Browsing by Author "Leadbeater, Thomas"
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- ItemOpen AccessDevelopment of 18F radiochemistry for positron emission particle tracking (PEPT)(2023) Camroodien, Ameerah; Leadbeater, ThomasPositron emission particle tracking (PEPT) is a non-invasive tracer-based measurement technique used to obtain dynamic information about multiphase systems. The basis of the technique is to radiolabel a phase-representative tracer particle with a positron emitting radionuclide. Electron-positron annihilation produces pairs of back-to-back 511 keV gamma photons emitted from the locality of the tracer particle. Pairs of annihilation photons are detected within the field-of-view of a modified positron emission tomography (PET) scanner, and the instantaneous position of the tracer particle is determined in three dimensions using reconstruction of consecutive annihilations via an iterative algorithm. From the continuously measured tracer particle trajectory, the time differential is used to produce velocity (and acceleration) fields, and further trajectory analysis reveals the localised behaviour of the tracer in the system under study including residence times and kinematic properties. For optimum tracking the radioactivity in a single particle must be sufficient irrespective of the tracer size and material. Typically activities in the range 250 µCi – 2 mCi are required, often loaded on to particles in the 10 mm - 100 µm diameter range. The physical properties of the tracer must accurately reflect those of the bulk media under study such that the measured PEPT data is reflective of the bulk motion. In this research a novel radiochemical technique in producing tracer particles from commercially available medical grade 18F-fluorodeoxyglucose (18F-FDG) (half-life 110 minutes) was developed. In the context of the radioisotopes and accelerated beams available through the National Research Foundation (NRF) accelerator facility iThemba LABS (Cape Town, South Africa), these methods offer the best, perhaps only, mechanism to utilising 18F-based radiotracers for PEPT in South Africa. Furthermore, this work reduces the need for daily isotope production by a dedicated cyclotron beam and the need for specialist equipment. Provided a suitable source of medical 18F-FDG can be sourced, PEPT is therefore possible without the site constraints of an accelerator facility, enabling wider applications of the technique. Compared to the 68Ga-based tracer particles (half-life 68 minutes) pioneered by PEPT Cape Town, 18F based 1 tracers have the distinct advantages of a longer half-life allowing for extended experimental timescales, and an increased signal to noise ratio from the pure β + emission with no additional gamma transitions. Commercially available medical grade 18F-FDG was utilised as the initial 18F stock. A column chromatography method was developed to separate the glucose-complex of 18F-FDG in preparation of the reactive solution, made of 18F in trifluoroacetic acid (TFA) complex, and used for radiolabelling. The anion exchange resins Purolite A870 and A200 were investigated for their ability to exchange non-radioactive counter ions for the required species. Ion exchange techniques were used to label small phase-representative tracer particles (<1 mm diameter) by controlled uptake of 18F. With limiting abundance effects, a loading solution composition of 1:2.5 of TFA:18F-FDG provided greatest uptake and activities of up to 100 µCi and 80 µCi were achieved for the A870 and A200 resins of diameter ranging from 430 - 590 µm respectively. The radiolabelling method was adjusted to incorporate pretreatment of the reactive solution prior to radiolabelling and yielded improved radiolabelling performance, resulting in 300 µCi and 200 µCi activities for the A870 and A200 resins of diameter ranging from 550 - 610 µm respectively. While the activity is still low with regards to optimum tracking, the methods developed here show potential for future tracer particle production. In some applications, particularly those with low attenuation and photon scattering, and/or small scale systems, the particles developed here are ideal.
