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Presented by: S. M. Hosseini Pooya (PhD)National Radiation Protection Department

Iran Nuclear Regulatory Authority

Development of a diffusion chamber for passive

discriminative measurements of

radon/thoron concentration in dwellings

F. Torabi Nabil, S. M. Hosseini Pooya, M. Taheri, M. Shamsaie Zafarghandi

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Introduction Radon (222Rn) has the most contribution in annually natural exposure of the human via inhalation of the gas. Thoron (220Rn) may have the same effects (even be more hazardous) due to the long lived of its daughters . As well most of the radon detectors are also sensitive to the thoron and may be affected on accurate measurements by the presence of thoron in media. Thus some of the researches are focused on the designing of discriminative systems for separated measurements of these isotopes. Most of the suggested systems have complicated designs (in active methods) and etching conditions (in passive methods). A home-made passive diffusion chamber which is based on SSNTD, has been used for many years for passive radon measurements in dwellings in Iran. In this research first the operation and calibration method of the chamber have been described for both radon and thoron measurements. Then the design is developed for separated measurements of radon/thoron, and the measurement characteristics of the detector are discussed in detail.

Structure of home-made diffusion chamber: for Radon measurements

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4

Electrochemical Etching (ECE) 4

H2O) 45% + C2H5OH 40% + KOH 15%Etchant PEW :Voltage:800 VFrequency: 2 kHzTime: 3 hrTemp.:25 ° C

0.0 0.5 1.0 1.5 2.0 2.5 3.00

25

50

75

100

Alpha Fluence: 1000 / cm2

Etchant: 15% KOH + 40% C2H

5OH + 45% H

2O

ECE Conditions: 32 kV.cm-1 , 2 kHz , 3 hr , 25oC

Alpha Particle Energy (MeV)

Det

ectio

n E

ffici

ency

(%)

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Calibration Method

The volume of calibration chamber for Radon is 50 litersand for Thoron is 4 liters

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0 500 1000 1500 2000 2500 3000 3500 40000

1000

2000

3000

4000 Y=0.83186X

R2=0.90131

Trac

k de

nsity

(tr

acks

cm-2

)

Time-integrated thoron conc. (Bq lit-1 day)0 20 40 60 80 100 120 140 160 180

0

500

1000

1500

2000

2500 Y=13.54957X

R2=0.99989

Trac

k de

nsity

(tr

acks

cm-2

)

Time-integrated radon conc. (Bq lit-1 day)

S= 13.55 (Track.cm-2.Bq-1 .l .day-1) S= 0.83 (Track.cm-2.Bq-1 .l .day-1)

The calibration curve of the chamber for

Radon

The calibration curve of the chamber for

Thoron

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Results of the chamber response in the 4th International Intercomparison of Passive

Radon Detectors (by NIRS –Japan, 2011)

Home-Made Radon Diffusion Chamber

Reference values by NIRS Measured values by diffusion chamber

Activity Exposure time (h)

Rn conc. (Bq. m-3)

Int. Rn conc. (kBq.h.m-3)

Rn conc. (Bq. m-3)

Int. Rn conc. (kBq.h.m-3) Difference

Low 100 1022 ± 59 102 ± 6 1033.6 ± 70 103.36 ± 7.2 1%

Medium 50 8818 ± 221 441 ± 11 8409 ± 720 420.43 ± 36 4.60%

High 100 8753 ± 270 875.3 ± 27 8478 ± 760 847.88 ± 75 3.20%

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Developed diffusion chambers for discriminative measurement of radon and

thoron

A. Double-filter diffusion chamber & 212Po detector

B. Double-film diffusion chamber

Double-filter diffusion chamber + 212Po detector

Developed diffusion chamber: Radon detectorthe outside detector:

212Po detector

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Detector holder

Detector(Lexan 250µm)

Attenuator(Lexan 75µm)

• It attenuates only the particles of 8.78 MeV of 212Po to the energies between 0.5 to 1MeV and absorbs all of the other particles of the daughters.

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75% C2H4(NH2)2+25% H2O

attenuator The thickness of 75±2 μm has been calculated by Trim Code

The thickness of 75±2μm is achievable by soaking of the 250μm Lexan films in the Aqueous solution of Ethylenediamine for 4 hours at 25 ⃘C as :

Sensitivity of the diffusion chamber for radon measurements

The MDL values of the diffusion chamber for radon

measurements0 20 40 60 80 100 120 140 160

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Trac

k de

nsity

(trac

ks c

m-2)

Time-integrated radon conc. (Bq lit-1day)

0 5 10 15 20 25 300

2

4

6

8

Det

ectio

n Li

mit (B

q lit

-1)

Exposure time (day)

S= 8.95 (Track.cm-2.Bq-1 .l .day-1)

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tSC netTDMDL .

2

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Sensitivity of the outer Lexan film for 212Po measurements

0.0 0.1 0.2 0.3 0.4 0.50

1000

2000

3000

4000

5000

6000

Trac

k de

nsity

(trac

ks c

m-2

)

Exposure(Bq Lit-1 day)

S=7.96 (Track.cm-2.Bq-1 .m3 .day-1)

The MDL values of the outer Lexan film for 212Po

measurements

0 5 10 15 20 25 300.000

0.002

0.004

0.006

0.008

0.010

Det

ectio

n Li

mit

(Bq

lit-1

)Exposure time (day)

tSC netTDMDL .

2

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Calculation of Thoron Concentration The outer Lexan film only can measure the concentration of

212Po

Tn

Po

Tn F

CC

21264100913.0

CTn(Thoron concentration)can be calculated by:

The average value of FTn (Equilibrium Factor) is 0.03 in dwellings,

0.913 is the Equilibrium Coefficient,

100/64 is the reverse value of branching ratio in decay chain of 212Po

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Detector holder

Detector

Filter holder

Detector holder

DetectorFiber glass filter

Cap & fiber glass filter

Double-film diffusion chamber

The upper film detects Radon+ Thoron The lower film detects only the Radon

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0 5 10 15 20 25 300

10

20

30

40

50D

etec

tion

Lim

it (B

q lit

-1)

Exposure time (day)

upper film sensitivity=2.06

lower film sensitivity=1.60

S(upper film)=2.06S(lower film)=1.60

Sensitivity of the diffusion chamber for radon measurements

The MDL values of the diffusion chamber for radon

measurements

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0 1000 2000 3000 4000 5000 6000 7000 80000

100

200

300

400

upper film

Trac

k de

nsity

(tr

acks

cm

-2)

Time-integrated thoron conc. (Bq day lit-1)

0 5 10 15 20 25 300

200

400

600

800

1000

1200

1400

Det

ectio

n Li

mit

(Bq

lit-1

)Exposure time (day)

Sensitivity of the diffusion chamber for Thoron measurements

The MDL values of the diffusion chamber for Thoron measurements

S=0.056(Track.cm-2.Bq-1 .l.day-1)

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Conclusion

A home-made passive diffusion chamber which is based on Lexan SSNTD, has been developed for simultaneous measurements of radon/thoron in dwellings.

The sensitivity of the chamber shows that it is appropriate for seasonal measurements of radon in an extended range of concentrations.

The sensitivity of the double-film chamber shows that it is appropriate for seasonal measurements of thoron in areas with high thoron concentrations, while the double-filter method is appropriate for that of low concentration .

The advantages of the method are including its small size, light weight and simple etching process.

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Thank you!