Electronic Theses and Dissertations (PhDs)

Permanent URI for this collectionhttps://hdl.handle.net/10539/38021

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Surface Brillouin scattering studies of high-temperature elasticity
(University of the Witwatersrand, Johannesburg, 1999-03) Stoddart, Paul Randall; Comins, J. Darrel
A novel technique has been developed for studying the elastic proper ties of opaque solids at high temperatures. The method is based on surface Brillouin scattering (SBS) and has the advantages of being contact-free and non-destructive. The elastic constants can be extracted from SBS measurements of the directional dependence of the surface wave velocities. An optical furnace was designed to provide the special scattering geometry required for these measurements. The technique has been evaluated on silicon and a single-crystal nickel-based superalloy, with measurements up to 800°C and 200°C respectively. Above these temperatures, measurements were precluded by a marked deterioration in the surface quality. The elastic constants for silicon compare favourably with the established ultrasonic values, particularly in terms of the changes as a function of temperature. Additional measurement were performed on silicon at temperatures up to 900°C in order to examine the well-known central mode feature. These results shed light on a major outstanding problem in SBS, because they reveal the presence of a second quasielastic mode that may be associated with scattering from diffusive excitations. Further measurements at high and low temperatures are proposed to confirm the mechanism. Silicon was also used as a test system to clarify certain aspects of the theory and practice of SBS that have not been properly dealt with before, such as the effects of surface anisotropy and of the extended collection aperture. This indicates that SBS provides effective elastic constants for the outer 300 nm of the sample surface and thus may be influenced by surface damage and surface contamination. In the case of the superalloy, the difficulties encountered in gathering data at higher temperatures suggests that modifications to the furnace arrangement are required. The larger relative error in the velocities also created problems in the extraction of the elastic constants. This difficulty was satisfactorily overcome by using the longitudinal threshold in the Lamb shoulder to fix the value of c₁₁. Although the work described here has been limited to temperatures below 900°C, it is clear that SBS provides a powerful method for probing the elastic properties of opaque solids at elevated temperatures.
An unsupervised search of non-thermal diffuse emission in extended sources
(University of the Witwatersrand, Johannesburg, 2022-01) Thwala, Siphiwe Anthony; Beck, Geoff
Low-surface brightness diffuse radio emission is a useful probe for studying the connection between non-thermal processes in radio sources and their environment, as well as gaining insights on galaxy formation. The presence of this emission in different astrophysical and cosmological signals is not well understood in the literature due to challenges in detection caused by the low-surface brightness of diffuse objects and their origins, particularly in relation to galaxy feedback and formation. We explored the utility of publicly available data to detect, characterise, and study diffuse emission in extended radio sources at the scales of radio galaxies and galaxy clusters. We performed multiple analyses on extended radio continuum sources found in large publicly available radio surveys. The analyses range from in-depth multi-frequency examinations of individual sources to novel approaches for grouping and classifying radio sources with unsupervised machine learning. The analysis of individual sources brought together a range of datasets and techniques to provide insight about their nature. To group and classify continuum radio sources at scale, a novel unsupervised machine learning approach was designed to combine self-organising maps with convolutional neural networks for automatically detecting and clustering similar sources in radio surveys. To the best of our knowledge, this is the first implementation of an architecture that allows for the training of a machine learning model using multi-frequency and multi-scale radio continuum data cubes, as input, to automatically detect and cluster similar sources in radio surveys. This comes at a time when radio astronomy is undergoing a transformation and data mining methods are critical for optimum scientific utilisation of data from telescopes like MeerKAT, JVLA, MWA, and LOFAR (as well as the upcoming SKA). The different works showcase the most interesting radio sources with diffuse emission observed in these investigations.
Transfer reactions to populate the pygmy dipole resonance in 96Mo
(University of the Witwatersrand, Johannesburg, 2023) Khumalo, Thuthukile Charmane; Pellegri, L.; Wiedeking, M.
The presence of a low-lying dipole strength in neutron-rich nuclei has been established and its location in the vicinity of the neutron threshold (Sn) has implications in nucleosynthesis and specifically in neutron-capture reaction rate calculations. Additionally, a correlation of this low-lying dipole strength with neutron-skin thickness has been discussed. Since its observation, there has been a great deal of work in an attempt to understand its nature, both theoretically and experimentally. Some of the characteristics of this low-lying dipole strength include isospin mixing, which allows the use of different experimental probes to study it. In addition, compared to the IVGDR, the degree to which the low-lying dipole states are collective is under scrutiny and remains an open question of interest. This study was aimed at addressing the question of collectivity of these dipole states and one-nucleon transfer reactions were the chosen probes as they have been shown to be powerful in probing the single-particle property of nuclei. In particular the (p,d) and (d,p) reactions have been instrumental in such measurements. To allow the investigation from both neutron addition and removal, the 96Mo nucleus is particularly attractive as it can be populated via both mechanisms, with the availability of stable targets as a bonus. In addition, the (d,p) has been successfully used recently used for PDR related measurements on 120Sn and 208Pb with results alluding to a strong single-particle contribution, hence conducting the investigation on 96Mo provides access to a different mass region. 