- ItemOpen AccessEnhancing PEPT: high fidelity analysis techniques with augmented detection systems(2024) Van Der Merwe, Robert; Leadbeater, Thomas; Peterson, StevePositron emission particle tracking (PEPT) is a non-invasive, tracer-based technique used in the study of dynamic systems, such as particulate and fluid flows. Relying on positron imaging principles, typical PEPT systems operate with millimetre precision at tracking speeds of up to 10 m/s, with applications in fields from engineering to medicine. Performance is constrained by the efficiency of conventional fixed geometry detector systems and achievable activity of tracer particles, creating challenges when addressing phenomena on the micro-scale. Previous work with a pair of pixelated cadmium zinc telluride (CZT) room temperature semiconductors (9680 pixels of 1.8 x 1.8 x 0.5 mm3) exhibited potential in micro-scale PEPT, but achievable location rates and field of view (FOV) were limiting. To address these issues, a modular bismuth germanate oxide (BGO) scintillator array, consisting of 1024 detector elements (512 pixels of 6.75 x 6.25 x 30 mm3 and 512 pixels of 4.1 x 4.0 x 30 mm3), has been developed and characterised for use in a hybrid system, combining semiconductor and scintillator de vices. Optimal detection system geometry was determined through numerical modelling of system sensitivity, with the BGO array covering a FOV of 120×174×102 mm3 and the high-resolution semiconductor FOV of 62 × 42 × 20 mm3 placed centrally. This design maximises absolute efficiency through the scintillators and spatial resolution through the semiconductors. A coincidence timing resolution of 5.37 ± 0.17 ns and an energy resolution of 30.51 ± 0.48% at 511 keV was measured for the BGO devices, enabling optimisation of coincidence gates and energy level discriminators respectively. Using a novel 3D positioning stage and a 20.11 ± 0.26 kBq Na-22 calibration source, measurements of system sensitivity, spatial resolution and accuracy were performed. Sensitivity profiles were found in agreement with simulation, with a maximal central sensitivity of 34.8 ± 0.6 cps/kBq. Sub-millimetre system accuracy was achieved in all axes except between the BGO detector faces, in which an expected warping effect was identified. Sub-millimetre spatial resolution, σ, was achieved for a maximum location rate per unit activity, L ′, of 0.45 Hz/kBq, with an identified σ = 1.5 √ L′ trade-off to be optimised for specific use cases. The results of this work demonstrate the applicability of PEPT to the study of micro-scale phenomena and outline the path towards hybrid implementation
- ItemOpen AccessEnhancing PEPT: high fidelity analysis techniques with augmented detection systems(2024) Van Der Merwe, Robert; Leadbeater, Thomas; Peterson, StevePositron emission particle tracking (PEPT) is a non-invasive, tracer-based technique used in the study of dynamic systems, such as particulate and fluid flows. Relying on positron imaging principles, typical PEPT systems operate with millimetre precision at tracking speeds of up to 10 m/s, with applications in fields from engineering to medicine. Performance is constrained by the efficiency of conventional fixed geometry detector systems and achievable activity of tracer particles, creating challenges when addressing phenomena on the micro-scale. Previous work with a pair of pixelated cadmium zinc telluride (CZT) room temperature semiconductors (9680 pixels of 1.8 x 1.8 x 0.5 mm3) exhibited potential in micro-scale PEPT, but achievable location rates and field of view (FOV) were limiting. To address these issues, a modular bismuth germanate oxide (BGO) scintillator array, consisting of 1024 detector elements (512 pixels of 6.75 x 6.25 x 30 mm3 and 512 pixels of 4.1 x 4.0 x 30 mm3), has been developed and characterised for use in a hybrid system, combining semiconductor and scintillator de vices. Optimal detection system geometry was determined through numerical modelling of system sensitivity, with the BGO array covering a FOV of 120×174×102 mm3 and the