Doctoral Degrees (Physics)

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Analyzing power and cross section distributions of the 12C (p,pα)8Be cluster knockout reaction at an incident energy of 100 MeV
(Stellenbosch : University of Stellenbosch, 2010-03) Mabiala, Justin; Cowley, A. A.; Fortsch, S. V.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The (p, pα) reaction on 12C was investigated experimentally using polarized incident protons of 100 MeV. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25◦ to 110◦, were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well. The observed agreement allows the extraction of distorted momentum distributions from experimental data. These distributions are very consistent over a wide range of angle pairs at which cross section energy-sharing distributions vary considerably. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of spin-orbit distortions were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-α cross section enters as a multiplicative factor in the three-body (p, pα) cross section expression. Spectroscopic factors derived from the data are fairly consistent with the trend of the theoretical predictions. Analyzing power data also follow the trend of free p-4He scattering data, and comparisons with DWIA predictions are in reasonable agreement. The theory reproduces also very well analyzing power angular distributions of the projectile-cluster two-body scattering at large angular momentum of the residual nucleus. This indicates that a quasifree knockout mechanism dominates the reaction. The two-body interaction response between the projectile and the α cluster was found to resemble the scattering of protons from a free α particle to a remarkable degree, the present results strongly imply the existence of preformed α clusters in 12C.
Application of the relativistic random-phase and distorted wave impulse approximations to quasielastic proton-nucleus scattering
(Stellenbosch : University of Stellenbosch, 2010-12) Van Niekerk, David Douglas; Van der Ventel, B. I. S.; Hillhouse, G. C.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: In this dissertation a fully relativistic model for polarized inclusive quasielastic proton-nucleus scattering is developed. Using a standard relativistic impulse approximation (RIA) treatment of quasielastic scattering and a two-body SPVAT form of the current operator, it is shown how the behaviour of projectile and target can be decoupled. Subsequently, different models for projectile and target can be adopted and combined to examine a variety of relativistic effects. The most simplistic model of the target is provided by a mean-field nuclear matter approximation to the relativistic meson-nucleon model, quantum hadrodynamics (QHD). Here relativistic effects manifest as an effective mass, which is lower than the free mass, of the constituent nucleons. This model is improved upon by including many-body correlations through medium-modification of meson propagators in the relativistic random-phase approximation (RPA). Since it is generally accepted that the strong nuclear force and the extended range of the nuclear potential lead to distortion effects on the projectile and ejectile (seen as a modulation of the wave functions), our formalism is geared towards the use of relativistic distorted waves (RDWIA). The distorted waves are written as partial wave expansions and are solutions to the Dirac equation with potentials. The inclusion of distortions, however, greatly increases the computational burden and we show how a number of analytical and numerical techniques can be used to facilitate the process of calculation. It is also shown how the standard relativistic plane wave treatment (RPWIA) can, instead, be easily employed to obtain a baseline for determining the impact of distortions. A calculation is performed for the reaction 40Ca(!p, !p !) at a beam energy of 500 MeV. Here it is found that the effect of correlations on the RPWIA calculation can be seen as a quenching of the cross section that is expected to become more pronounced at lower energies or for higher density targets. A RDWIA calculation shows additional reduction and if target correlations are included this effect is enhanced. To our knowledge this is the first calculation that attempts to include both these effects (RPA and RDWIA) in the context of quasielastic proton-nucleus scattering.
Aspects of femtosecond pulse shape transfer via difference frequency mixing
(Stellenbosch : Stellenbosch University, 2015-12) Botha, Gerda Nicolene; Uys, Hermann; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: We investigate the generation of shaped femtosecond pulses in the infrared spectral regime via a nonlinear process called di erence frequency mixing. First we develop a detailed model of the process, incorporating pulse propagation during di erence frequency mixing, in the slowly varying envelope approximation. Di erence frequency mixing (DFM) is numerically simulated for several wavelengths, nonlinear crystals and Type I and Type II frequency mixing. The di erent factors in uencing the shape transfer e ciency of a shaped pulse to a di erent wavelength regime, as well as the conversion e ciency, is identi ed and investigated thoroughly by doing a parametric study. The numerical modeling demonstrates that the e ciency, with which a shaped pulse in the near-infrared is transferred to another wavelength regime, depends strongly on the refractive index of the nonlinear medium for the interacting pulses and is optimal when the velocity of the generated pulse equals that of the shaped input pulse. We show that it is possible to control the temporal pulse duration of the generated pulse by using speci c input angles and so manipulating the e ective refractive index of the nonlinear material for the input and generated pulses. It was found that it is possible to temporally broaden or narrow the generated pulse relative to the input pulses. We compare the developed numerical model to experimental measurements. A liquid crystal spatial light modulator (SLM), inserted in a 4f setup, is used to generate the shaped pulses. Experimentally we demonstrate high- delity shape transfer by mixing 795 nm and 398 nm femtosecond pulses in a BBO crystal. The temporal broadening and narrowing of the generated pulse is also shown and compared to the numerical simulations showing excellent agreement with measured results.