97Mo(p,d)96Mo and 95Mo(d,p)96Mo transfer reactions were performed in normal kinematics using the MAGNEX magnetic spectrometer at INFN-LNS. The 25 MeV/u proton beam and 5 MeV/u deuteron beam from the Tandem accelerator interacted with the 97Mo and 95Mo targets, respectively. The MAGNEX spectrometer was utilised to analyse the scattered particles based on their momentum prior to being detected at the focal-plane. Excitation energy spectra were obtained and angular distributions were computed for the bound states and the higher excitation energy region of interest (above Ex = 4 MeV). These were fitted, using the MDA with DWBA calculations considering different single-particle configurations from a simplistic shell model. Comparing spectra from the two reactions, same excitation energy regions were populated. The results from the MDA of the (p,d) data, show a strong single-particle component in the Ex region that was analysed, with one particular configuration that excites 1− states dominating. The QPM was used for the theoretical interpretation and below 6 MeV, the configuration ((2d5 2 )+1 N(1g9 2 )−1) that populates 2+ states dominates but in the experimental data, this configuration was found to be suppressed as the momentum matching conditions were optimized for l=1 momentum transfer. When considering the QPM predictions involving only the sp configurations of momentum transfer of l=1, 2 and 3, an agreement with the data was found. Extraction of reliable angular distributions from the (d,p) was not possible thus future (d,pγ) experiments are envisaged
Study on the influence of Nuclear Deformation on the Pygmy Dipole Resonance in Samarium isotopes
(University of the Witwatersrand, Johannesburg, 2023) Jivan, Harshna; Sideras-Haddad, Elias; Pellegri, Luna
The past decade has seen an increase in studies dedicated to understanding the low-lying electric dipole (E1) response, commonly referred to as the Pygmy Dipole Resonance (PDR). These studies revealed that the PDR has a mixed isospin nature, and that the use of complimentary probes is needed to fully understand this response. Since majority of studies on the PDR focused on spherical nuclei, the influence that deformation has on the PDR response is yet to be understood. Preliminary relativistic proton scattering studies on 154Sm performed at RCNP (Japan), showed potential evidence for a splitting in the PDR responses similar to that of the Giant Dipole Resonance with deformation. A tentative interpretation suggested that this splitting could be connected to the splitting of the resonance structure with respect to the K quantum number. Theoretical studies considering the deformed HFB+QRPA model however, suggest that this splitting is connected to the isospin mixed character of these states as observed in spherical nuclei. The isoscalar responses of the spherical 144Sm and axially deformed 154Sm isotopes were investigated for the first time using the inelastic scattering of alpha particles at 120 MeV. The comparative experiments were performed at iThemba LABS in South Africa, coupling together for the first time, the K600 magnetic spectrometer in zero-degree mode with the BaGeL (Ball of Germanium and LaBr3:Ce detectors) array. The particle-gamma coincidence measurement was used to obtain the cross section for the population of the pygmy states. In both nuclei, the region of the PDR was excited and the E1 multipolarity of the transitions was supported by the angular correlation between the α-particles and the co-incident γ-rays measured. The total exclusive cross section measured for 144Sm amounted to 24.3 ± 3.8 mb/sr while for 154Sm to 18.8 ± 2mb/sr. The experimental results were compared with the prediction of the RQTBA and the deformed HFB+QRPA theories, respectively. The theoretical cross sections were extracted within a semiclassical coupled-channel approach. The fragmentation observed in the experiment for the 144Sm was underestimated by the calculations, although good agreement with the total cross section measured was found. In the case of the deformed 154Sm however, the experimental cross section accounted for only 52% of the predicted cross section in the same excitation region. The isoscalar response extracted in this thesis was compared with the isovector strength obtain from an experiment performed at RCNP using the relativistic proton scattering at forward angles. The double hump observed in the isovector channel was not found in the case of the isoscalar strength. This implies that the difference obtained between these two experiments is related to the “isospin splitting” of the PDR rather than a splitting of thestrength connected with the K quantum number.
The application of weakly supervised learning in the search for heavy resonances at the LHC
(University of the Witwatersrand, Johannesburg, 2023-06) Choma, Nalamotse Joshua; Ruan, Xifeng; Mellado, Bruce
The discovery of the Higgs boson at the Large Hadron Collider by the ATLAS and CMS experiments has made the search for new physics beyond the Standard Model a priority in the field of High Energy Particle Physics. New resonances have yet to be discovered using inclusive and model-dependent searches, which means they may be driven by subtle topologies. Rapid improvements in Machine Learning techniques have led to their increasing application in High Energy Particle physics. Unlike supervised learning, which is known to assume full knowledge of the underlying model, semi-supervised learning, in particular weakly supervised learning, allows the extraction of new information from data with partial knowledge. The goal of this study is to set up searches for heavy resonances at the electroweak scale with topological requirements performed in both inclusive and exclusive regions of phase-space tailored to a particular production mode. These resonances could be generated with different production mechanisms. In this work, we describe search procedures based on weakly supervised learning applied to mixed samples and used to search for resonances with little or no prior knowledge of the production mechanism. This approach has the advantage that sidebands or control regions can be used to effectively model backgrounds without relying on models. The effectiveness of this method is measured by the production of the Standard Model Higgs boson, which decays into a pair of photons in both inclusive and exclusive regions of phase-space at the LHC. Having confirmed the ability of the method to extract various Standard Model Higgs boson signal processes, the search for new phenomena in high mass final states will be set up at the LHC. Subsequently, the approach is used in the search for new resonances in the Zγ final state with Z → e +e − or Z → µ +µ −, using the Monte Carlo simulated signal samples for 139 fb−1 of integrated luminosity for Run 2 collected at the LHC. The weakly supervised learning approach is implemented and compared to the performance of the fully supervised approach, which is then used to calculate the production limit for Higgs-like particles for Zγ where the significance of the signal is maximal.
