PROTON AND ALPHA INDUCED REACTIONS ON LIGHT NUCLEI FOR NUCLEAR STRUCTURE AND ASTROPHYSICS

Abstract

Understanding the nuclear interaction is one of the fundamental quests of nuclear physics. Researchers had long been shooting various probes on the nucleus and detecting the outgoing particles to gain insight into its structure. Studying light nuclei using low-energy protons and alpha particles as probes has long been challenging for researchers. This is due to the complex nature of the interaction of the projectile with the nucleons in the target nucleus. The nuclear structure effects become dominant, and the overall system can be regarded as a many-body quantum system. In this work, we have performed inelastic scattering experiments using proton and alpha particles on light nuclei, namely, 10B, 12C and 16O. The γ-rays produced from the low-lying states have been detected using large volume NaI:Tl and LaBr3:Ce scintillation detectors. The γ-ray cross sections were calculated from the relevant photo peaks observed in the γ-ray spectra. This cross-section data is important for determining elemental abundance usingγ- ray astronomy and material isotopic composition techniques such as Particle Induced Gamma Emission (PIGE). For the sake of theoretical analysis, the underlying physical process can be divided into two steps. First is the excitation of the target nucleus from the incoming projectile. Second is the de-excitation of the target nucleus via emission of γ-rays. The Optical Model Potential (OMP) framework was used for the excitation of the target nucleus. Nuclear reaction codes like TALYS, which can perform optical model calculations, are available in the literature. It has been shown in this work that TALYS, in the default configuration and with different modes available within, is unable to reproduce the experimental elastic scattering cross-sections. A rigorous optimization of OMP parameters is required to reproduce the elastic scattering crosssections. The optimisation has been achieved using ROOT and BASH-based scripts. Several resonances in the optical potential were included, low-lying channels were coupled, and target deformations were incorporated in the calculations to reproduce the resonances observed in the γ-ray cross sections. Precise measurements of cross-sections require excellent detection systems. Lanthanum halides, especially LaBr3:Ce, are among the most widely used inorganic scintillators for γ-ray measurements due to their excellent energy resolution and detection efficiency. Researchers are continuously trying to improve upon the detection properties of these scintillators by altering their composition. LaBr2.85Cl0.15:Ce is the result of such attempts for improvement. We have thoroughly characterised a 1′′×1′′ LaBr2.85Cl0.15:Ce crystal for γ-rays up to 4.4 MeV. Various crystal properties were studied: linearity of the response, energy and timing resolutions, internal activity, and photo-peak efficiencies. Realistic simulations were carried out for the detector’s response for γ-rays using the Monte Carlo simulation toolkit GEANT4. The LaBr2.85Cl0.15:Ce crystal has also been characterised for fast and thermal neutrons using an Am-Be neutron source. Similar measurements were carried out using the widely used NaI:Tl and LaBr3:Ce crystals for comparison. The response to fast neutrons was also simulated using GEANT4 and compared with the measured spectrum. The fast neutron detection efficiency of the three detectors has been obtained from the measured spectra. Continuing the characterisation of detectors, we have measured of the intrinsic energy resolution of mono-energetic single Compton electrons in CeBr3 crystals using Compton Coincidence Technique (CCT). This technique can be used to study the non-proportionality and intrinsic energy resolution of the scintillation crystals to gain insight into the scintillation mechanism. It shows the contribution of the scattering of electrons inside the scintillator to the overall energy resolution of the detector. On the instrumentation side, a liquidless cooled target setup has been designed and developed. Target cooling becomes important for nuclear astrophysics experiments where a large amount of current is incident upon the target for low cross-section measurements. This leads to the heating of the target which can cause target degradation. Cooling continuously removes excess heat from the target making is less prone to degradation.