Characterisation of heavy mineral sands and soils by radiometry and its use in mineral benefication and agriculture
(Stellenbosch : University of Stellenbosch, 2011-03) De Villiers, Dawid; Stander, J. A.; Rozendaal, Abraham ; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Radioactivity is well known and well understood, but its usefulness in industrial applications to optimise processes or increase economic viability is not yet fully utilised by many industries. This study focuses on the measurement of natural radioactivity and its application in heavy mineral separation and vineyard soil classification. The gamma radiometry set-up consisted of a high purity germanium detector, a Marinelli beaker as sample container and associated electronics. It was calibrated for laboratorybased measurements by minimising the background radiation with the use of lead castle and energy and resolution calibrations. Furthermore, detection parameters were optimised; these included the counting time, the selection of gamma rays used for analysis of a sample, the peak area calculation for the detector dead time and the detector efficiency. Given that the samples had different densities and volumes, the detector efficiency had to be corrected for volume and density effects. After implementation of the corrections and optimisations the detection system was tested and found able to accurately measure radioactivity concentrations. The systematic measurement errors for 238U were 5.1 % in the case of the heavy mineral sands and 34.3 % for the vineyard soils, 4.5 % for the 232Th concentrations and 4.7 % for 40K concentrations. Statistical errors were kept below 2 %. The application of radiometry has not been done before at any South African heavy mineral separation plant. For this reason radiometry is suggested as an easier, faster and cheaper alternative to X-ray fluorescence (XRF) for effective implementation of grade control for zircon to improve the cost benefit of the heavy mineral separation process. Zircon is an example of a heavy mineral that is worldwide in demand with a consumption of more than a million tonnes per year. It is used in a wide range of industrial applications and products that include tiles, sanitary ware and plasma displays. South Africa is the second largest producer of zircon in the world and also has the second largest reserve of available zircon, making this mineral a viable source of income for several years to come. Radioactivity, in the form of uranium and thorium, and other impurities such as iron oxide and titanium oxide are found in the crystal lattice of zircon. For it to be a sellable product, the sum of the uranium and thorium concentrations must be less than 500 parts per million for prime or first grade zircon and less than 1000 parts per million for second grade zircon. At present the concentrations of uranium and thorium in zircon concentrates are measured on a whole rock basis by XRF during and at the end of the processing cycle before the final products are ready to be shipped. This is not an ideal situation as the grab samples are taken periodically and are not necessarily representative of the stream or final assignment and has resulted in significant losses by the producer. The solution is to accurately measure the uranium and thorium concentrations fast or immediately, preferably the measurements must be made online and in real time so that processing decisions can be implemented quickly to optimise the final product. Heavy mineral sand samples were obtained from the various separation processes in a Mineral Separation Plant and their 238U and 232Th concentrations determined. The results indicated that the samples’ uranium to thorium ratios together with their total concentrations can be used to differentiate between the samples (i.e. separation processes). The measurement results were compared with those obtained with XRF. The correlations with radiometry were excellent for the uranium (r2 = 0.992), thorium (r2 = 0.998) and total concentrations (r2 = 0.998). Radiometric measurements were also conducted by decreasing the counting time from 3600 s to 1 s to investigate its effect on the accuracy of the results. Correlations between the different times and 3600 s ranged from excellent to good. The obtained results are then used to recommend that radiometry is used in a Mineral Separation Plant to verify that the zircon and zirkwa meet the specifications, to optimise the entrance feed and the other separation processes and to monitor the tailings streams. Finally the practical aspects of the implementation of radiometry are discussed. As a second application was radiometry applied in an agricultural pilot study to demonstrate the applicability of radiometry as a possible useful tool in soil classification. The creation of a vineyard is a long term and expensive investment and its yield and quality will be influenced by many factors such as the type of soil, viticultural preparations and climate. Information on the different soil types in a vineyard is therefore indispensable for the optimisation of land use with respect to vine cultivar, wine quality and production. Soil samples were obtained from Kanonkop, Simonsig and Spier vineyards and their 238U, 232Th and 40K concentrations determined, assuming that fertilisers would have no effect on the results. The difference in 40K concentrations were related to the clay fraction of the soil and demonstrated that the Kanonkop and Simonsig soils are fine-grained and clay-rich compared to the sandy coarse-grained soils of Spier. The uranium and thorium concentrations were indicators of whether the mineralogy of the soil is the same as the underlying bedrock as well as soil maturity. The measurement results were compared with those obtained with XRF. The correlation with radiometry were poor for the uranium concentrations (r2 = 0.314), as many of the samples concentrations were below the XRF detection limit. The correlations were excellent for both thorium (r2 = 0.985) and potassium (r2 = 0.999). As a positive result from the findings of the study was an in-situ measurement performed by Newman et al. for the radiometric mapping of a Simonsig vineyard for soil classification.