Implementation of the DAQ software for the ALTI module in the ATLAS TileCal and the search for new physics in the four lepton final state
(University of the Witwatersrand, Johannesburg, 2023-06) Tlou, Humphry Sijiye; Wilkens, Henric; Ruan, Xifeng; Mellado, Bruce
The discovery of the Standard Model (SM) Higgs boson in 2012 presents new challenges and opportunities for the Large Hadron Collider (LHC) experiments. After a long period of operation, the LHC experiments needed to maintain and upgrade their detectors in order to continue and conduct research beyond the SM. As part of the upgrades, the Tile Calorimeter (TileCal) participated in Phase-I of the upgrades (December 2018 - March 2022). TileCal, the central hadronic calorimeter (|η| < 1.7) of the ATLAS experiment uses a set of Trigger and Data Acquisition (TDAQ) software to readout, transport and store physics data resulting from collisions at the LHC. In preparation for the Phase-I upgrades, the ATLAS Local Trigger Interface (ALTI) module was designed for the ATLAS experiment at CERN for TDAQ purposes. It is a 6U VME electronics board, which is a part of the Timing, Trigger and Control (TTC) system. It integrates the functionalities of four legacy modules, currently used in the experiment: Local Trigger Processor, Local Trigger Processor interface, TTC VME bus interface and the TTC emitter. The ALTI module provides the interface between the Level-1 Central Trigger Processor and the TTC optical broadcasting network to the front-end electronics of each of the ATLAS sub-detectors. This thesis discusses the development, validation and integration of the TileCal specific ALTI software in the TileCal online software by the author. A set of ALTI boards were installed in the back-end electronics of the sub-detector and fully validated for the ATLAS detector at CERN. Performance testing and maintenance of the ALTI modules and software were performed during the second half of the upgrade period, in preparation for Run 3 (2022–2025) data-taking period. The thesis also discusses the search for the presence of a new heavy resonance produced via gluon-gluon fusion and decaying to the four-lepton (4ℓ) final state, in association with missing transverse energy (EmissT), with ℓ = e, µ (where ℓ is the lepton, e is the electron and µ is the muon). The search uses 2015–2018 proton-proton collision data at √s = 13 TeV, corresponding to an integrated luminosity of 139 fb−1, collected by the ATLAS detector. The data are interpreted in terms of two models, firstly the R → SH → 4ℓ + EmissT , where R is a scalar boson, which decays to two lighter scalar bosons (S and H). The S decays to a pair of neutrinos and the H decays into 4ℓ, through ZZ bosons. The second model is the A → Z(νν)H(ZZ) → 4ℓ + X, where A is considered to be a CP-odd scalar which decays to a CP-even scalar H and the Z boson. The Z boson decays to X, which can be a pair of neutrinos or jets, and the H decays to the 4ℓ final state.
Digital toolbox for the generation and detection of vectorial structured light
(University of the Witwatersrand, Johannesburg, 2023-06) Singh, Keshaan; Dudley, Angela; Forbes, Andrew
Light has been an invaluable tool in the development of the modern world, with the myriad of applications increasing along with our degree of control over it. From the development of coherent light sources, to the shaping of amplitude and phase, this development has not ceased for the past half century. The field of structured light, borne out of the necessity and desire for control over light, has been growing steadily in recent years. In the spatial domain, the control over light’s polarization (i.e., the local planes in which the electric and magnetic fields oscillate) has been the most recent avenue of improvement, providing enhancements to a variety of applications ranging form microscopy and communication to materials processing and metrology. This class of light, commonly referred to as vectorial light, often requires specialised equipment in order for its its creation before its numerous benefits can be exploited. These tools often incur high costs and suffer from limitations relating to the diversity of vectorial light they can create, wavelength dependence and slow refresh rates. This thesis follows the development of a series of digital tools for the versatile generation and analysis of vectorial light using low-cost core technologies which can operate at high rates over a broad wavelength range. We follow the development of the generation tool in the context of its application in generating novel accelerating polarization structures, emulating vectorially apertured optics, generating probes to measure birefringence and chirality and creating synthetic spin dynamics. The development of the analysis tool is explored by investigating its application in performing automated digital Stokes polarimetry measurements, completely characterizing the internal degrees of freedom of arbitrary vectorial light and acting as a polarization and wavelength independent wavefront sensor. We then demonstrate how these tools can be used, in conjunction, to investigate the fundamental invariance of vectorial light to perturbing channels and how this invariance can be exploited in a highly robust novel communication scheme. In addition to demonstrating the applicability and versatility of these vectorial light tools, the applications offered a means to highlight areas for the optimization for the design. This culminated in the ongoing prototyping of versatile, fast, broadband devices which operate stably and have a small physical footprint.
The Large N Limit of Heavy Operator Excitations
(University of the Witwatersrand, Johannesburg, 2023-07) Carlson, Warren Anthony; De Mello Koch, Robert
Operators with bare dimension of order N are studied. These are restricted Schur polynomials labeled by Young diagrams with two long rows or two long columns and are heavy operators in the large N limit. A dramatic simplification of the action of the dilatation operator on these states is found, where the diagonalization of the dilatation operator reduces to solving three-term recursion relations. The solutions to these recursion relations reduce the spectrum of the dilatation operator to that of decoupled harmonic oscillators, showing that these systems are integrable at large N. Then, generating functions for bound states of two giant gravitons are constructed and an extension to more than two giant gravitons is sketched. These generating functions are integrals over auxiliary variables that encode the symmetrization and anti-symmetrization of the fields in the restricted Schur polynomials and give a simple construction of eigenfunctions of the dilatation operator. These generating functions are shown to be eigenfunctions of the dilatation operator in the large N limit. As a byproduct, this construction gives a natural starting point for systematic 1/N expansions of these operators. This includes the prospect to generate asymptotic representations of the symmetric group and its characters via the restricted Schur polynomials. Finally, the asymptotic expansion of the three-point function is computed in three BMN limits by varying one parameter in the large N limit. It is argued that these asymptotic expansions encode non-perturbative effects and are related by a parametric Stokes phenomenon.
Modification of boron nitride nanostructures induced by medium energy ion irradiation
(University of the Witwatersrand, Johannesburg, 2023-08) Lisema, Lehlohonolo Innocent; Madhuku, Morgan; Derry, Trevor
This research focused on using Chemical Vapour Deposition (CVD) to synthesize boron nitride nanostructures, particularly nanotubes, and selectively introducing defects into them through ion implantation. Boron ion implantations were carried out at ambient temperature at 150 keV energy and fluences 1x1014 and 5x1014 ions/cm2. The synthesized samples were analyzed using scanning electron microscopy (SEM), Raman spectroscopy, and Grazing incidence X-ray diffraction (GIXRD). Ion implantation was found to introduce defects into the surface of the samples, resulting in increased stress levels and a higher local density that favoured more crystallized nanostructures. SEM images showed clear evidence of BN nanostructures and boron nitride nanotubes (BNNTs), with the latter appearing as long, thin structures with diameters ranging from ⁓30-90nm. After ion implantation, the Raman spectra of samples implanted with ion fluence 5×1014 ions/cm2 at 1000oC, show an amorphous h-BN peak, and a narrower, intense E2g vibrational mode of h-BN is observed around 1366 cm-1 for samples synthesized at 1100oC and 1200oC. Raman analysis did not show any E2g mode of vibration of h-BN for all samples at implanted with ion fluence 1×1014 ions/cm2. The samples synthesized at 900 ºC had no active 1366 cm-1 Raman peak present. Grazing incidence X-ray diffraction (GIXRD) spectra revealed a prominent peak between 54 and 56 ° 2θ, corresponding to the (004) h-BN reflection, which was used to determine the average a and c lattice parameters 0.249 ± 0.0002 nm and 0.662 ± 0.001 nm, respectively, yielding an interplanar distance of 0.166 ± 0.0001 nm representing the stacking direction of the BN layers. The majority of the samples had broad peaks, indicative of a nanocrystalline material. The only exception was the sample grown at 1200 °C, which was found to have a Scherrer crystallite size >100 nm. In contrast, the rest of the samples had an average size of 3.5 ± 0.3nm. The average crystalline domain size values confirmed that after ion implantation, the phonon lifetime would be longer due to a large domain size, indicating that the BN nanostructures were more crystallized. The fluence of 5x1014 ions/cm2 showed to be the optimal growth condition for BNNTs. Overall, BNNTs and BN nanostructures were effectively synthesized at 900°C, 1000°C, 1100°C, and 1200°C CVD temperatures, and insights into the influence of ion implantation on the composition as well as properties of BN nanostructures are presented. The most noteworthy finding of the experiment was the substantial increase in the size of the Raman derived crystallite domains in the 1100°C and 1200°C samples following ion implantation with boron ions at a fluence of 5x1014 ions/cm2.
