1. Stellenbosch University
Measuring low concentrations of naturally occurring uranium by analysing the keV gamma ray peak of 214Pb.
Jacques Bezuidenhout
Stellenbosch University
South Africa

2. 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.

3. Gamma ray surveys(in situ)

4. Primordial nuclides
Uranium (238U), Thorium (232Th) and Potassium (40K)

5. Method A Typical Laboratory Gamma Ray Spectrum 214Pb (351.3 keV)
214Bi
( keV)
208Tl
( keV)
Energy (keV)
Counts

6. Uranium decay
238U gamma lines

7. Uranium decay 214Bi 212Pb 1764.4 keV 351.9 keV Low efficiency
Gamma ray emitting daughters of uranium
214Bi
212Pb
keV
351.9 keV
Low efficiency
High efficiency
High resolution
Low resolution
15.4% Intensity
35.8% Intensity

8. Method 1. Sampling - More than a 100 samples
- Polypropylene pill containers of 100 ml
- Sample mass ranging from kg to 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

9. Method Net count rate in the ROI:
ROI’s is i = 0, 1, 2, and 3 (351.3 keV, keV, keV and 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

10. Method A Typical Laboratory Gamma Ray Spectrum 214Pb (351.3 keV)
214Bi
( keV)
208Tl
( keV)
Energy (keV)
Counts

11. Method
Solved efficiencies

12. Measuring uncertainties
Method
Solve AU’, AK, AU and ATh
Measuring uncertainties
Detection limits

13. Results
212Pb peak vs. 214Bi peak
R2 =

14. 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

15. Future - In Situ measurements - Fossil areas - 234Pa measurements
- 232Th measurements
Protactinium (Pa), Actinoid ... /ˌproʊtækˈtɪniəm/ PROH-tak-TIN-ee-əm

17. Uranium decay
γ-emitters

18. Baviaansberg
Palaeoshoreline