Charge carrier effects in free standing Si membranes investigated by linear and second harmonic optical techniques
(Stellenbosch : Stellenbosch University, 2016-12) Ndebeka, Wilfrid Innocent; Rohwer, Erich G.; Neethling, Pieter N.; Steenkamp, Christine M.; Stafast, Herbert; Stellenbosch University. Faculty of Science. Dept. of Physics
ENGLISH ABSTRACT : Silicon (Si) based materials have been of technological importance throughout the years and today silicon remains a leading material in microelectronics industries. The silicon/silicon dioxide (Si/SiO2) interface has been one of the most studied systems in the realms of material and condensed matter physics. Despite the great outpouring research and huge technological success, there has been little effort to construct a comprehensive, unified microphysical model of the Si/SiO2 interface. The optical properties, both linear and nonlinear, of the interface region are of interest since they vary considerably from those of the adjacent bulk phases. Second harmonic generation is used as a versatile tool to investigate surfaces and buried interfaces of centrosymmetric materials such as Si. Specifically, the electric field induced second harmonic (EFISH) generation process is used as a valuable technique to investigate the mechanisms of charge transfer and trap generation and population, both by electrons and holes being pumped across the interface. In this work, the transmission of the fundamental laser beam (90 fs pulse trains at 80MHz repetition rate, 800 nm) through 10 m to 30 m thick silicon (Si) membranes as well as simultaneous measurements of the electric field induced second harmonic (EFISH) generated in reflection from and transmission through the Si membranes are reported for the first time. The transmission of the laser power through Si membranes, irradiated at an incident angle of 45 , shows an increase with increased power, then reaches a maximum, and finally continues to decrease considerably as the incident power is increased. A similar behavior is observed with the EFISH signals in transmission of the Si membranes. The nonlinear behavior of the fundamental transmitted signal is attributed to free charge carrier absorption (FCA) in Si. The experimental setup and these results are discussed in this thesis.
Charge dynamics in hybrid and organic-inorganic light harvesting thin films followed with femtosecond transient absorption spectroscopy
(Stellenbosch : Stellenbosch University, 2017-12) Minda, Iulia; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT : In order to bridge the gap between traditional sources of electricity and the increasing global demand for it, as a society we must move towards renewable sources of energy such as solar radiation. Photovoltaic devices (PVs) harness solar power and convert it to electrical power. In order to be commercially viable, they need to be efficient, cost effective, simple to fabricate and environmentally friendly. To address these requirements, the class of emerging PVs arose, which includes dye sensitised solar cells (DSSCs) and perovskite solar cells. Femtosecond transient absorption spectroscopy (TAS) is an experimental technique which allows us to follow the ultrafast photoinduced charge dynamics in real time in light harvesting thin films and PVs. By assigning time and rate constants to various processes governing the charge generation and extraction in solar cells, we construct charge dynamics models, and therefore learn the fundamental photophysics reasons behind what makes the power conversion efficiencies (PCEs) of some solar cells superior. In particular, this study focused on the charge transfer processes in indoline dye (DN216) sensitised electrodeposited ZnO solar cells, and the charge recombination dynamics in FA0.85MA0.15PbI2.55Br0.45 perovskite thin films. To construct the simplest fully consistent charge dynamics models, we match the visible and near-infrared spectroscopic signals of our samples to the allowed electronic transitions, and follow their temporal evolutions on the femtosecond and picosecond time scales. From our measured time and rate constants we observed that ZnO based DSSCs are less efficient than their TiO2 counterparts because the electron injection from the photoexcited indoline dye into the ZnO CB doesn’t just occur directly (< 200 fs), but also stepwise via neutral (∼ 2 ps) and ionic (∼ 10 ps) intermediate charge transfer states, resulting from surface trap states characteristic of electrodeposited ZnO. Moreover, FA0.85MA0.15PbI2.55Br0.45 is an excellent hybrid photoabsorber in record efficiency perovskite solar cells because even at high charge carrier densities of 1019 cm−3 , the third order non-radiative Auger recombination mechanism is not dominant. Furthermore we determined the associated geminate, non-geminate and Auger recombination rate constants as A = 5 × 109 s −1 , B = 10−10 s −1 cm3 and C = 50 × 10−32 s −1 cm6 .