Development of TileCoM firmware and software for the off-detector electronics of the ATLAS Tile Calorimeter at the HL-LHC
(University of the Witwatersrand, Johannesburg, 2023-08) Gololo, Mpho Gift Doctor; Argos, Fernando Carrio; Mellado, Bruce
In 2010 the LHC started to operate as the energy frontier particle accelerator in the world, situated close to Geneva and 100 m below the French and Swiss border in a circular tunnel of 27 km. The HL-LHC which is an upgrade of the LHC is envisioned to maximize the instantaneous luminosity of L = 1 × 1034 cm−2s −1 by a factor of 5 to fully exploit the physics potential at the energy frontier. During 10 years of operation, an improved TDAQ system architecture will have the capability to accommodate the trigger rates and the amount of data generated from the HL-LHC. TileCal is the ATLAS central hadronic calorimeter, a sampling calorimeter with iron as passive medium and plastic scintillator tiles as active medium. The ATLAS TileCal Phase-II upgrades will prepare the ATLAS experiment for the HL-LHC and includes new requirements in terms of radiation levels, an increase in data bandwidth, and clock distribution. To meet the requirements of the HL-LHC, a completely new readout electronics is designed to support the data acquisition system of TileCal. As part of the new readout electronics, this thesis is focused on the design of the TileCoM and Tile GbE Switch. The Tile GbE Switch PCB is manufactured by two South African companies, Trax Interconnect and Jemstech. The PCBs are fully func tional and have been integrated with new readout electronics. Three main function alities are implemented on the TileCoM in software and firmware implementation as key elements of the TDAQ and DCS of the ATLAS TileCal at the HL-LHC. The TileCoM and Tile GbE Switch are successfully integrated with the ATLAS Phase II TileCal upgrade electronics. This is achieved by successful remote control and monitoring of the ATLAS TileCal Phase-II upgrade electronics. This thesis shows monitoring results based on voltage, current and other parameters.
Hunting dark matter with faint radio halos
(University of the Witwatersrand, Johannesburg, 2023-10) Sarkis, Michael David; Beck, Geoff
The nature of Dark Matter (DM), the elusive substance that constitutes a significant amount of the total matter in the universe, remains an unsolved problem in modern physics despite a decades-long search effort. One approach to this problem has been to search for faint emission signatures that are produced indirectly from the DM present in large astrophysical structures, and thus infer properties about theoretical DM models from observational data. In recent years, the results from studies that use this type of indirect search have produced stringent constraints on the most popular DM particle candidate parameter spaces, ruling out swathes of viable DM models. These compelling results have been enabled by the arrival of sophisticated interferometric radio telescopes, which are excellent DM hunters due to their high sensitivity and resolution. In this thesis, we focus on the use of the latest data from the MeerKAT radio interferometry telescope, through the first public release of the MeerKAT Galaxy Cluster Legacy Survey, to search for DM emissions in a set of nearby galaxy clusters. Each step of this process, from the creation of theoretical DM emission models to the statistical analysis of the observational data, has been described in detail in this thesis. With this data, we find an almost universal improvement to results found with corresponding modelling scenarios in the literature. Since this work is among the first to use MeerKAT data in astrophysical DM searches, these results present a strong argument for continued work in this field. Another central focus of this thesis is the accurate modelling of the physical processes involved in the production of the DM-induced radio emissions, as the quality of current radio data requires theoretical models that are sufficiently accurate to describe the emission at such high resolutions. One aspect of the modelling that has lacked this accuracy has been the solution to the diffusion-loss equation, which is a crucial factor in determining indirect DM emissions. A new algorithm for solving this equation, which provides higher accuracy and computational efficiency than previous public methods, has thus been developed and presented in this thesis. These aspects of DM indirect detection study will become ever more important as we approach the inauguration of the Square Kilometre Array (SKA), which will provide data with unprecedented potential with which to continue the hunt for DM.
