Design, construction and characterisation of a low-level radioactivity counting system based on gamma-ray spectrometry with LaBr3:Ce scintillator detectors

Bashir, Munirat (2020-04)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

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.

AFRIKAANSE OPSOMMING: Die aktiwiteitskonsentrasies in monsters van radioaktiewe materiaal wat natuurlik voorkom (NORM) word normaalweg gemeet met 'n gammastraal spektrometer wat uit 'n enkele detektor (meestal HPGe of NaI:Tl) bestaan en omhul is in 'n loodskerm. In hierdie werk is 'n passiewe waterskerm, bedoel om die agtergrondstraling wat die detektors kan bereik te verminder, ontwerp met behulp van GEANT4 Monte Carlo simulasies en dan gebou. Volume grondmonsters wat in Marinelli bekers geplaas is, is gemeet in enkelen gelyktydige-modus met behulp van vier LaBr3:Ce detektors sonder afskerming; twee LaBr3:Ce detektors sonder afskerming en dieselfde twee LaBr3:Ce detektors binne die geboude waterskerm. Die monsters is ook gemeet met behulp van 'n NaI:Tl detektor binne die waterskerm en 'n standaard HPGe detektor, wat met lood afgeskerm is, om die resultate van metings met die LaBr3:Ce detektors te vergelyk en te bekragtig. 'n Nuwe metode vir die vermindering van agtergrond, deur gebruik te maak van foton-vlugtyd, is aangewend saam met die gelyktydige meting van die twee fotonenergiee. Beide die gesimuleerde en gemete resultate toon dat die waterskerm gammastrale van energie 2614.5 keV ( 208Tl / 232Th-reeks) met 90 % verminder. Hierdie energie is die maksimum vol-energie piek in die gammastraalspektra van NORMs. Die aktiwiteitskonsentrasies van 238U en 232Th-reeks en 40K radionukliede in IAEA-375 grond is bepaal deur middel van die NaI:Tl detektor binne die waterskerm. Die teenwoordigheid van 138La en 227Ac in die kristal van die LaBr3:Ce detektor verminder die minimum waarneembare aktiwiteit (MDA) by gammastraal energiee van 1460.8 keV (40K) en 2614.5 keV. Die gemete interne aktiwiteit van 138La in die LaBr3:Ce detektorkristal is 263.8 ± 26.8 Bq kg−1 , wat vergelykbaar is met die berekende aktiwiteit van 293.3 Bq kg−1 . Die aktiwiteitskonsentrasie van 40K in IAEA-375 grond en strandsand is laer as die MDA, soos gemeet met die LaBr3:Ce detektor geometrie. Die aktiwiteitskonsentrasies van 238U en die 232Th-reeks radionukliede in IAEA-375 grond is in gelyktydige-modus bepaal met behulp van die vier LaBr3:Ce detektors sonder afskering. Die resultate van die gemete aktiwiteitskonsentrasies van 238U en 232Th-reeks en 40K radionukliede in IAEA-375 grond het binne eksperimentele onsekerheid met geserti seerde waardes ooreengestem. Die aktiwiteitskonsentrasies van 238U en die 232Th-reeks radionukliede in strandsand is in beide enkel- en gelyktydige-modus bepaal. Die resultate van die gemete aktiwiteitskonsentrasies van 238U en 232Th-reeks radionukliede in strandsand, soos gemeet met LaBr3:Ce en NaI:Tl detektors, het binne die 1σ tot 2σ vlak ooreengestem met die gemeet met die HPGe detektorstelsel. Die waterskerm (50 cm dikte) het goed gevaar om agtergrond gammastrale van 2614.5 keV met 90 % af te skerm, en gammastraal energiee minder as 2614.5 keV met meer as 90 % af te skerm tydens lae-vlak radioaktiwiteitsmetings. Die interne aktiwiteit van die LaBr3:Ce detektor verhoog die MDA by 1460.8 keV en 2614.5 keV, en dit beperk die meting van 40K radionukliede met 'n lae aktiwiteitskonsentrasie in enkel-modus. Die gebruik van die gelyktydige-modus saam met foton vlugtyd onderdruk die verstrooide- / agtergrondstraling beduidend.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108111
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