A cluster study of the nuclei 212Po and 218Rn
(Stellenbosch : University of Stellenbosch, 2009-12) Ibrahim, Taofiq Toyin; Wyngaardt, Shaun M.; Perez, S. M.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: A binary cluster model is used to investigate the properties of the ground state band of 212Po, modelled as a 208Pb-alpha core-cluster system. The results obtained using a microscopic corecluster potential are compared to those obtained with a purely phenomenological potential. The two potentials were found to exhibit similar surface behaviour and thus give similar predictions for the ground state alpha decay half-life. They however generate very different energy spectra, with the results from the phenomenological potential clearly superior. We optimize the phenomenological potential parameters, and propose an additional short range interaction to improve the underbinding generally found for the J¼ = 0+ ground state. We then investigate two possible scenarios for generating the negative parity states in 212Po. We find that both are necessary in order to produce low-lying negative parity states which are able to decay via electric dipole transitions to the positive parity states of the ground state band. Finally we present a novel calculation of the properties of the low-lying positive and negative parity states of 218Rn described as a doubly closed 208Pb core plus a 10Be cluster.
Combinatorics and dynamics in polymer knots
(Stellenbosch : Stellenbosch University, 2014-04) Rohwer, Christian Matthias; Muller-Nedebock, Kristian K.; Scholtz, Frederik G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: In this dissertation we address the conservation of topological states in polymer knots. Topological constraints are frequently included into theoretical descriptions of polymer systems through invariants such as winding numbers and linking numbers of polynomial invariants. In contrast, our approach is based on sequences of manipulations of knots that maintain a given knot's topology; these are known as Reidemeister moves. We begin by discussing basic properties of knots and their representations. In particular, we show how the Reidemeister moves may be viewed as rules for dynamics of crossings in planar projections of knots. Thereafter we consider various combinatoric enumeration procedures for knot configurations that are equivalent under chosen topological constraints. Firstly, we study a reduced system where only the zeroth and first Reidemeister moves are allowed, and present a diagrammatic summation of all contributions to the associated partition function. The partition function is then calculated under basic simplifying assumptions for the Boltzmann weights associated with various configurations. Secondly, we present a combinatoric scheme for enumerating all topologically equivalent configurations of a polymer strand that is wound around a rod and closed. This system has the constraint of a fixed winding number, which may be viewed in terms of manipulations that obey a Reidemeister move of the second kind of the polymer relative to the rod. Again configurations are coupled to relevant statistical weights, and the partition function is approximated. This result is used to calculate various physical quantities for confined geometries. The work in that chapter is based on a recent publication, "Conservation of polymer winding states: a combinatoric approach", C.M. Rohwer, K.K. Müller-Nedebock, and F.-E. Mpiana Mulamba, J. Phys. A: Math. Theor. 47 (2014) 065001. The remainder of the dissertation is concerned with a dynamical description of the Reidemeister moves. We show how the rules for crossing dynamics may be addressed in an operator formalism for stochastic dynamics. Differential equations for densities and correlators for crossings on strands are calculated for some of the Reidemeister moves. These quantities are shown to encode the relevant dynamical constraints. Lastly we sketch some suggestions for the incorporation of themes in this dissertation into an algorithm for the simulated annealing of knots.
Confined counterions surrounding a Macroion : a field theoretic approach
(Stellenbosch : Stellenbosch University, 2011-12) Boonzaaier, Leandro; Muller-Nedebock, Kristian K.; Scholtz, Frederik G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Several experiments [1, 2, 3, 4] have shown that e ective attractive interactions exist between con ned like-charged macromolecules. Theoretical approaches have not reached consensus as to precisely what the mechanism for the attraction is, but it is agreed that comprehending the role of the counterion arrangement around macromolecules is crucial for understanding the e ective macromolecule interactions. It is generally assumed that attraction only occurs in the limit of strong electrostatic coupling and is driven by correlation e ects that are neglible in a mean- eld approach, which is valid in the weak-coupling limit. However, in some experimental situations attraction occurs even in the limit of weak-coupling. We consider a eld-theoretic approach that includes uctuations to study the Coulomb interactions of con ned counterions with a single exible charged spherical macromolecule that can expand or collapse uniformly by changing its radius. We show how the linearised eld-theory (valid in the weak-coupling limit) is mapped onto the square-well potential of Quantum Mechanics. The con nement leads to bound states being present in the spectrum at all times. Bound states are non-perturbative and we investigate the role they play in the physics of the system. Some of the e ects are rather counter-intuitive. Firstly, upon expanding the macromolecule in a xed con nement volume, the uctuation part of the free energy favours a decrease in the free energy. Secondly, upon increasing the temperature to high but nite values, the uctuation contribution does not dominate the free energy as would be expected. The mathematical origins of these e ects are dicussed in detail and as part of the analysis we introduce a novel regularisation scheme for computing the functional determinant arising in the model considered where the cut-o is speci ed unambiguously in terms of physical parameters.