Optimization of gallium oxide (ga2o3) nanomaterials for gas sensing applications
(University of the Witwatersrand, Johannesburg, 2024) Gatsi, Nyepudzai Charsline
Gas sensors are needed for monitoring different gases in indoor and outdoor environments, food quality assessment, and health diagnostics. Among materials studied for these applications, semiconducting metal oxides (SMOs) have generated a lot of interest due to their excellent sensitivity, simple circuit, and low cost. One-dimensional (1𝐷) 𝐺𝑎2𝑂3 nanomaterials are part of the promising candidates explored for the sensing of different gases due to their excellent electrical conductivity, high catalytic behavior, and chemical and thermal stability. This study reports the optimization of crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for use in the detection of various gases. A set of unmodified and noble metal modified 1𝐷 𝐺𝑎2𝑂3 nanomaterials were synthesized by microwave-assisted hydrothermal method followed by heat-treatment at different temperatures and their gas sensing performances were systematically studied. The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman analysis, scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), photoluminescence (PL), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) methods. The effects of heat-treatment temperatures on phase transformations and gas sensing performances of various 𝐺𝑎2𝑂3 polymorphs were investigated. The 𝛼 − 𝐺𝑎2𝑂3, 𝛽 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures were synthesized and evaluated for gas sensing. The 𝛽 − 𝐺𝑎2𝑂3 sensing layers presented selective response coupled with fast response/recovery times towards carbon monoxide (𝐶𝑂) compared to the 𝛼 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures. The observed variations in the gas sensing performances of these three crystal structures were attributed to controlled properties of different 𝐺𝑎2𝑂3 polymorphs. Furthermore, the 𝛽 − 𝐺𝑎2𝑂3 polymorph was prepared in the form of regular and hierarchical nanorod-based morphological features which demonstrated different gas sensing behaviors. The 𝛽 − 𝐺𝑎2𝑂3 regular nanorods showed better capabilities of detecting isopropanol than the nanobundle-like and nanodandelion-like features, and these differences were attributed to changes in textural, porosity, and compositional properties related to different morphologies. The effects of incorporating 𝐴𝑔 and 𝐴𝑢 noble metal nanocrystals on regular 𝛽 − 𝐺𝑎2𝑂3 nanorods surfaces on their gas sensing behaviour were also investigated. The results revealed that surface modification of 𝛽 − 𝐺𝑎2𝑂3 nanorods with 0.5 and 1.0 𝑚𝑜𝑙% 𝐴𝑔 and 𝐴𝑢 noble metals significantly lowered the sensor operating temperature compared to that of unmodified 𝛽 − 𝐺𝑎2𝑂3 nanorods towards the detection of ethylene. In addition, surface incorporation of 1.0 𝑚𝑜𝑙% 𝐴𝑔 dramatically increased the sensor sensitivity and selectivity and reduced the response/recovery times towards ethylene gas, and these positive changes were attributed to the electronic and chemical sensitization effects stimulated by the catalytic activity of 𝐴𝑔 nanocrystals incorporated on the surface of 𝛽 − 𝐺𝑎2𝑂3 nanorods. This study unambiguously optimized the crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for the detection of carbon monoxide, ethylene and isopropanol gases. These sensors may potentially be used in real-time detection of carbon monoxide and isopropanol for indoor air quality monitoring to improve human health. In additional they have also demonstrated capabilities for the precise and economical detection of ethylene around plants and fruits, which could be beneficial to the horticultural and agricultural industries
Search for new resonances in the four-lepton channel and implementation of the LED integrator panel for the PROMETEO system in the ATLAS Tile Calorimeter
(University of the Witwatersrand, Johannesburg, 2024) Mtintsilana, Onesimo; Kumar, Mukesh; Mellado, Bruce
The Large Hadron Collider (LHC) has transformed our understanding of fundamental particles and forces, notably with the seminal discovery of the Higgs boson in 2012, which completed the Standard Model (SM) of particle physics. Despite its success, the SM leaves numerous unanswered questions, motivating a quest for new physics. This thesis explores three main avenues: Firstly, it investigates the possibility of an extended Higgs sector or alternative SM extensions, focusing on heavy ZZ resonances that decay into four leptons. Using a dataset of 139 fb−1 from proton-proton collisions at the LHC, this study explores both gluon-gluon fusion and vector-boson fusion production mechanisms. Although no significant signal for a new resonance is observed, upper limits on the production cross section of spin-0 or spin-2 particles are established. These limits provide constraints on specific theoretical models, such as Type-I and Type-II two-Higgs doublet models for spin-0 resonances, and the Randall-Sundrum model for spin-2 resonances. Intriguingly, the combined results of ATLAS and CMS for Run 2 and Run 3 data in the final state of 4 leptons exhibit an excess around 250 GeV, reaching a significance of 2.4σ which is in the region of interest of the multi-lepton anomalies.. In the second part, the analysis extends to heavy boson decays resulting in a final state with four leptons, specifically focusing on the R boson or the A boson decays into a combination of the SM Higgs boson and another boson, denoted S, which further decays into dark-matter candidates. No evidence contradicting SM predictions is found, yielding stringent upper limits on the production cross-sections of these hypothesised bosons and their branching ratios at a 95% confidence level. Lastly, the thesis highlights advancements in Higgs boson studies and new particle discovery potential in the upcoming High-Luminosity LHC era starting in 2029, emphasising improvements to the ATLAS detector electronics, particularly the integration of a new LED Integrator Panel within the Prometeo portable readout module system, enabling precise calibration and monitoring of individual detector components
Efficiency Enhancement in Photovoltaic Devices Using Light Management and Morphology
(University of the Witwatersrand, Johannesburg, 2024-04) Kumalo, Sandile; Quandt, Alexander; Wamwangi, Daniel
Meeting the ever-increasing global demand for energy is society’s principal challenge for attaining economic growth and dynamic technological progress. Novel materials and technologies to extend photoabsorption and harness the solar emission spectrum are critical for producing solar-based electricity on a large scale. Current techniques and nanostructure-based approaches can revolutionise the production of solar electricity. In this work, experimental light management strategies through plasmonic nanostructures in silicon-based thin film solar cells were explored to augment power conversion efficiency (PCE). These devices incorporated plasmonic, magnetoplasmonic, and coreshell nanostructures coated with SiO2. It was demonstrated that magnetoplasmonic nanoparticles enhance interactions with both the charge of electrons and the unpaired spin with the B-field component of the electromagnetic spectrum. Furthermore, core-shell structures passivate the surface of the nanoparticles, significantly enhancing PCE. The highest PCE (10.7%) was observed for Au@SiO2 nanoparticles, attributed to a bonding plasmon mode at the interface between the nanoparticles and the surrounding bulk material. Additionally, F e3O4@SiO2 nanoparticles primarily enhanced the short-circuit current (Jsc), due to magnetic interactions with superparamagnetic nanoparticles. A detailed investigation into the Curie temperature (Tc) of various magnetic nanoparticles revealed that 4 nm F e3O4 nanoparticles possess the highest Tc of 906.1 K, indicating strong magnetic stability under operational conditions. For Ni@Fe core-shell nanoparticles, a decrease in Tc with increasing Ni content was observed, highlighting the critical role of composition in tuning magnetic properties. Morphological analysis through TEM imaging revealed uniform dispersion and spherical morphology for Au nanoparticles, crucial for consistent plasmonic properties. The addition of SiO2 shells to both Au and Ag nanoparticles significantly improved their optical absorption characteristics due to the modification of the local dielectric environment. Furthermore, a study on the bulk heterojunction of organic solar cells demonstrated that processing solvents play a pivotal role in optimising active layer performance. It was found that solvent mixtures, particularly 2-MEA and toluene in a 7:3 ratio, significantly enhance device efficiency by promoting better phase separation and charge transport, achieving a PCE of 5.77%. These findings showcase the significant potential of nanostructures and solvent processing in improving the efficiency of photovoltaic devices. The enhanced PCE and stability of devices incorporating plasmonic and magnetoplasmonic nanoparticles, along with optimised solvent processing techniques, provide valuable insights for future research and development in solar energy technologies.