high-resolution semiconductor FOV of 62 × 42 × 20 mm3 placed centrally. This design maximises absolute efficiency through the scintillators and spatial resolution through the semiconductors. A coincidence timing resolution of 5.37 ± 0.17 ns and an energy resolution of 30.51 ± 0.48% at 511 keV was measured for the BGO devices, enabling optimisation of coincidence gates and energy level discriminators respectively. Using a novel 3D positioning stage and a 20.11 ± 0.26 kBq Na-22 calibration source, measurements of system sensitivity, spatial resolution and accuracy were performed. Sensitivity profiles were found in agreement with simulation, with a maximal central sensitivity of 34.8 ± 0.6 cps/kBq. Sub-millimetre system accuracy was achieved in all axes except between the BGO detector faces, in which an expected warping effect was identified. Sub-millimetre spatial resolution, σ, was achieved for a maximum location rate per unit activity, L ′, of 0.45 Hz/kBq, with an identified σ = 1.5 √ L′ trade-off to be optimised for specific use cases. The results of this work demonstrate the applicability of PEPT to the study of micro-scale phenomena and outline the path towards hybrid implementation
- ItemOpen AccessModeration of high-energy fast neutrons in beryllium from a tokamak fusion reactor and heat transfer to the cooling water system(2020) Ellis, Benjamin; Leadbeater, Thomas; Hutton, TanyaA modeling demonstration of the moderation of 14.1 MeV primary neutrons in beryllium emitted from a D-T fusion nuclear reaction. The energy deposited from neutron-beryllium interactions which produces heat in the blanket of a fusion tokamak. A review of literature and data available for neutron-beryllium interactions is provided to support the MC software of a simplified model of the ITER first wall and blanket. Energy deposited in regions of the model using FLUKA are used to calculate a polynomial heat flux profile through the model. One dimensional conductive heat transfer through the model is performed and the cooling capacity of the coolant channels via convective heat transfer is explored.
- ItemOpen AccessMulti-photon decay mode spectroscopy of positronium(2022) Johnson, Storm; Leadbeater, Thomas; Jones, PeteAn approximation for the branching ratio of the four-photon decay of parapositronium (BR4γ) was measured using a multi-gamma-ray spectrometer. For the first time in such measurements, the spectrometer consisted of an array of eight identical LaBr3:Ce scintillator detectors, each of which combines good energy resolutions (5% and 10% at 511 keV for the signals from the eighth dynode and anode of the photomultiplier tube, respectively) with an excellent timing resolution (∼ 300 ps). These energy resolutions were minimised through an optimal selection of the digital signal processing parameter settings. The detectors were situated in a planar geometry, where the source-to-detector distance of the detector system was selected such that the effect of peak pulse pile-up was minimised (to less than 3%), while maximising the full-energy peak detection efficiency at 511 keV (to 3%). For this work, locally-produced 22Na radioactive sources were used as positron emitters, which enabled the formation of positronium and subsequent gamma decays. Energy calibration measurements were performed using a 152Eu source, where the prominent energy peaks of (121.8, 244.7, 344.3, 778.9, 964.1, 1408.0) keV were used for calibration. For the BR4γ measurement, 5×1011 events were accumulated over a measurement period of 60 days, which resulted in low statistical uncertainties for the coincident counting between detector pairs (less than 1%). Through simplifying assumptions that neglected the background corrections and efficiency normalisations for each of the 2γ and 4γ decays, a first order approximation of BR4γ was determined as the ratio between measured 4γ events (N4γ) and measured 2γ events (N2γ), such that BR4γ ∼ N4γ N2γ = 4.8 (19) × 10−7. This measured value of BR4γ differs from previous measurements and accepted literature values by a factor of 3.
