RADON SOURCE DETERMINATION BY MEANS OF RADON … · Terrestrial Environmental Physics 7 of 23 Rn...

Terrestrial Environmental Physics

RADON SOURCE DETERMINATION BY MEANS

OF RADON ISOTOPES PROGENY MEASUREMENTS

IN SOIL GASV. Goliáš*, D. Pittauerová*♦, R. Procházka*,

Z. Třískala* & M. Fejgl*☼

* Charles University in Prague♦ University of Bremen

☼ Institute of radiation protection of the Czech Republic

Daniela Pittauerová

Terrestrial Environmental Physics2 of 23

Rn source determination by means of Rn progenymeasurements in soil gas

Contents

• Radon and daughters• Test site

• Construction of apparatus• Experimental• Model

• Results




Radon and daughters

B. Bourdon, et al. (2003): Reviews in Mineralogy and

Geochemistry 2003 52: 1-21.




Radon

• Radioactive noble gas, Z=86, formed by decay of radium• Radon risk: Rn and its solid progeny carcinogenic• Causes significant disequilibrium in natural decay series

(the only gas)• 3 isotopes:

– 222Rn (238U series – T1/2 = 3,8 d)– 220Rn „thoron“ (232Th series - T1/2 = 55,6 s)– 219Rn „actinon“ (235U series - T1/2 = 3,96 s)




Radon and daughtersU-238 U-235

Ra-226 1600 y

Ra-223 11.4 d

Rn-222

3.8 d Rn-219

3.96 s

Po-218 3.1 min

Po-214 0.14 ms

Po-210 138 d

Po-215 1.87 ms

Bi-214 19.9 min

Bi-210 5 d

Bi-211 2.15 min

Pb-214 26.8 min

Pb-210 22.3 y

Pb-206 stable

Pb-211 36.1 min

Pb-207 stable

Tl-207 4.77 min

Alpha decay Beta decay

„Radon“ „Actinon“




Objectives

• Original goal: construction of an apparatus proposed for determination of long-lived radionuclides (234Th, 226Ra and 210Po)

• Determination of other alpha emitting radionuclides in soil gas– electrostatic deposition of radionuclides from

soil gas– alpha spectrometry




Test site, location

• Joachimsthal, Erzgebirge, Czech Republic• Polymetallic Ag-Co-Ni-Bi-U deposit• 450 years old waste rock dump after Ag mining, Geister

vein• area of about 5000 m2: detailed radiometric and

geophysical survey– known geological background (rocks, faults, veins)– volume activity of 222Rn in soil gas (up to 1300 kBq/m3)– specific activity of radionuclides (238U and 232Th series),

surface and depth distribution




Geister site, Joachimsthal







Waste rock dupm cross-section

up to 4500 Bg/kg 226Ra




Construction of apparatus for electrostatic deposition of radionuclides

from soil gas

• Different approach:

– Collection of positively charged aerosol particles not directly at the detector, but rather on a circular stainless steel target with a diameter of 22 mm




Construction of apparatus for electrostatic deposition of radionuclides

from soil gas• Field sampling apparatus: compact source of HV-, filter

body, actual sampling chamber + emanometric pump




Sampling

• The soil gas was sampled by emanometric probe fromdepth of 1.2 m from the entire soil profile




Alpha-spectroscopy

• Samples: stainless steel disks with radionuclides deposited• Si-barrier detector with an active surface area of 450 mm2

located in a vacuum chamber • The signal from the detector was processed by a ORTEC

142A preamplifier and Canberra series 10 plus multichannel analyser

• Energy calibration: 241Am-239Pu• High resolution (FWHM 50 - 60 keV ) was achieved thanks

to usage of vacuum chamber




Alpha-spectroscopy




Short-term deposited samples

• The first measured spectrum of short term deposited sample AM8 with prevailing 218Po

• A progression of the time series measured on short term deposited sample AM 8

E [MeV]

3 4 5 6 7 8 9

Cou

nt p

er c

hann

el

0

10

20

30

40

50

60

70

218PoAM 8

214Po




Modelling

• 218Po a 214Po activities measured on long-term deposited sample AM9 compared to mathematical model

• Escape of some 214Po atoms from the sample –recoil by alpha decay

t [min]

0 20 40 60 80 100 120 140 160

A [B

q]

0

10

20

30

Asat.

Amax.

214Po

218Po

r=0.25

r=0

AM9

ts t1

end of sampling




211Bi activity

• Sample from the horizon of active dump material

• Sample from the vein outcrop

Col 1 vs Col 2 Col 1 vs Col 2 Col 1 vs Col 2 Col 1 vs Col 2 Col 1 vs Col 2

E [MeV]

5 6 7 8

Cou

nt p

er c

hann

el

0

10

20

30

40

50

218Po

214Po

211Bi

211Bi

GD 5

E [MeV]

5 6 7 8

Cou

nt p

er c

hann

el

0

20

40

60

80

100

AM 9

218Po

214Po




211Bi/214Po ratio

• differs in different geological situations• is very sensitive to the input 219Rn/222Rn ratio




211Bi/214Po ratio

• Radon age: time when the mixture of radon and actinonwas separated from their source




Rn agesample 211Bi/214Po sigma + sigma -

sigma

Rn age

[ s ] [ s ] [ s ] GD 4 0.155 0.022 -7.66 0.94 1.63

GD 5 0.0607 0.0051 -3.19 0.46 0.92

GD 7 0.041 0.020 0.50 2.27 6.09

GD 9 0.049 0.020 0.95 1.96 4.95

AM 9 0.0131 0.0013 5.64 0.55 1.14

GD 13 0.046 0.011 -1.07 1.23 2.79




Long-lived radionuclides

• Long-lived radionuclides (234Th, 226Ra a 210Po) measured after returning from the field

But• their activities under the detection limit of the alpha

spectrometer (below 0.1mBq)




Conclusions

• Developed apparatus is suitable for sampling of short time progeny of radon from soil gas.

• The method is useful for determining relative depth of the radon source and for distinguishing surface sources from deep sources.

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RADON SOURCE DETERMINATION BY MEANS OF RADON … · Terrestrial Environmental Physics 7 of 23 Rn...

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