Construction of a spectroscopic terahertz time-domain ellipsometer
(Stellenbosch : Stellenbosch University., 2020-04) Smith, Shane Raymond; Neethling, Pieter H.; Rohwer, Erich G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Please refer to full text for abstract.
Control of CO2 vibrational dynamics via shaped-pulse coherent anti-Stokes Raman spectroscopy
(Stellenbosch : Stellenbosch University, 2017-03) Attie, Hendriks; Hermann, Uys; Stellenbosch University. Faculty of Science. Dept. of Physics
ENGLISH ABSTRACT : In this work we investigate the coherent control of carbon dioxide (CO2) vibrational dynamics using Coherent anti-Stokes Raman Scattering (CARS). During CARS, vibrational modes are excited via stimulated Raman scattering (SRS). Subsequently a narrowband probe field interacts with the molecular ensemble providing not only information about the modes populated, but also on the evolution of the wave-packet created during excitation. By spectrally shaping one of the SRS pump fields the vibrational dynamics can be controlled. In this work it was assumed that the pump pulse structure which will lead to a desired dynamics is unknown. To find that structure, a learning algorithm was developed which utilizes a spatial light modulator (SLM) in a 4f-optical con guration to spectrally shape the pump. Both a time-frequency representation of the shaped pulse (called the von Neumann basis) and a standard Fourier domain representation were bench-marked during optimization of a second harmonic generation (SHG) signal in a BBO crystal to ascertain which will suit the optimization problem best in terms of convergence rate and parameter space size. It was found that the von Neumann basis converged faster than the standard Fourier domain representation while still operating on a larger parameter space and therefore it was used in all subsequent work. In addition, we developed a quantum mechanical theoretical model of the CARS process to ensure proper understanding of our measurements. We demonstrated experimentally that mode excitation selectivity can be achieved using the pump fields extracted by the learning algorithm, and we explore the underlying selectivity mechanisms. Control of the relative phase of oscillation of different vibrational modes is also observed. Our work demonstrates coherent quantum control of all relevant aspects of the molecular vibrational dynamics of CO2.
Coulomb excitation of the 2+ state in 14C and the quadrupole deformation of states in 194Pt
(Stellenbosch : Stellenbosch University, 2019-12) Brits, Christiaan Petrus; Wiedeking, Mathis; Hadynska-Kl¸ek, Katarzyna; Papka, Paul; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The safe Coulomb excitation experiment of 14C and 194Pt which took place at Florida State University took advantage of the unique beam capabilities, the availability of highefficiency large volume LaBr3(Ce) detectors and the S3 double sided silicon strip detector. Using the advantageous experimental environment the B(E2: 2+1 ! 0+1 ) value of the firstexcited 2+1 state of 14C and the quadrupole deformation of states in 194Pt are investigated. B(E2: 2+1 ! 0+1 ) values of neutron-rich even-even C isotopes have been reported up to 20C and provide important information on the evolution of the underlying structural mechanism towards the drip line. They also provide critical constraints for theoretical models as is the case for the experimentally determined B(E2: 2+1 ! 0+1 ) value for 14C which exhibits persistent inconsistencies with that obtained from theoretical models. The B(E2: 2+1 ! 0+1 ) value in 14C cannot be reproduced by theoretical models making it indispensable to enhance our theoretical understanding of the C isotopic chain in general. The 2+1 state was not observed in the particle- data, which may indicate that 14C undergoes single-particle excitation and has a smaller B(E2: 2+1 ! 0+1 ) value than previously thought. The A 190 region is very interesting since it has oblate, prolate, -soft and spherical shaped nuclei. It is predicted that Pt evolves from prolate deformed 180􀀀186Pt to -soft 188Pt and triaxial 190Pt to oblate 192􀀀198Pt and finally to spherical 204Pt. From this work, the quadrupole deformation of the 0+1 , 2+1 , 4+1 , 2+2 , 4+2 states are successfully measured with increased accuracy. Additionally it is determined that 194Pt is triaxial oblate which supports the theoretical prediction of the shape evolution in Pt.
