3. Electronic Theses and Dissertations (ETDs) - All submissions

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Browsing 3. Electronic Theses and Dissertations (ETDs) - All submissions by Faculty "Faculty of Science"

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    Neutron radiation damage of plastic scintillator UPS 923A and a data driven method in the Z h + E'Iiiss search at the LHC
    (2018) Mthembu, Skhathisomusa Dumani
    Plastic scintillators, exposed to radiative environments from high energy charged particles traversing through them in the LHC at CERN, undergo radiation damage. This damage is characterized in terms of the transmittance, luminescence and light yield optical properties of the plastic are tested for changes resulting from exposure of these plastic scintillator samples to fast neutrons of varying fluences. The neutron irradiation was carried out at the IBR-2 reactor of the Frank Laboratory for Nuclear Problems at the Joint Institute for Nuclear Research, at fluences of 1.8-1014 , 1.7-1013 and 3.8-1012 neutrons/cm2 , and compared to a control sample. Results show a small change in the transmittance of the higher energy visible spectrum, and a noticeable change in the light yield of the samples as a result of the damage. There is no change observed on the luminescence as a result of radiation damage at studied fluences, and higher energy fast neutron irradiatioin is needed for more conclusive results. Another contribution to this research is a data-driven method employing an effective field theory approach, with the addition of two real scalars, H and X, in troduced and added to SM particles to study the associated Higgs boson production with top quarks at the Large Hadron Collider, at a centre of mass energy of Js = 8 TeV. A benchmark scenario, where the parameters of the model are constrained for mH = 275 GeV and mx = 60 GeV, is considered. From this, a comparative study was performed for the production of the Higgs boson, in association with a single top and top-pairs in the standard model, regarding the processes with h being replaced by the heavy scalar H, further decaying through the channel H ➔ h XX . This study is performed for a single lepton channel first, and then for same-sign leptons, electrons or muons in dilepton events. Different observables, such as b- and c-tagged jet multiplicities, total missing energy, and total scalar sum of transverse momenta of all jets and leptons, are presented. Considering the introduced scalars H and X, an analysis for new physics searches Ill in the Z h channel with missing energy final states at the LHC. The analysis in volves the consideration of a CP-odd, gluon-fusion-produced scalar A decaying into a heavy CP-evcn neutral scalar H with Z boson as A ➔ ZH. H further decays into the CP-even SM Higgs boson h in association with dark matter candidate x - a source of missing energy. The signal final state products are described by Z ➔ z+z- (where z± = e±, µ±) and h ➔ bb decay modes is presented. The masses of the par ticles presented are considered as mh = 125 GeV, mx = 60 GcV, 2mh < mH and mA > mH. Background, from quantum chromodynamic events, is suppressed by imposing selections on the observables. The decreased event numbers for single top, diboson, top pair, W + jets and Z + jets are then compared for same-flavour and for different-flavour decay schemes. This comparison is plotted as a ratio between the two decay schemes.
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    Petrography, geochemistry and mica mineral chemistry of granites and granophyres from the mutue fides-stavoren tin field, the Lebowa granite suite
    (2021) Ray, Michael Eden
    The Mutue Fides-Stavoren Tin Field in the Palaeoproterozoic Bushveld Complex, lies approximately 30 km northwest of Marble Hall in Mpumalanga. The lithologies comprise granites from the upper portions of the Lebowa Granite Suite, granophyres from the Rashoop Granophyre Suite and quartzites from the Pretoria Group of the Transvaal Supergroup. The granites are characterised by localised zones of endo- and exogranitic cassiterite mineralisation. Although these tin deposits have been mined, mainly artisanally since the early 1900s, there is limited data on the petrology and geochemistry of the granite suite of the area and no mineral chemistry. This study provides petrographic and geochemical data from samples of outcrop, half-core, and mine-dump material of the granites and granophyres and presents mica and chlorite mineral composition data for the first time from this area. Geochemically the granites can be distinguished from other granite facies of the Lebowa Granite Suite and a new name - the Gaasterland Granite - has been proposed for this granite facies. This distinction is most clearly shown by their LILE compositions (5-40ppm Sr, 350-770ppm Ba and 220-410ppm Rb) in contrast to the Nebo Granite (30-140ppm Sr, 500-2500ppm Ba and 125-250ppm Rb). The petrographic and geochemical evidence suggests that the granophyres, which are host to significant cassiterite mineralisation, formed as an early intrusive phase. This contrasts with previous interpretations that the granophyres formed either as a result of metamorphism of Transvaal sedimentary rocks during the intrusion of the underlying Rustenburg Layered Suite or as a late phase intrusion between the Lebowa Granite Suite and Transvaal sedimentary succession. A sequence of late-stage hydrothermal alteration processes was identified in the granites and granophyres. Early pervasive microclinisation of perthitic III feldspars was followed by minor albitisation in which a ‘snowball texture’ was developed. Subsequent greisenisation of the granite resulted in an assemblage of intensely sericitised alkali-feldspar, white mica, quartz and interstitial purple fluorite. Greisen halos around veins in the granophyre consist of dark green to honey brown mica, quartz, white and purple fluorite, topaz, chalcopyrite, pyrite and hematite. Cassiterite crystallised during the greisenisation and late silicification stages of alteration with a final stage of chloritisation. Mineral compositional data revealed there is a clear distinction between magmatic and hydrothermal mica compositions within the Lebowa Granite Suite. During differentiation magmatic mica evolved from an Fe-Ti-rich annite upwards through the granite sheet to hydrothermal phengite micas that are depleted in Fe and enriched in Al. A late stage hydrothermal annite-siderophyllite within the Stavoren Granophyre is highly depleted in TiO2 and enriched in Cl and FeO (38.0 - 41.0 wt%). Such evolved mica compositions within the roof portion of the Lebowa Granite Suite is indicative of hydrothermal alteration and tin mineralisation. Such mineralised sectors are limited in the Lebowa Granite Suite because the sheeted architecture of the intrusions limited the potential for concentration of hydrothermal fluids in apical portions thus restricting extensive rock-fluid interaction and consequent ore deposition.
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