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Measuring low concentrations of naturally occurring uranium by analysing the 351.9 keV gamma ray peak of 214 Pb. Jacques Bezuidenhout Stellenbosch University South Africa. Naturally Occurring Radioactive Nuclides. - Earth’s sciences - Exploration - Environmental monitoring

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Jacques Bezuidenhout Stellenbosch University South Africa

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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


Gamma ray surveys(in situ)


Primordial nuclides

Uranium (238U), Thorium (232Th) and Potassium (40K)


214Pb

(351.3 keV)

Counts

40K

(1460.8 keV)

214Bi

(1764.5 keV)

208Tl

(2614.5 keV)

Energy (keV)

Method

A Typical Laboratory Gamma Ray Spectrum


Uranium decay

238U gamma lines


Uranium decay

Gamma ray emitting daughters of uranium


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


214Pb

(351.3 keV)

Counts

40K

(1460.8 keV)

214Bi

(1764.5 keV)

208Tl

(2614.5 keV)

Energy (keV)

Method

A Typical Laboratory Gamma Ray Spectrum


Method

Solved efficiencies


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


Uranium decay

γ-emitters


Baviaansberg

Palaeoshoreline


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