Cross section measurement of light ions using (p,xp) and (p,xn) reactions
(Stellenbosch : Stellenbosch University, 2020-12) Kenfack Jiotsa, Doris Carole; Papka, Paul; Jones, Pete; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The production cross section of light neutron-rich nuclei obtained from a proton induced reaction using light stable nuclei was extracted. The relevancy of such study, away from the valley of stability is oriented toward the understanding of the origin of the elements. Targets of natLi, 9Be and natB were bombarded by a proton beam in order to produce 6;8He and 8;9Li. The proton beam of energy 50 MeV and 66 MeV was turned on and off during predefined intervals of time using a beam chopper. The beam-on sequence corresponding to the activation time was set long enough to reach secular equilibrium. The beam-off sequence was set to allow at least six decays of the product with the longest half life. The decay rate, hence the production rate of exotic nuclei of interest was evaluated using the Fermi-Kurie plot. The production cross section was deduced and results were compared to theoretical calculations from TALYS. Measurements were conducted at the Separated Sector Cyclotron facility at iThemba Laboratory of Accelerator Based Sciences (LABS), South Africa. The detection system was composed of two electron spectrometers, mounted following the E-E technique. Each telescope comprised a 5 mm plastic scintillator used to estimate the energy loss and a thin window germanium detector (LEPS) for residual energy measurement. Such combination of detectors ensured accurate particle identification, while allowing high-resolution measurement. This project was designed in order to evaluate the feasibility of the production of light neutron rich beams at iThemba LABS using the ISOL (Isotope Separation On-Line) method. Light targets produce lesser species than usual heavier targets, which makes the on-line separation easier. Moreover, nuclei such as boron can be produced in carbide, nitride and oxide forms, known to sustain high temperatures and can be used as a substitute to the uranium carbide in the current design of the ISOL source at iThemba LABS.
Design, construction and characterisation of a low-level radioactivity counting system based on gamma-ray spectrometry with LaBr3:Ce scintillator detectors
(Stellenbosch : Stellenbosch University., 2020-04) Bashir, Munirat; Newman, Richard T.; Jones, Pete; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The activity concentrations in naturally occurring radioactive material (NORM) samples are conventionally measured using a gamma-ray spectrometer with a single detector (mostly HPGe or NaI:Tl) enclosed in a lead shield. In this work, the GEANT4 Monte Carlo toolkit was used to design a passive water shielding to reduce background radiation from the measurement environment reaching the detectors which was then constructed. Volume soil samples placed in Marinelli beakers were measured in singles and coincidence modes using four LaBr3:Ce detectors without shielding; two LaBr3:Ce detectors without shielding and the same two LaBr3:Ce detectors inside the constructed water shield. The samples were also measured using a NaI:Tl detector inside the water shield and a standard HPGe detector shielded with lead to compare and validate the results from measurements with the LaBr3:Ce detectors. A novel method of background reduction was employed by using photon time-of- ight in addition to measurement of the two photon energies in coincidence. Both the simulated and measured results show that the water shield attenuates 2614.5 keV ( 208Tl/232Th series) gamma rays by 90 %. This energy is the maximum fullenergy peak centroid in the gamma-ray spectrometry spectrum of NORMs. The activity concentrations of 238U and 232Th series and 40K radionuclides in IAEA-375 soil were determined using the NaI:Tl detector inside the water shield. The presence of 138La and 227Ac in the LaBr3:Ce detector's crystal increases the minimum detectable activity (MDA) at 1460.8 keV (40K) and 2614.5 keV gamma-ray energies. The measured internal activity of 138La in the LaBr3:Ce detector crystal is 263.8 ± 26.8 Bq kg−1 which is comparable to the calculated activity of 293.3 Bq kg−1 . The activity concentration of 40K in IAEA-375 soil and beach sand measured using the LaBr3:Ce detector geometries is below the MDA. The activity concentrations of 238U and 232Th series radionuclides in IAEA-375 soil were determined in coincidence mode using the four LaBr3:Ce detector without shielding. The measured activity concentrations of 238U and 232Th series and 40K radionuclides in IAEA-375 soil were comparable to certi ed values to within measurement uncertainty. The activity concentrations of 238U and 232Th series radionuclides inside beach sand were determined in both singles and coincidence modes. The results of the measured activity concentrations of 238U and 232Th series radionuclides in beach sand using LaBr3:Ce and NaI:Tl detectors agree with those measured using the HPGe detector system to within 1σ to 2σ level. The water shield (50 cm thickness) performed well in shielding 2614.5 keV background gamma-ray energy by 90 % and energies less than 2614.5 keV by more than 90 % during low-level radioactivity measurements. The internal activity of the LaBr3:Ce detector increases the MDA at 1460.8 keV and 2614.5 keV, which limits the measurement of 40K radionuclide with low activity concentration in singles mode. The use of a coincidence method plus photon time-of- ight signi cantly suppresses the scattered/background radiation.
Development and characterisation of a Nd:YAG pumped wavelength tunable VUV light source
(Stellenbosch : Stellenbosch University, 2016-03) Rigby, Charles Ian; Steenkamp, Christine M.; Rohwer, Erich G.; Stellenbosch University. Faculty of Science. Dept. of Physics
ENGLISH ABSTRACT : Please refer to the full text abstract.