Development and Reliability Testing of a new Low-Voltage Power Supply for the ATLAS Hadronic Tile-Calorimeter Phase-II Upgrade
(University of the Witwatersrand, Johannesburg, 2024-06) Mckenzie, Ryan Peter; Solans, Carlos; Mellado, Bruce
The Large Hadron Collider (LHC), located at the Conseil Européan pour la Recherche Nucléaire (CERN) also known as the European Laboratory of Particle Physics, is a tworing-superconducting-hadron accelerator and collider located on the Franco-Swiss border. The LHC was successfully commissioned in 2010 for proton–proton collisions and is expected to deliver 500 f b−1 before Long Shutdown three (LS3) that is schedule to commence in 2026. Its successor, the HL-LHC, will provide a levelled instantaneous luminosity of L = 5x 1034 cm−2 s−1 and is projected to deliver an integrated luminosity of more than 4000 fb−1 to its two general purpose detectors, known as A Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS), over a span of 10 years. The main motivation to upgrade the LHC is to fully exploit its physics potential. Through a series of machine and detector upgrades, it is possible to increase the instantaneous luminosity. This could unlock many of the physics processes that are today inaccessible to the LHC because of the lack of statistics. The primary impacts of the HL-LHC on the detector environment are a direct consequence of an increase in delivered instantaneous luminosity. The ATLAS experiment will undergo its Phase-II Upgrade during Long-Shutdown 3 to ensure peak performance during high-luminosity operations. ATLAS is composed of several specialized sub-detectors one of which is the hadronic Tile-Calorimeter (TileCal). The TileCal will undergo numerous upgrades on of which will be to the Low-Voltage (LV) power distribution system that services its on-detector electronics. The on-detector finger Low-Voltage Power supplies form the second stage of the LV system. Their primary functional device is a transformer-coupled buck converter, known as a Brick, which is responsible for converting bulk power to that required by the on-detector electronics. All legacy Bricks will be replaced with a new version that employs several design changes to enable their reliable operation within the HL-LHC detector environment. In this thesis, the development of the Phase-II Upgrade Brick is presented with an emphasis placed on its thermal performance and reliability. A thermal analysis of the proposed upgrade Brick versions is presented with design changes occurring as a result. Due to the design change incorporating a new active component an irradiation campaign is conducted to qualify it for use within the high-luminosity detector environment. A reliability analysis of the Phase-II upgrade Brick is conducted necessitated by the change of many critical components. The quality assurance procedure of the Bricks that is undertaken post-production is presented with particular attention placed on their Burn-in testing.
The Use of Semi-Supervised Machine Learning Techniques in the Search for New Bosons with the ATLAS Detector
(University of the Witwatersrand, Johannesburg, 2024-06) Lieberman, Benjamin; Mellado, Bruce
Since the completion of the Standard Model, with the discovery of the Higgs Boson, there has been a significant shift in the exploration of new physics to explain deviations between model simulated data and that produced at the Large Hadron Collider. These investigations are greatly aided by the integration of advanced machine learning techniques. Machine learning methods offer powerful solutions for complex collider physics challenges. However, these models often depend on simulations that might not fully align with actual physical phenomena. In order to remove this model dependency, semi-supervised classifiers can be used. This solution, however, is not without challenges. In this thesis, an evaluation of the use and limitations of semi-supervised classifiers in particle physics is presented. This is achieved by using a well constrained di-lepton final state dataset to assess the efficacy and ability of the technique, compared to its supervised counterpart. Following this baseline study, the S(150) ! Zg final state, motivated by the multi-lepton anomalies at the LHC, is used to perform narrow resonance searches with semi-supervised Neural Network (NN) classifiers. This work details the methodology and outcomes of a frequentist study aimed at quantifying the extent of a trials factor introduced by semi-supervised NN classifiers. This involves an in-depth analysis of the rate of statistical fluctuations generated in the training of semi-supervised techniques. A secondary component of this study is the evaluation of machine learning based data generators, with emphasis on the Wasserstein Generative Adversarial Network (WGAN), to produce large quantities of realistic data for physics analyses. The results of this investigation into semi-supervised techniques, firstly validates the efficacy and ability of these techniques to classify particle events. This is followed by the frequentist study results, which substantiation that the trials factor remains suitably managed with the use of semi-supervised NN classifiers. The insights derived from this research pave the way for enhancing the reliability of upcoming resonance searches, underscoring the potential of semi-supervised learning in searches for narrow resonances.
