Radon Monitor Calibration using NIST Radon Emanation Standards:
Steady Flow Method
P. Kotrappa and L.R.Stieff
5714-C Industry Lane, Frederick, MD 21704, USA
and
P. Volkovitsky
National Institute of Standards and Technology
Gaithersburg, MD 20899, USA
ABSTRACT
The National Institute of Standards and Technology (NIST) Polyethylene-Encapsulated
226
Ra/222Rn Emanation (PERE) Standards (old SRM 4968 and new SRMs 4971, 4972,
and 4973) provide precise radon emanation rate, certified to a high degree of accuracy
(approximately to 2%). Two new SRM 4973 standards containing total of 1036 Bq
(0.028 μCi) of 226Ra, emanate 0.114 Bq (3.08 pCi) of 222Rn per minute. Air passing over
such sources at a flow rate of 1 L min will have a radon concentration of 114 Bq m3
(3.08 pCi L1). This paper describes a practical calibration system and the actual
calibration verification data obtained at different flow rates, for E-PERM® passive radon
monitors, Femto-Tech® and Alpha Guard® Continuous Radon Monitors. Use of such an
affordable and easy to use system by the manufacturers and users of radon measurement
devices will bring uniform standards with traceability to a NIST standard source and is
considered an important step in standardizing radon measurement methods.
NOTE: This paper is in the process of publication in the Journal of Radiation Protection
Dosimetry (2005)
INTRODUCTION
The new series of the National Institute of Standards and Technology (NIST)
Polyethylene-Encapsulated 226Ra/222Rn Emanation (PERE) Standards (SRM 4971, 4972,
and 4973) (1) provide precise radon emanation rate, certified to a high accuracy of about
2% (2 sigma uncertainty). Calibrating commercially available radon monitors, using such
sources, is an important step in standardizing the calibration procedures for radon
monitors. The low strength sources, SRM 4971 and 4972, with 226Ra activity of 5 and 50
Bq are suitable for use in accumulation mode. Detailed procedures for using such sources
are described in a related publication (2) .One of the problems encountered in using the
sources in accumulation mode was enclosing the detectors in airtight enclosures and
knowing the precise air volume of the enclosure. The stronger sources (SRM 4973) with
226
Ra activity about 500 Bq can be used in a continuous flow mode, overcoming the
problems encountered in using the sources in accumulation mode. The present work
describes the design and performance of a practical calibration system. As an illustration,
the system is further used to verify the calibration of some popular radon monitoring
systems.
MATERIALS AND METHODS
Calculation of expected radon concentration
Two factors that determine the emanation rate of a NIST standard are the 226Ra activity
and the emanation fraction. Both of these factors are certified with small uncertainties.
Equation (1) gives the radon emanation rate:
R (222Rn) = f · A (226Ra) · λ
(1)
Here R (222Rn ) is the radon activity emanation rate by the standard in Bq s
f is the emanation fraction,
A (226Ra) is the activity of 226Ra in Bq,
λ is the decay constant of 222Rn in s (2.09822 · 10 s),
Radon concentration in air flowing over the source, C (222Rn), is calculated using equation
(2).
C (222Rn) = R (222Rn) / F
Here
C(
222
Rn)
(2)
3
is the concentration in Bq m ,
R (222Rn) is the radon emanation rate in Bq s,
F is the flow rate in m3 s.
For SRM 4973, the emanation rate R can be calculated with two sigma uncertainty of
about 2%. In our tests air goes through a calibration chamber with a volume of 40 liters.
This volume is large enough to hold most practical radon monitors. It can have
penetrations for power chords. Reasonable sealing is required.
For a flow rate of 1 L min1
A (226Ra) = (1036 ± 6) Bq;
f = 0.8736 ± 0.0063;
= (2.09822 ± 0.00016)·10 s;
R (222Rn) = (0.001899 ± 0.000017) Bq s;
F = 1 L min = (1.667± 0.017)·10 m3 s
Radon concentration in 40 L of the air at a flow rate of 1 L min, C = (113.9 ± 1.6) Bq m3.
