School of Physics - Theses
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ItemEstimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment( 2022)Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.
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ItemDirect observation of charged particle tracks in crystals( 1995)From a general point of view in material science, it is interesting to know, how swiftly moving, heavy ions (such as those ejected from spontaneously occuring nuclear fission events inside natural minerals) interact with solid bulk matter. For example, in geological mineral deposits (such as zircon, apatite, and sphene) one often finds that naturally radioactive isotopes (uranium, thorium etc.) are included in varying degrees of abundance. In particular, single 238 U atoms have a small but finite probability to disintegrate through a spontaneous nuclear fission event, thereby ejecting two highly charged and comparably heavy nuclear fragments with about 250Me V kinetic energy. This fact became known only about 30 years ago when straight-line, needle-like contrast features were observed in electron micrographs of mica (S&B 59, P&W 62). These so called "fission tracks" originate through the damage incurred on the host material containing naturally radioactive isotopes by the fast moving, charged and heavy fission-fragments penetrating the matrix in two oppositely directed straight lines. The length of the tracks is related to the penetration range of the ions, which in turn is determined through the initially available kinetic energy, the atomic number, and charge-state of the projectile as well as the mean density of the target material. (Because the energy level difference between the initial and final state of the nuclear decay event is constant on average, a fixed amount of kinetic energy is available for the two fragments. This fact requires the fission tracks to have a more or less constant longitudinal dimension of around 50µm.) Their thickness, however, could not be determined accurately up to very recently because the image quality remained poor due to the technical limitations in electron-optical engineering in the first few generations of electronrnicroscopes which were commercially marketed after their pioneering development by Ruska some sixty years ago (Rus 49). With the improved performance of the recent generation of high-voltage microscopes a renewed attempt was made to determine a) the electron optical conditions of making fission tracks (or at least short sections thereof) visible in their latent form through transmission electron microscopy, employing a variety of contrast mechanisms, b) the geometrical parameters of the track morphology in the particular instance of 208Pb20+ ions penetrating slabs of zircon crystals at an initial kinetic energy of 14MeV/u, c) the structural conditions as well as the nature and mechanism of the damage formation. Lastly, it was also hoped to be able to image the damaged regions showing atomic-size detail in High Resolution Electron Microscopy (HRTEM), thus making the native structure of the damaging effects of nuclear radiation penetrating bulk matter directly visible for the first time.
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ItemBlack holes and higher dimensions( 2007)Higher dimensional black holes are studied in the extra large dimensions scenario. Bulk fermion quasi-normal modes and bulk fermion Hawking emission is calculated. It is found that bulk emission dominates brane emission for d > 5. To address the Planck phase an effective field theory is investigated. Lepton family number violating processes are elucidated and the corrections to the muon magnetic moment from these channels are calculated. Bounds are placed on the couplings of the theory. A discrete symmetry between quarks and leptons, and left- right- chirality fields, is orbifolded in 5 dimensions. Using split fermions a one generational standard model extension is found. An investigation of entanglement in black holes and accelerated motion is presented. It is found that in a certain system the acceleration between two spinors enhances the rate of their disentanglement.