Low-temperature electronic transport of metal doped carbon nanotube molecular hybrids and Nitrogen-doped nanocrystalline diamond
(University of the Witwatersrand, Johannesburg, 2024-08) Sodisetti, Venkateswara Rao; Bhattacharyya, Somnath
This thesis explores the magnetism and spin-related properties in carbon-based molecular hybrid materials, with a focus on expanding our understanding of low-dimensional carbon structures and their potential electronic applications. The investigation spans from one-dimensional systems, such as carbon nanotubes (CNTs) functionalized with single-molecule magnets (SMMs), to three-dimensional systems like nitrogen-doped ultra nanocrystalline diamond (UNCD). In these carbon structures, electronic transport is intricately tied to microstructural features, such as grain boundaries and impurity clusters, which hold significant potential for the development of all-carbon electronic devices. The research begins with a detailed examination of the chemical functionalization of multi-walled carbon nanotubes (MWCNTs) through controlled acid treatment to achieve precise metal doping. Using Raman spectroscopy and complementary techniques like ICP-MS and ToF-SIMS, we successfully demonstrate how functionalization levels influence the magnetic properties of CNT hybrids loaded with magnetic metals from the lanthanide series (Gd, Tb, Dy). The study reveals that low percentages of metal doping (0.5% to 1.0%) preserve the magnetic bistability of SMMs post-grafting, while higher doping levels lead to complex magnetic behaviors including super paramagnetism, quasi-ferromagnetism, and potential Kondo lattice behavior inCNT-heavy metal systems. We also explore the spin-phonon coupling in Gd-filled double-walled CNTs, where the onset of superparamagnetic properties at low temperatures is coupled with phonon mode stiffening observed via Raman spectroscopy. This enhanced coupling offers promising pathways for developing efficient molecular qubits through the modulation of spin-phonon interactions in one-dimensional systems. The second part of the thesis investigates into the microwave plasma-assisted chemical vapor deposition (MWCVD) growth of nitrogen-doped nanocrystalline diamond (NCD) thin films on different substrates. By pioneering upgrades to the MWCVD system, I was able to achieve reliable growth of high-quality nanocrystalline diamond thin films. Notably, I observed a novel nanostructure, termed Diaphite-a previously unreported feature, in these NCD films, consisting of nanodiamond grains coherently linked with graphene-like rings. This structure, along with the non-equilibrium growth conditions induced by nitrogen doping and secondary nucleation, presents unique polymorphic features in artificially grown diamonds. Detailed low-temperature transport measurements on four different samples—ranging from 7.5% to 20% nitrogen doping—uncovered complex transport phenomena such as 3D weak localization (WL), variable-range hopping (VRH), and unusual magnetoresistance (MR) behavior. In particular, the 7.5% N2-doped UNCD film on quartz exhibited 3D weak localization (WL) at low fields and anti-weak localization (AWL) at higher fields, with distinct magnetoresistance characteristics depending on the direction of the applied magnetic field. The 20% N2-doped films on both quartz and silicon showed more metallic-like behavior, with magneto-resistance characterized by a B1/2 dependence at low temperatures, suggesting an intricate relationship between doping level, microstructure, and electron transport. These findings significantly expand our understanding of the role that microstructural and chemical modifications play in determining the electronic and magnetic properties of carbon-based materials. This work provides a foundational platform for future research into carbon electronics, offering potential breakthroughs in spintronics, molecular transistors, quantum computing, and other advanced electronic applications.
First principle study of inorganic metal halide perovskites for solar cells application
(University of the Witwatersrand, Johannesburg, 2024-08) Maleka, Prettier Morongoa; Maphanga, Regina R.; Ntwaeaborwa, Odireleng Martin
All-inorganic halide perovskites have received significant attention as semiconductor materials due to their outstanding opto-electronic properties, which have achieved power conversion efficiency (PCE) of up to 25% in perovskite solar cells. Their exceptional characteristics include long diffusion lengths for electrons and holes, tuneable band gap, high absorption coefficients, small effective masses, high carrier mobility, and simple reproducible process. Despite these excellent properties, metal halide perovskites have drawbacks that negatively affect the PCE and stability of the perovskite solar cell devices. This study investigated all-inorganic halide perovskite, CsPbI3, by employing the first-principle density functional theory (DFT) method. Firstly, the effect of mixing halides on X-site was investigated to probe the structural stability and opto-electronic properties. The structural, electronic, optical, mechanical and thermodynamics properties of CsPbI3 – xBrx were investigated using three exchange correlation functionals, namely, LDA, GGA-PBE and SCAN meta-GGA. The findings revealed that mixed halide perovskites have an ideal direct energy band gap for suitable photovoltaic applications. For GGA-PBE and SCAN meta GGA exchange correlation functionals, the determined energy band gap ranges from 1.33 eV and 1.877 eV, whereas the LDA band gap ranges between 0.960 eV and 1.137 eV. The electronic band gaps predicted by GGA-PBE and SCAN meta GGA exchange correlation, which offer better precision compared to LDA suggest that Br-doped CsPbI3 – xBrx perovskite is suitable for photons absorption from near-infrared to visible regions of the spectrum. The modification of the band gap is an essential feature of photovoltaics, as it enables the optimization of solar cell performance. In addition, the systems CsPbI3, CsPbI2Br, CsPbIBr2, and CsPbBr3 exhibited exceptional mechanical and thermodynamic stability. Secondly, perovskites that are considered for photovoltaic applications contain toxic element lead (Pb) on the B-site, which limits application of these perovskites in photovoltaic devices. In this study, substitution of toxic Pb with a smaller percentage of selected transition metals was investigated in order to alleviate the toxicity problems. Thus, CsPbI3 doped with 12.5 % concentration of transition metal, Mn, Fe, Ni and Zn was investigated using DFT. The results showed that transition metal doped-CsPbI3 perovskites enhanced the absorption of this material, although they are all indirect band gaps semiconductors. All the materials were found to be mechanically stable. Lastly, cluster expansion which is a method that is capable of describing the concentration dependent thermodynamic properties of materials while maintaining DFT accuracy, was used to predict new (CsPbI/Br)3 structures. The cluster expansion method generated 42 new stable (CsPb)xIyBrz (where x = 1 to 3 and y and z = 1 to 8) structures and these were ranked the meta-stable structures based on their formation enthalpies. Monte Carlo calculations showed that CsPbI0.5Br0.5 composition separates into different phases at 300K, but changes to homogeneous phase at 700 K, suggesting that a different phase of CsPbI3 may exist at higher temperature. Among the 42 predicted structures, randomly selected structures around iodide rich, 50:50 and bromine rich sites were studied further by determining their electronic, optical, mechanical and thermodynamic properties using DFT. The materials possess similar properties as cubic Br doped CsPbI3 perovskites. The mechanical properties of these compounds revealed that they are ductile in nature and mechanically stable. In summary, the thesis present a novel work on introduction of impurities into CsPbI3 perovskite material as well as compositional engineering to alter its electronic and optical properties for solar cells application.
