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)

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

Primordial nuclides

A Typical Laboratory Gamma Ray Spectrum

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

214Pb (351.3 keV)

40K (1460.8 keV)

214Bi (1764.5 keV)

208Tl (2614.5 keV)

Energy (keV)

Cou

nts

Method

Uranium decayU-238 gamma lines

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

92.3

8

92.8

0

99.8

5

131.

30

152.

72

186.

21

226.

50

227.

25

241.

99

249.

22

293.

79

295.

22

351.

93

369.

50

568.

90

569.

50

609.

31

699.

03

705.

90

733.

39

742.

81

768.

35

796.

10

805.

80

831.

50

876.

00

883.

24

898.

67

925.

00

925.

90

926.

72

934.

06

946.

00

1120

.28

1238

.11

1377

.66

1393

.90

1407

.98

1509

.22

1729

.59

1764

.49

1847

.42

2204

.21

Gamma energy [keV]

Inte

nsi

ty [

%]

238U gamma lines

Uranium decay

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

Method1. 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

Net count rate in the ROI:

3

1,,,

njnniji AeR

The net count rate Ri,j:

bij

jiji R

t

NR ,

,,

Linear system of i x n (4 x 3) simultaneous equations

Method

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.

A Typical Laboratory Gamma Ray Spectrum

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

214Pb (351.3 keV)

40K (1460.8 keV)

214Bi (1764.5 keV)

208Tl (2614.5 keV)

Energy (keV)

Cou

nts

Method

Solved efficiencies

1,1

1,01,0 A

Re

1,1

1,11,1 A

Re

2,2

2,02,0 A

Re

2,2

2,12,1 A

Re

2,2

2,22,2 A

Re

3,3

3,22,2

2,03,0

3,0 A

AA

RR

e

3,3

3,22,2

2,13,1

3,1 A

AA

RR

e

3,3

3,23,2

2,03,2

3,2 A

AA

RR

e

2,2

2,32,3 A

Re

3,3

3,22,2

2,33,3

3,3 A

AA

RR

e

Method

Measuring uncertainties

21

,

,

1

ss

bnn

nnA tm

RR

en

Detection limits

21

,

,

22

ss

bn

nnn tm

R

eL

ThUKU AeAeAeR 3,0'2,01,0'

ThUKK AeAeAeR 3,12,11,1

ThUU AeAeR 3,22,2

ThUTh AeAeR 3,33,2

Solve AU’, AK, AU and ATh

Method

0.0

50.0

100.0

150.0

200.0

250.0

0.0 50.0 100.0 150.0 200.0 250.0

Uranium activity from ROI 2 [Bq/kg]

Ura

niu

m a

cti

vit

y f

rom

RO

I 0

[B

q/k

g]

Results212Pb peak vs. 214Bi peak

R2 = 0.9789

Results

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

212Pb peak vs. 214Bi peak

Future

- In Situ measurements- Fossil areas- 234Pa measurements- 232Th measurements

γ-emitters

Uranium decay

Baviaansberg

Palaeoshoreline