusion region: an approximately head-on collision can cause the coalescence of HIs leading to the formation of an excited compound nucleus in complete fusion reaction (CF). The excited compound nucleus decays via emission of light nuclear particle(s) and/or characteristic rays leading to the evaporation residues. Fission is also considered as one of the decay modes for excited compound nucleus, where it principally splits into two fragments having more or less equal masses. The nal reaction products may also be populated via the emission of light nuclear particle(s) and characteristic -rays from the ssion fragments. In this thesis, a series of experiments have been performed at the Inter University Accelerator Centre (IUAC), New Delhi to explore di erent aspects of heavyion reactions around the Coulomb barrier and to understand the underlying dynamics. In the rst set of experiments, to study heavy-ion induced fusion- ssion reaction in pre-actinide region and low incident energies, the production crosssections of ssion-like events have been measured in 12C + 169Tm system at Elab 6.4, 6.9, and 7.4 MeV/A employing the activation technique followed by o ine

-spectroscopy. In the second set, to understand the localization of input angular momentum window, and to explore the possibility of populating high-spin states via ICF, the gamma-transition intensity patterns of di erent reaction residues were measured in 12C + 169Tm system in an energy range Elab 5-7.5 MeV/A using particle- -coincidence technique. Finally, in the third set of experiments, quasi-elastic (QEL) scattering have been measured in 7Li + 116;118Sn systems at Elab 2.14-4.14 MeV/A (30% below to well above the barrier) using a stateof- the-art detection system HYbrid Telescope ARray (HYTAR) in General Purpose Scattering Chamber (GPSC) beamline. The self-supporting 116;118Sn targets were fabricated using ultra-high vacuum (UHV) facility available in the Target laboratory at IUAC. Various advanced characterization techniques were used to determine their thickness and establish their purity and stability in HI beam irradiation. The barrier distributions have been extracted from the measured QEL excitation functions to have an insight on the e ect of breakup channels on fusion. The experimental QEL excitation functions and the corresponding barrier distributions are analyzed in the framework of theoretical model code CCFULL-SC.