Study of 28Mg and 22Ne using fusion-evaporation and Doppler shift techniques

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(Thesis) Ph.D.
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Investigations of nuclear structure via high precision measurements of energies and lifetimes of excited nuclear states has been the focus of an ongoing experimental program carried out at TRIUMF, Canada’s particle accelerator centre, using the TIGRESS Integrated Plunger (TIP) infrastructure. As part of this work, methods have been developed to determine energies and lifetimes of excited states in nuclei populated using the fusion-evaporation reaction mechanism and measured using the Doppler-shift attenuation method (DSAM). These methods include a comparison of experimental data to Monte-Carlo simulations in order to determine lifetimes of observed states. Methods were validated using data collected for the benchmark nucleus 22Ne at the ISAC-II facility at TRIUMF. Additionally, an array of 128 CsI(Tl) charged particle detectors has been constructed at Simon Fraser University. This new ‘CsI ball’ array replaces a 24-detector array used in previous TIP experiments and offers improved charged particle detection efficiency due to its increased solid angle coverage (nearly 4π in the lab frame). A subset of the CsI ball array was used to study excited states of 28Mg in an experiment at ISAC-II/TRIUMF, with the goal of investigating the evolution of nuclear shells and searching for evidence of the lowering in energy of pf negative parity orbitals predicted in this region. For the first time 28Mg was investigated using a fusion-evaporation reaction, leading to preferential population of states at high spin and excitation energy where the influence of the pf negative parity orbitals is expected. Analysis methods developed for 22Ne were applied to the 28Mg DSAM data to extract lifetimes of observed states. 3 new excited states of 28Mg were identified. Multiple candidates for negative parity states were also observed, providing an explicit indication of pf negative parity orbital population. Underlying principles and theories of nuclear structure, a technical overview of the CsI ball array and other TIP infrastructure, along with results from the 22Ne, CsI ball commissioning, and 28Mg experiments are presented. A comparison of the data to the predictions of various theoretical models is shown and implications for the structure of 28Mg are discussed.
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Thesis advisor: Starosta, Krzysztof
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