Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
ELECTRET ION CHAMBERS (EIC) TO MEASURE RADON
EXHALATION RATES FROM BUILDING MATERIALS
P. Kotrappa and F. Stieff(1)
Rad Elec Inc., 5714-C industry Lane
Frederick, MD 21704, USA
301-694-0011 Fax: 301-694-0013
Pkotrappa@aol.com
ABSTRACT
Electret ion chambers (EIC) have been used for measuring radon in water. This method
uses a four liter jar with rubber seals as radon leak tight accumulator. In view of
increased interest in measuring radon exhalation rates from building materials, the
method is now extended for this purpose. Required equations are derived to compute the
exhalation rates for granite samples. As an illustration, a set of granite slabs are
characterized by using different accumulation times. Fluxes measured on typical
commercially available granites range from 20 to 30 Bq m-2 d-1. These are similar to the
published results for granite samples. This is an additional useful application for users of
EIC.
INTRODUCTION
Four liter jars with rubber seals have been used as radon leak tight accumulators. These
have been successfully used for measuring radon in water and also for characterizing
NIST (National Institute of Standards and Technology) emanation standards. In view of
increased interest in measuring radon exhalation rates from building materials, the
method is now adapted for this purpose. Required equations are derived to compute the
exhalation rates by measuring average radon concentration for a given accumulation
period. As an illustration, a set of granite slabs are characterized by using different
accumulation times.
(1) The authors are developers of, and have a commercial interest in the electret ion
chamber featured in this paper.
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
MATERIALS AND METHODS
An accumulator is simply a container of known volume that can be sealed radon leak
tight. The sample and the detectors are enclosed inside the container that serves as an
accumulator. Integrating radon detectors such as EIC (electret ion chamber) are used for
measuring integrated averages over the accumulation time. See Figure-1.
Typical accumulator successfully used for several applications is simply a glass jar with a
nominal volume of 4 liters, with a sealable rubber collar (Kotrappa, 1993; Kotrappa,
1994).
Radon concentration (Aldenkamp, 1992) at any time T is given by a well known equation
(1):
C ( Rn) =
0.1814T
( FXA)
(1 − e −
)
VX 0.1814
Equation (1)
When C (Rn) in equation (1) is integrated and divided by the total time, it leads to time
averaged radon concentration (Kotrappa, 1994) after accumulation time of T days. This
leads to equation (2)
( F × A ) 1 − e − 0.1814T
1 −
C ( Rn) Av =
V × 0.1814 0.1814T
Equation (2)
F is the radon flux in Bq m-2 d-1
A is the area in m2
(F x A) is the exhalation rate in Bq d-1
0.1814 is the decay constant of radon in d-1
C(Rn) is the radon concentration at any accumulation time of T (days) in Bq m-3
C(Rn) Av is the integrated average radon concentration in Bq m-3
T is the accumulation time in days
V is the air volume of the accumulator in m3
If we call K as the constant inside the bigger bracket in equation (2), equation (2) can be
rewritten as equation (3) and equation (4).
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
Note K depends only on time of exposure in day units. For those who do not have access
to spread sheet, a table can be built to provide K values for different T values. Such table
in conjunction with equation (4) is used for hand computation.
C ( Rn) Av =
(F × A )
V × 0.1814
×K
Equation (3)
(F x A)= C ( Rn) Av x V x 0.1814 / K
Equation (4)
All parameters on right hand side of equation (4) are either measurable or computable.
Exhalation rate (FxA) is calculated using equation (4). Further dividing exhalation rate by
the area of the sample leads to the flux.
Table-1 gives average radon concentration for different accumulation times for
exhalation rate (F x A) of 1 Bq d-1. Figure 2 gives a graphical representation of the build
up of time averaged concentration for stated accumulation time.
PROTOCOLS
Accumulator and sample size
This protocol describes the method of using sealable jars and EIC radon monitors for
measuring radon emanation rates from granite slabs. Sample size of 7.8 cm long, 8.9 cm
wide and 3.1 cm thick, or of any other suitable sizes are usable in this method.
Sealable glass jars have been used successfully used for measuring radon in water
(Kotrappa, 1993) and in using NIST (National Institute of Standards and Technology)
sources (Kotrappa, 1994) for calibrating passive detectors. These are of nominal volume
of 4 L, with arrangement to seal and suspend an electret ion chamber. Sample is
introduced inside the jar, held by small adhesive pivots at the bottom of the jar. Figure 2
gives a sketch of the arrangement.
Air Volume of the accumulator
One of the parameter needed is the air volume inside the jar. This is obtained by
subtracting air volume occupied by the sample and the detector from the volume of the
jar. Precisely measured air volume of the jar with one EIC is 3.843 L 9Kotrappa, 1994).
The volume of the sample is 0.215 L. Therefore, the net air volume is 3.628L.
