Precision Elastic Scattering and Reaction Cross-section Measurements of Radioactive Li-8 with Ni-58, Pb-208 and Bi-209
Creators: name: Sengupta, Chandrima
Publisher: name: The Australian National University
Year: 2020

description: Experimental and technological developments in the last few decades have made it possible to explore the evolution of nuclides from stability to the drip lines, where the nuclei are no longer bound and decay by spontaneous nucleon emission. With this, nuclear physics research has been driven towards understanding the structure, properties and reaction mechanisms of these exotic nuclei and new properties related to nuclear structure and reactions have been discovered. Particularly interesting structure phenomena have been observed to arise in light weakly bound nuclei such as Li-8. Li-8 consists of a weakly bound neutron and a cluster structured core of Li-7, and this structure has a strong influence on nuclear reaction mechanisms, especially at bombarding energies near the barrier. This thesis work is focussed on Li-8, to understand the effect of its complex structure on nuclear reactions at near and above barrier energies. Elastic scattering provides an important benchmark for nuclear reaction models and allows extraction of the reaction cross-section. This thesis thus focusses on elastic scattering of Li-8 on medium and heavy mass targets. The weak intensity of radioactive ion beams (RIBs) makes precision measurements challenging. To achieve a better understanding, an efficient RIB capability has been developed at the Australian National University. This produces RIBs by in-flight transfer, and the desired RIB is separated and brought to a focus on the secondary target using a solenoidal separator. A high efficiency double-sided silicon detector array surrounds the secondary target and is used to measure the scattered particles. A unique aspect of this RIB capability is the two position sensitive tracking detectors that can handle MHz rate and are used to track the trajectories of RIB on its way to the secondary target. As the solenoid has finite acceptance angles, the RIB exiting the solenoid also has finite angle of incidence and spot size on the secondary target. This has entailed the development of software routines to reconstruct the trajectories of RIB on an event-by-event basis, which is essential for precise determination of the scattering angle. We then measured elastic scattering of Li-8 on Ni-58, Pb-208 and Bi-209 at energies near and above the barrier. With the aid of the reconstruction routines, a reliable elastic scattering angular distribution can be obtained for each energy and target. Normalisation of this elastic scattering angular distribution to Rutherford scattering is then performed with a Monte Carlo simulation, which incorporates the beam spot size on an event-by-event basis. Once the elastic scattering angular distributions have been obtained, detailed optical model calculations are performed to extract the interaction potential and obtain reaction cross-sections. The systematics of elastic scattering angular distributions and reaction cross-sections enables a comparison with neighbouring lithium isotopes and aids the development of nuclear reaction models for light weakly bound nuclei.