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
1
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.
2
Structure of home-made diffusion chamber:
for Radon measurements
3
4
Etchant PEW: %15 KOH + %40 C2H5OH + %45 H2O)
100
Detection Efficiency (%)
Voltage:800 V
Frequency: 2 kHz
Time: 3 hr
Temp.:25 ° C
75
50
25
Alpha Fluence:
Etchant:
2
1000 / cm
15% KOH + 40% C 2H5OH + 45% H2O
ECE Conditions: 32 kV.cm
-1
o
, 2 kHz , 3 hr , 25 C
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Alpha Particle Energy (MeV)
4
The volume of calibration chamber
for Radon is 50 liters
and for Thoron is 4 liters
5
The calibration curve
of the chamber for
Radon
Y=0.83186X
2
R =0.90131
4000
Y=13.54957X
2
R =0.99989
-2
Track density (tracks cm )
-2
Track density (tracks cm )
2500
The calibration curve
of the chamber for
Thoron
2000
1500
1000
3000
2000
1000
500
0
0
0
20
40
60
80
100
120
Time-integrated radon conc. (Bq lit
S=
140
-1
160
0
180
1000
1500
2000
2500
3000
3500
4000
-1
Time-integrated thoron conc. (Bq lit day)
day)
13.55 (Track.cm-2.Bq-1 .l .day-1)
500
S=
0.83 (Track.cm-2.Bq-1 .l .day-1)
6
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.
Difference
(kBq.h.m-3)
Low
100
1022 ± 59
102 ± 6
1033.6 ± 70 103.36 ± 7.2
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%
1%
7
8
Developed diffusion chamber:
Radon detector
the outside detector:
212Po detector
9
10
Sensitivity of the diffusion chamber
for radon measurements
2000
1600
1400
1200
1000
800
The MDL values of the diffusion
chamber for radon
measurements
600
400
200
0
0
20
40
60
80
100
120
140
160
Time-integrated radon conc. (Bq lit-1day)
S=
8.95 (Track.cm-2.Bq-1 .l .day-1)
8
-1
Detection Limit (Bq lit )
Track density (tracks cm-2)
1800
6
C MDL 
4
2 TD net
S .t
2
0
0
5
10
15
20
Exposure time (day)
25
30
11
Sensitivity of the outer Lexan
film for 212Po measurements
The MDL values of the outer
Lexan film for 212Po
measurements
0.010
-1
Detection Limit (Bq lit )
-2
Track density(tracks cm )
6000
5000
4000
3000
2000
1000
0
0.0
0.1
0.2
0.3
Exposure(Bq Lit
0.4
-1
0.5
day)
C MDL 
2 TD net
S .t
0.008
0.006
0.004
0.002
0.000
0
5
10
15
20
25
30
Exposure time (day)
S=7.96 (Track.cm-2.Bq-1 .m3 .day-1)
12
 The outer Lexan film only can measure the concentration of
212Po
0 . 913 
C Tn 
100
64
FTn
 C 212 Po
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
212
Po
13
 The upper film detects Radon+ Thoron
 The lower film detects only the Radon
14
The MDL values of the
diffusion chamber for radon
measurements
50
upper film
-1
Detection Limit (Bq lit )
sensitivity=2.06
lower film
40
sensitivity=1.60
30
20
10
0
0
5
10
15
20
Exposure time (day)
S(upper film)=2.06
S(lower film)=1.60
25
30
15
Sensitivity of the diffusion chamber
for Thoron measurements
The MDL values of the diffusion
chamber for Thoron measurements
upper film
1400
-1
Detection Limit (Bq lit )
-2
Track density (tracks cm )
400
300
200
100
0
1200
1000
800
600
400
200
0
0
1000
2000
3000
4000
5000
6000
7000
8000
-1
Time-integrated thoron conc. (Bq day lit )
0
5
10
15
20
25
30
Exposure time (day)
16
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.
17
Thank you!
18