Measuring spectral capability of the MARS camera.

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dc.contributor.authorWijesooriya, Manoj
dc.date.accessioned2021-06-24T22:42:16Z
dc.date.available2021-06-24T22:42:16Z
dc.date.issued2021en
dc.description.abstractThe primary goal of my thesis is to improve the image quality of the MARS scanners. I have taken a leading role in much of the assembly and testing of the MARS quad cameras, V5 cameras, and the v6 MARS scanner hardware. One of the first tasks of my thesis is to characterise the energy response of the MARS V5 and V6 cameras at the pixel level. I have introduced a new x-ray fluorescence geometry, which was used to generate x-ray fluorescence photons with a significantly low level of contamination compared to previous methods. The main objective of the characterisation is to improve the energy resolution of the MARS camera and the overall spectral resolution of the MARS imaging chain. The improved spectral performance will help differentiate closely related tissue types, compounds, and elements with high accuracy and precision. Therefore, the outcome of this thesis can be used to improve the specificity and sensitivity of the MARS scanner in a clinical setting. First, a Monte Carlo simulation was conducted to study the efficiency and effectiveness of the pinhole-based inverse broad beam geometry(IBBG). The simulation illustrates that the contamination from primary and Compton photons is significantly low compared to other XRF generation methods. Based on simulation results, the IBBG was utilized to generate XRF using compressed pellets of molybdenum, barium fluoride, europium oxide, tungsten oxide rather than metal foils. The results demonstrate that IBBG is capable of generating strong and accurate XRF peaks. It also proves that salts of metals are more effective in generating XRF than their metal counterparts. In the next part of the thesis, we report on a per-pixel energy calibration procedure using the fluorescence peaks produced from the experiments. The result shows that individual pixels have a range of gain even though the chip was equalized. The difference between global energy response and pixel energy response generates a variation in counts from pixel to pixel. Since the current reconstruction code does not take this energy resolution into account, the reconstructed images are degraded compared to what would be possible. So, I have introduced a count correction method to minimize the photon count variation due to gain variation. Once the correction scheme was applied, the raw energy resolution of the detector was improved by 18%. The design of the Medipix chip integrates several digital to analog converters (DACs), which play an essential role in the spectral performance of the chip. These DACs require application- specific optimization. The final part of the thesis demonstrates the importance of these optimizations. Molybdenum and europium fluorescence peaks were used to investigate the spectroscopic performance of the Medipix3RX chip at different Ikrum DAC settings, bias voltages and temperatures. Results show that Ikrum needs to be kept at specific values to obtain the best possible energy resolution from the detection system. The study also suggests that bias voltage should be kept mid-range to achieve the best possible FWHM. Finally, the chapter shows how temperature is negatively affecting the operation of the Medipx3RX chip. To summarize, this thesis has investigated possible strategies to improve the MARS imaging chain performance and take full advantage of the spectral capabilities of the MARS camera.en
dc.identifier.urihttps://hdl.handle.net/10092/102095
dc.identifier.urihttp://dx.doi.org/10.26021/11150
dc.languageEnglish
dc.language.isoen
dc.publisherUniversity of Canterburyen
dc.rightsAll Rights Reserveden
dc.rights.urihttps://canterbury.libguides.com/rights/thesesen
dc.titleMeasuring spectral capability of the MARS camera.en
dc.typeTheses / Dissertationsen
thesis.degree.disciplinePhysicsen
thesis.degree.grantorUniversity of Canterburyen
thesis.degree.levelDoctoralen
thesis.degree.nameDoctor of Philosophyen
uc.bibnumber3054743
uc.collegeFaculty of Scienceen
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