Development of a high flux neutron radiation detection system for in-core temperature monitoring
(Stellenbosch : Stellenbosch University, 2012-03) Singo, Thifhelimbilu Daphney; Papka, Paul; Wyngaardt, Shaun M.; Dobson, Robert T.; Smit, F. D.; Stellenbosch University. Faculty of Sciences. Dept. of Physics.
ENGLISH ABSTRACT: The objective of this research was to develop a neutron detection system that incorporates a mass spectrometer to measure high neutron flux in a nuclear reactor environment. This system consists of slow and fast neutron detector elements for measuring fluxes in those energy regions respectively. The detector should further be capable of withstanding the harsh conditions associated with a high temperature reactor. This novel detector which was initially intended for use in the PBMR reactor has possible applications as an in-core neutron and indirect temperature-monitoring device in any of the HTGR. Simulations of a generic HTGR core model were performed in order to obtain the neutron energy spectrum with emphasis on the behavior of three energy regions, slow, intermediate and fast neutrons within the core at different temperatures. The slow neutron flux which has the characteristic of a Maxwell- Boltzmann distribution were found to shift to larger values of neutron flux at higher energies as the fuel temperature increased, while fast neutron flux spectra remained relatively constant. In addition, the results of the fit of the slow neutron flux with a modified Maxwell-Boltzmann equation confirmed that in the presence of the neutron source, leakage and absorption, the effective neutron temperatures is above the medium temperatures. From these results, it was clear that the detection system will need to monitor both slow and fast neutron flux. Placing neutron detectors inside the reactor core, that are sensitive to a particular energy range of slow and fast neutrons, would thus provide information about the change of temperature in the fuel and hence act as an in-core temperature monitor. A detection mechanism was developed that employs the neutron-induced break-up reaction of 6Li and 12C into α-particles. These materials make excellent neutron converters without interference due to γ-rays, as the contributions from 6Li(γ,np)4He and 12C(γ,3α) reactions are negligible. The mass spectrometer measures the 4He partial pressure as a function of time under high vacuum with the help of pressure gradient provided by a high-vacuum turbomolecular pump and a positive-displacement fore-vacuum pump connected in series. A cryogenic trap, which contains a molecular sieve made of pellets 1.6 mm in diameter, was also designed and manufactured to remove impurities which cause a background in the lighter mass region of the spectrum. The development and testing of the high flux neutron detection system were performed at the iThemba Laboratory for Accelerator Based Sciences (LABS), South Africa. These tests were carried out with a high energy proton beam at the D-line neutron facility, and with a fast neutron beam at the neutron radiation therapy facility. To test the principle and capability of the detection system in measuring high fluxes, a high intensity 66 MeV proton beam was used to produce a large yield of α-particles. This was done because the proton inelastic scattering cross-section with 12C nuclei is similar to that of neutrons, with a threshold energy of about 8 MeV for both reactions. Secondly, the secondary fast neutrons produced from the 9Be(p,n)9B reaction were also measured with the fast neutron detector. The response of this detection system during irradiation was found to be relatively fast, with a rise time of a few seconds. This is seen as a sharp increase in the partial pressure of 4He gas as the proton or neutron beam bombards the 12C material. It was found that the production of 4He with the proton beam was directly proportional to the beam intensity. The number of 4He atoms produced per second was deduced from the partial pressure observed during the irradiation period. With a neutron beam of 1010 s−1 irradiating the detector, the deduced number of 4He atoms was 109 s−1. When irradiation stops, the partial pressure drops exponentially. This response is attributed to a small quantity of 4He trapped in the present design. Overall, the measurements of 4He partial pressure produced during the tests with proton and fast neutron beams were successful and demonstrated proof of principle of the new detection technique. It was also found that this system has no upper neutron flux detection limit; it can be even higher than 1014 n·cm−2·s−1. The lifetime of this detection system in nuclear reactor environment is practically unlimited, as determined by the known ability of stainless steel to keeps its integrity under the high radiation levels. Hence, it is concluded that this high flux neutron detection system is excellent for neutron detection in the presence of high γ-radiation level and provides real-time flux measurements.
Development of a kHz optical remote sensing system for in situ insect monitoring
(Stellenbosch : Stellenbosch University, 2016-03) Gebru, Alem Kindeya; Rohwer, Erich G.; Neethling, Pieter N.; Sorensen, Mikkel B.; Stellenbosch University. Faculty of Science. Dept. of Physics. Laser Research Institute
ENGLISH ABSTRACT : In this work we have developed a kHz optical remote sensing system for in situ insect monitoring applications. This is an active and passive remote sensing system based on laser and sunlight. This system showed potential for monitoring pollinators in agricultural fields. It enables the implementation of improved vector control mechanisms and pest management. The passive remote sensing setup called dark field spectroscopy uses sunlight as an illumination source. Considering the passive remote sensing techniques, it is shown that one can determine flight direction, retrieve spectral information, and resolve wing-beat frequency (and harmonics) and iridescence features of fast insect events. With regards to active remotes sensing technique, a number important range resolved quantitative assessments of insects such as size, speed and wing-beat frequency can be performed. It is shown that the CW-LIDAR based on the Scheimpflug principle improves the range resolution beyond the diffraction limit. The reason for this is because of the fact that the sampling frequency is in the order of kHz and insects behave like blinking particles similar to super resolution microscopy called stochastic optical reconstruction microscopy (STORM) where molecules blinks between bright and dark states. Generally, this dissertation highlights the potential of applied optical remote sensing techniques to remotely identify insects and understand their impact onan ecosystem.