- ItemOpen AccessThe consequence of station blackout in a nuclear power plant(2021) Olifant, Tshepo; Leadbeater, ThomasThe main purpose of the nuclear power plant is to generate electricity and to supply it to the grid and its auxiliaries systems. Since nuclear safety overrides the production of electricity and therefore nuclear power plants are designed to protect the public and the environment from the radiation release. In order to achieve this design criteria, the nuclear safety regulations requires that the plant must have safety features incorporated in the design to remove the decay heat in a safe manner and to prevent the release of radiation to the environment. Some of the safety systems rely on constant supply of electricity to achieve their function of cooling the reactor core and to safely shutdown the plant. The total loss of Alternating Current power supply to the essential and non-essential equipment of the plant is called Station Black Out. It is critical that the system that removes decay heat in the reactor is effective to prevent the core melt. One of the design basis accidents that the nuclear power plant is design to withstand is loss of coolant accident during Station Black Out. The loss of coolant accident is more likely to occur through the primary pump seals if they do not receive adequate cooling. In this research, the seal leak rate through the primary pump was quantified using a simulator model of a Pressurised Water Reactor type, the quantified leak rates were compared to the recent Station Black Out accident that happened at a Boiling Water Reactor (Fukushima Nuclear plant Unit 1) in Japan. The simulation results of primary pump seals leakage together with the values obtained from Fukushima event were analysed and compared with primary pumps seal leakage flow rates from Westinghouse analysis. Westinghouse assumptions and analysis predicts the primary pump seal leakage during Station Black Out accident to a value of 54m³/h. The Westinghouse predictions were independently verified by Energy Technology Engineering Centre in response to Nuclear Regulation Council concerns regarding seal leakage rates on the Westinghouse Reactor Coolant Pump designs during the Station Black Out. The Energy Technology Engineering Centre calculations developed finite element structural models for all three seals to predict thermal and pressure distortion. The seal leakage of 45m³/h was determined using two-phase correlations based on Dukler constant flow slip flow model [1]. The Pressurised Water Reactor simulation performed in this research predicts a seal leakage of 39m³/h which is not far off from the studies. The radiation release due to Station Black Out was analysed and estimated using a source term code called Rascal. The amount of radiation released by a Pressurised Water Reactor plant determined to be higher than Boiling Water Reactor. The research identified the lessons drawn from the Fukushima accident. The research also highlighted the importance of effectiveness of safeguard systems that are designed to mitigate the accident during SBO.
- ItemOpen AccessValidation and verification of FLUKA for neutron shielding problems(2022) Dondolo, Petrus; Hutton, Tanya; Leadbeater, ThomasMonte Carlo-based radiation transport codes provide an opportunity to simulate situations with various levels of activation and different induced nuclides. However, to test their reliability, it is important to verify the simulation codes by comparing them with experimental data. In this study, validation of simulation models with experiments was performed with the purpose of determining the reliability of the simulation/experimental results. Concrete is the most generally used shield material as it is inexpensive and adjustable for any construction design. Radiation shielding properties of concrete may vary depending on the concrete composites. In this thesis, the fluences (i.e. the flux integrated over time) of neutrons impinging on the shielding nuclear material were studied using FLUKA Monte Carlo package. The rectangular blocks of shielding nuclear materials such as concrete ingredients: cement, sand and water were irradiated with a beam of 14 MeV neutrons and the shielding properties of these materials were investigated using FLUKA Monte Carlo simulation code. The simulation set-up replicates the experimental measurements performed within the nuclear laboratory in the Department of Physics at the University of Cape Town. The comparison of the effective removal cross-section shows a good agreement between experiments and FLUKA. The results from these two approaches show general agreement for sand and cement, but show some minor deviations for water and concrete. The source of these deviations is discussed, along with potential solutions. FLUKA has been well benchmarked and validated against other Monte Carlo codes. The discrepancies obtained on water and concrete may have occurred from the material properties in the input file. Comparisons of results are presented and the discrepancies and agreements between the two methods are discussed for these target materials. The effective removal cross section of a concrete mix was measured by simulation to be 0.1038 +/- 0.0005 cm-1 and by experiment to be 0.1230 +/- 0.0002 cm-1 of 14 MeV neutrons. This illustrates a broad agreement between experiment and simulation in the case of concrete ingredients. Validation and comparison of measured and simulated neutron irradiation on concrete ingredients shows good agreement, supporting the use of FLUKA for estimating the neutron transmission into the shielding material.