Stellenbosch University Measuring low concentrations of naturally occurring uranium by analysing the 351.9 keV gamma ray peak of 214Pb. Jacques Bezuidenhout Stellenbosch University South Africa
Naturally Occurring Radioactive Nuclides - Earth’s sciences - Exploration - Environmental monitoring - Classification of soil and rocks - Fossil areas - Mining areas exploration, environmental monitoring and the classification of soil and rocks.
Gamma ray surveys(in situ)
Primordial nuclides Uranium (238U), Thorium (232Th) and Potassium (40K)
Method A Typical Laboratory Gamma Ray Spectrum 214Pb (351.3 keV) 214Bi (1764.5 keV) 208Tl (2614.5 keV) Energy (keV) Counts
Uranium decay 238U gamma lines
Uranium decay 214Bi 212Pb 1764.4 keV 351.9 keV Low efficiency Gamma ray emitting daughters of uranium 214Bi 212Pb 1764.4 keV 351.9 keV Low efficiency High efficiency High resolution Low resolution 15.4% Intensity 35.8% Intensity
Method 1. Sampling - More than a 100 samples - Polypropylene pill containers of 100 ml - Sample mass ranging from 0.110 kg to 0.180 kg. 2. Measuring System - NaI(Tl) detector (7.62 x 7.62 cm) coupled to a scintiSPEC MCA - Surrounded by 15 cm thick lead shielding 3. Calibration and measurements - Efficiency calibration was done with 40K, 238U and 232Th reference materials. - Energy calibration was done from 0.3 to 2.7 MeV. 4. Region of Interest (ROI) - Four counting windows or ROI were selected. - Equation of Rybach (1971, 1988) was adapted for four regions
Method Net count rate in the ROI: ROI’s is i = 0, 1, 2, and 3 (351.3 keV, 1460.8 keV, 1764.5 keV and 2614.5 keV Calibration standards is j = 1, 2 and 3 of 40K, 238U and 232Th. The isotopes n = 1, 2 and 3 of 40K, 238U and 232Th. The net count rate Ri,j: Linear system of i x n (4 x 3) simultaneous equations
Method A Typical Laboratory Gamma Ray Spectrum 214Pb (351.3 keV) 214Bi (1764.5 keV) 208Tl (2614.5 keV) Energy (keV) Counts
Method Solved efficiencies
Measuring uncertainties Method Solve AU’, AK, AU and ATh Measuring uncertainties Detection limits
Results 212Pb peak vs. 214Bi peak R2 = 0.9789
Results 212Pb peak vs. 214Bi peak Coefficient of correlation: 0.9789 Average Statistically uncertainties: 214Pb => 7.1 Bq/kg 214Bi => 4.4 Bq/kg The natural background less than 3 counts/second. Average detection limits: 214Pb => 9.6 Bq/kg 214Bi => 36.8 Bq/kg Samples within detection limits: 214Pb => 76.6% 214Bi => 31.2% 214Pb peak results in shorter acquisition or more accurate uranium concentration
Future - In Situ measurements - Fossil areas - 234Pa measurements - 232Th measurements Protactinium (Pa), Actinoid ... /ˌproʊtækˈtɪniəm/ PROH-tak-TIN-ee-əm
Uranium decay γ-emitters
Baviaansberg Palaeoshoreline