Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "Bashir, Munirat"

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    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.