Time averaged radon concentration
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
Most suitable EIC is the SST E-PERM® (Kotrappa, 1990). Stick two small adhesive clay
(3 mm thick) pieces at the bottom of the sample. This keeps the sample above the bottom
of the jar, allowing radon to escape from the bottom part of the sample. Position the
sample at the bottom of the jar, suspend a pre-measured EIC, and close the jar and tighten
the seal. The measurement has started. At the end of the desired exposure period, remove
the EIC, calculate the radon concentration C ( Rn) Av . Set up a similar arrangement
without a sample to obtain background radon concentration C ( Rn) Av . The net radon
concentration is obtained by subtracting the background concentration from the
concentration measured with the sample.
Calculation of the exhalation rate and flux
Use equation (4) to calculate the exhalation rate from the sample. Divide the exhalation
rate by the area of the sample (0.0243 m2) to calculate flux.
Error associated with measurement is simply the errors expected in radon measurements.
Other errors are negligible. Methods of calculating errors are given in reference
(Kotrappa, 1990).
ILLUSTRATIVE MEASUREMENTS AND DISCUSSIONS
Total of five samples are obtained from commercial granite Supply Company, cut to the
required sample size. Results of the measurements done for accumulation time of 2, 3, 5
and 7 days, are given in Table 2. Fluxes measured are in the range of 20 - 30 Bq m-2 d-1.
DISCUSSION AND CONCLUSIONS
Fluxes measured are in the range of 20 to 30 Bq m-2 d-1. These are similar to the
published results for granite samples. Reference [5] gives values from 7 to 29 Bq m-2 d-1
and reference (Hazal-ur-Rehman, 2003) gives an average of 32.4 Bq m-2 d-1.
Reproducibility of measurements done on the same sample at different accumulation
times, are with in the expected range.
Large number of EIC users with E-PERM® system (Kotrappa, 1990) can easily use this
method for measuring the required parameters. Being a small accumulator, it is not
possible to use larger samples. However the equations given in this note can be used for
any other sealable accumulator and other integrating passive or active radon monitors. If
sample is likely to leave debris inside the accumulator, it is advisable to enclose the
sample in radon transparent bags such as Tyvek bags. Standard EICs do not have
significant sensitivity to thoron as such the results are truly for radon.
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
Figure -1 Radon Exhalation Measurement
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
Table-1 Average radon concentration for stated accumulation time for exhalation
rate of 1 Bq/day
(F X A)
-1
Bq d
1
1
1
1
1
1
1
1
1
1
Acc time
Days
1
2
3
4
5
6
7
8
9
10
Vol
3
m
0.00363
0.00363
0.00363
0.00363
0.00363
0.00363
0.00363
0.00363
0.00363
0.00363
K
0.08545563
0.16131633
0.22878692
0.28891133
0.34259494
0.39062373
0.43368073
0.4723601
0.50717923
0.53858921
Rn Av.
-3
Bq m
129.8
245.1
347.6
439.0
520.5
593.5
658.9
717.7
770.6
818.3
Rn Av. Bq/m3 for Exhalation of
1 Bq/day
Rn conc. Bq/m3
1000.0
800.0
600.0
400.0
200.0
0.0
0
5
10
15
Accumulation time in days
Figure 2 Build up of time averaged radon concentration for stated accumulation
time
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
Table-2 Results of measurements
Acc Time
days
3
3
3
5.88
5.88
5.88
Sample #
#1
#2
#3
#1
#2
#3
Rn Conc.
-1
pCi L
7.61
6.58
5.23
13.06
12.97
9.86
FxA
-1
pCi d
FxA
-1
Bq d
F
-2 -1
Bq m d
F
-1
-1
Bqd Kg
21.89
18.93
15.04
22.32
22.16
16.85
0.81
0.70
0.56
0.83
0.82
0.62
33.33
28.82
22.91
33.98
33.75
25.65
1.47
1.27
1.01
1.50
1.49
1.13
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008
REFERENCES:
1.
P.Kotrappa, J.C.Dempsey, R.W.Ramsey, and L.R.Stieff “A practical E-PERM™
(Electret passive environmental system for indoor Rn-222 measurement” Health
Physics 58:461-467 (1990)
2.
F.J.Aldenkamp, R.J. de Meijer, L.W.Put and P.Stoop
“An assessment of in situ radon exhalation measurements and the relations
between free and bound exhalation rates” Radiation Protection Dosimetry 45:449453 (1992)
3.
P.Kotrappa and W.A Jester “Electret ion chamber radon monitors measure
dissolved 222Rn in water” Health Physics 64:397-405 (1993)
4.
P.Kotrappa and L.R.Stieff “Application of NIST Rn-222 emanation standards for
calibrating Rn-222 monitors” Radiation Protection Dosimetry 55:211-218 (1994)
5.
N.W.El-Dine,A.Shershaby, F.Ahmed, and A.S Abdel-Haleem “ Measurement of
radioactivity and radon exhalation rate in different kinds of marbles and granites”
Appl Radiat. Isot. 55:853-860 (2001)
6.
Hazal-ur-Rehman, Al-Jarallah, Musazay, and Abu-Jarad “Application of the can
technique and radon gas analyzer for radon exhalation measurements” Appl
Radiat Isot. 59:353-358 (2003)
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