The Low–Temperature Properties of Boron–Implanted Diamond Materials
(University of the Witwatersrand, Johannesburg, 2024-08) Mahonisi, Nyiku Clement; Naidoo, Mervin
The physio-chemical properties of semiconducting diamond materials under extremely low temperatures have fundamental implications in Condensed Matter Physics. Highly doped boron diamonds have been shown to reach a superconductive state at critical temperatures (Tc) ranging from 4 K − 10 K, albeit such properties are ”at the moment” only attributed to heavily boron-doped synthesized samples via HPHT and CVD growth methods. Theoretical predictions have shown that by exceeding the current solubility limit of boron in diamond, an increase in Tc beyond the 4 K − 10 K is possible, even close to room temperatures. However, in order to gain such a feat, an increase in active boron concentration beyond the metal-to-insulator transition (MIT) is an absolute necessity, and hence, non-equilibrium doping fabrication processes such as CVD growth and ion implantation are required. In this study, we explore carefully the properties of degenerate diamond layers with p-type impurity bands via low energy and low fluence ion implantation. The study involves the utilization of the non-equilibrium ion implantation technique to fabricate boron layers that stretches from the diamond surface to some depth that is a few nm deep into the diamond matrix. A total of seven samples were processed uniquely with respect to ion energies, ion concentrations and annealing temperatures. Three samples (A, B and D) with varying parameters were implanted with both carbon and boron ions in regions where the carbon distribution overlaps with the boron distribution at concentrations close to the MIT level ∼ 4 × 1020 ions/cm3. The carbon implants were used to induce vacancy defects very close to the surface such that the boron ions that are subsequently implanted would simply diffuse into the carbon vacant sites. Furthermore, four samples (BE,ME and SE) were implanted with only boron ions also with varying processing parameters to establish a correlation between the two set of implanted samples. Lastly, one sample (C) was implanted similarly to the first set of samples , albeit, with only carbon ions in order to ascertain the boron-related defects. Initially, all the samples were annealed at 650◦C in order to recover the crystalline state. Spectral analysis clearly confirms the formation of boron-related defects for the carbon and boron implants with dynamic annealing at 200◦C and at 400◦C, respectively. That is, the Lorentzian-like component ∼ 1200 cm−1 and the asymmetric line shape ∼ 1300 cm−1. However, vacancy and interstitial peaks at ∼ 1500 cm−1 and ∼ 1620 cm−1, respectively, are also prevalent. Albeit, dynamic annealing limits the graphitization of the diamond structure with no detection of graphitic features. A concentration and thermal annealing dependency was also established. The second batch of samples were multi implanted with boron (i.e., within the same buried region for samples ME and SE and overlapping implanted regions for sample BE to form a box profile). Boron-related features were not wholly detected for all these samples. After multiple implantation processes it was evident that the diamond structure of samples BE and SE were adversely affected with prominent graphitic features forming. Very faint boron-related features appear for sample ME after nine multiple boron implantation processes with low energy and low fluence at room temperature (RT). However, the spectra show the onset of broadening effects associated structural damage. Expectantly, boron-related features are absent for the carbon implanted sample C with very minimum damage to the diamond spectra. Low resistance tri-layer metal contacts of Ti/Cr/Au were deposited onto the surfaces of the samples in a 4-point probe Van der Pauw configuration. Ohmic behavior was confirmed from electronic transport measurements. Thus, the conduction properties of the samples are also reported in this thesis. A very clear dependence on the fabrication method used to create the boron buried layers is demonstrated by the electronic response of the samples. Sample A experiences Space Charge Limited Currents (SCLCs) related to high localization effects that compensate the movement of free charge carriers through the diamond matrix such that the reported data is limited to a temperature range that is between 300 K → 100 K. The carrier concentration of sample B determined from Hall effect measurements indicate a contribution of both electrons and holes, likely due to the amphoteric vacancy states induced by carbon implantation. However, a careful increase in boron fluence and thermal annealing averts such effects. Samples BE and SE showcase a high level of conductivity with variable range hopping (VRH) mechanisms at low temperatures ∼ 10 K that suggests an increase of the localization length ξloc with low TES values. The overpopulation of boron ions within the nm−sized channels of these samples results in amorphous regions that contribute to the conduction properties of the materials. Hence, a very clear difference of the conduction properties of the samples is demonstrated.
Search for high-mass resonances in the Zgamma channel and Quality assurance of Scintillation detector modules of Tile Calorimeter Phase-I Upgrade of the ATLAS detector
(University of the Witwatersrand, Johannesburg, 2024-09) Mokgatitswane, Gaogalalwe; Ruan, Xifeng; Solovyanov, Oleg; Mellado, Bruce
This thesis presents a search for narrow, high-mass resonances decaying to a Z boson and a photon (Zy) in the final state. The analysis utilizes the full Run 2 dataset collected by the ATLAS experiment at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 140 fb-1 of proton-proton col- lisions at a center-of-mass energy of ps = 13 TeV. The search focuses on a mass range of 220 GeV and 3400 GeV, aiming to identify deviations from the expected background arising from Standard Model processes. A small excess is observed at 250 GeV within the area of interest, with a combined significance of 2.1 standard deviations, indicating the need for further investigation with more data. Upper limits are set on the production cross-section times branching ratio for resonances decaying to Zy across the investigated mass range. When considering spin-0 resonances produced through gluon-gluon fusion, the observed limits at a 95% confidence level range from 65.5 fb to 0.6 fb. For spin-2 resonances produced via gluon-gluon fusion (with quark-antiquark initial states), the limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb. The thesis also highlights the successful Phase-I upgrade of the Tile Calorimeter in the ATLAS detector, ensuring its continued performance. This involved the replacement of degraded Gap-Crack scintillators and Minimum Bias Trigger Scintillators (MBTS) with non-irradiated ones, along-side optimising their geometry, all in preparation for data taking during LHC Run 3. These upgrade endeavors encompassed the design of new Gap-Crack and MBTS counters, including extensions to higher rapidity, the assembly of these counters, their rigorous qualification, and characterization using radioactive sources (90Sr and 137Cs), along with their seamless integration onto the ATLAS detector.