Development of a multimodal nonlinear imaging system for biophotonic applications
(Stellenbosch : Stellenbosch University., 2020-04) Dwapanyin, George Okyere; Rohwer, Erich G.; Neethling, Pieter H.; Bosman, Gurthwin W.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Multiphoton microscopy techniques have gained wide prominence in biophotonics imaging applications since their inventions. Compared to conventional optical imaging, these nonlinear optical microscopy (NLOM) techniques are intrinsically confocal, and thus enables three-dimensional imaging with submicron spatial resolution. Additional advantages include decreased photodamage to tissue, increased depth of penetration as well as the ability to perform label-free imaging. Signal response in NLOM techniques depend nonlinearly on the peak intensity, therefore requiring a high peak intensity laser as source. Control of ultrashort pulses enables the generation of high peak intensity pulses with lower excitation pulse energies. This dissertation focuses on the development of a nonlinear microscopy system for biological applications based on the control of the spectral phase of broadband supercontinuum pulses generated in a polarization maintaining all normal dispersion photonic crystal ﬁbre. We further demonstrate, for the ﬁrst time, the real world application of a time domain ptychographic phase measurement technique known as i2PIE which allows for phase correction at the object plane, in microscopy, and how this phase control contributes to image enhancement in two photon excitation ﬂuorescence (TPEF) and second harmonic generation (SHG) imaging of biological tissue. By comparing this new technique to the commonly used multiphoton intrapulse interference phase scan (MIIPS) measurement technique, we show that i2PIE oﬀers an improved spectral phase measurement which can be used to generate shorter temporal pulses and ultimately produce higher peak intensities, even at lower pulse energies. Our results also show that for the same input pulse energies, i2PIE provides a higher contrast image and an improved signal to noise ratio compared to MIIPS. The results obtained from this work projects i2PIE as a promising phase measurement technique for the coherent control of ultrashort pulses used in nonlinear microscopy.
The development of a β-decay tape station and the search for vibrations in Ruthenium isotopes
(Stellenbosch : Stellenbosch University, 2021-03) Makhathini, Lucky Maxwell; Bark, R. A.; Papka, Paul; Rohwer, Erich G.; Van Zyl, J. J.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: This work gives details of the new facility that is introduced to iThemba LABS for nuclear structure studies, called the tape station. The technical description of this device is presented along with an experimental demonstration. A user-friendly control system for this device is developed and it can easily be integrated into the existing beam control system for the iThemba LABS beamline line control systems. Two versions of this device were successfully used to conduct experiments. The versatility of this device is demonstrated by performing multiple experiments, one with a shorter half-life and two with long half-lives. A method to produce "short-lived nuclei (radioactive nuclei)" at iThemba LABS is given. The low-lying states of 98,100Ru were investigated using the γ-ray spectroscopy following the β-decay of 98,100Rh. The source of activity for 162Yb was produced using the 147Sm(19F, 4n) reaction while in 98Ru and 100Ru were produced using the 89Y(12C, 3n)98Rh and 89Y(14N, p2n)100Rh, 89Y(14N, 3n)100Pd reactions at beam energies of 45 MeV and 47 MeV, respectively. The results obtained from experimental data are given and analysed. The 162Yb study demonstrated that this device is capable to be used to probe nuclei with shorter half-lives. While for 100Ru the data confirmed the γ-rays observed in the previous measurement, while in 98Ru new γ-rays were observed. The newly assigned γ and 0+2 bands in 98Ru fit well in the systematics of these excitations assigned in the heavier Ru isotopes while 100Ru differs. Beyond-mean-field calculations employing the self-consistent configuration mixing method suggest that the Ru isotopes are triaxiality deformed and shape coexistence occurs around 98Ru.
Development of diode end-pumped Nd:YLF lasers at 1314 nm for high power operation
(Stellenbosch : Stellenbosch University, 2016-03) Botha, Roelf Cornelis; Von Bergmann, Hubertus M; Combrinck, Ludwig; Strauss, Hencharl; Stellenbosch University. Faculty of Science. Dept. of Physics
ENGLISH ABSTRACT : Please refer to the full text abstract.

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