Experimental setup
Figure 1 shows the schematic of the experimental arrangement. Air is drawn from
ambient environment with low radon concentration, through a filter and a flow meter
with a regulator. The output of the pump is taken through a primary flow calibrator (3), the
NIST source holder into the calibration chamber. Volume of cylindrical calibration
chamber is about 40 liters. The lid of the calibration chamber can be opened and closed
for introducing detectors. Reasonable leak tightness is required. Power cord penetrations
are sealed using commercially available materials. The air from the calibration chamber
is let out to the room exhaust or to outside the building. The radon-laden air is injected at
the top and the let out from the tube that starts from near the bottom and out. This
arrangement is intended to provide mixing of the radon-laden air inside the chamber.
Experimental Procedure
The detectors that need to be calibrated or whose calibrations need to be verified are
installed inside the chamber and chamber is closed. Start the pump and adjust the desired
flow rate using flow meter with control. It is not desirable to use the primary flow
calibrator continuous mode over extended period. Because of this reason the exact flow
rate was determined in the beginning and at the end of calibration procedure. With a
good quality pump and a good quality flow controller used in the current work, once set,
the flow rates did not change more than ± 2 % over 2 to 7 days. At the end of the
calibration period, radon monitors are taken out and measured concentrations are
recorded.
Experiments at different flow rates
The calibration verification was done on two types of continuous radon monitors and one
type of passive integrating radon monitors. The Alpha Guard® (4) continuous radon
monitor manufactured by Genitron instruments in Germany and the Femto-Tech® (5)
continuous radon monitor manufactured by Femto-Tech Corporation in USA, are used in
this study. Both these monitors operate on the principle of pulsed ionization chamber and
use sophisticated data processing software to analyze the results. The E-PERM® (6)
electret ion chambers are widely used passive integrating radon monitors manufactured
by Rad Elec Inc. in USA. These are more fully described (2) in the associated publication
that described the use of NIST source in accumulation mode.
At the beginning of the test, the desired flow rate is set. Air flows through the NIST
source holder for at least two hours to establish the initial condition, before introducing
the air stream into the test chamber. In one experiment, two continuous radon monitors,
Alpha Guard® (AG) and Femto-Tech® (FT) are installed inside the calibration chamber.
The chamber is closed. These log the data in their memory for nearly 24 hours. Results
are read out at the end of the test period. Such a study is repeated for different flow rates
starting from about 0.1 L min to about 1 L min. In another experiment, sets of E-PERM®
(EP) radon monitors are installed inside the chamber for 2 to 3 days.
Precision of parameters
NIST certifies the radon emanation fraction with an uncertainty of ± 2 % at 2 sigma level.
The primary flow calibrator (3) used in this work is NIST traceable and certified with an
uncertainty of ±1 % at 1 sigma level.
RESULTS
Figure 2 shows the data from AG continuous radon monitor for different flow rates.
Steady state concentration is the mean of radon concentrations in the plateau region. This
is computed by the software. NIST radon concentration is calculated using equations (1)
and (2) and parameters of the NIST capsules. In these experiments, the inlet air comes
from the ambient air with a radon concentration of about 15 Bq m3. This background also
contributed into the detector readings in addition to the NIST capsules contribution. This
background was measured without the NIST capsules and subtracted from measurements
with capsules.
Table 1 gives the summary of test results. The first column gives the flow rates. The
second column gives calculated NIST predicted concentration. The third column gives
decay corrected predicted concentration. Column 4 gives the mean steady state radon
concentration as measured by AG unit after subtracting the background. Column 5 gives
the ratio of measured to predicted concentration. Column 5 and 6 give data for FT unit.
From Table 1, it follows that the continuous radon monitor results agree very well with
the NIST predicted radon concentration.
Because of the large volume (40 liters) of the exposure chamber and relatively low flow
rates (0.1 L min to 1 L min), finite time is required for the radon concentration to reach
the steady sate. Using data charts of Fig 2, it is possible to estimate the time to reach the
steady state (TTRSS) in 40-liter exposure chamber. Further as expected, the time to reach
the steady state depends upon the flow rate. The lower the flow rate, the longer is the
TTRSS.
The column 2 of Table.2 gives the estimated TTRSS in hours. It varies from about 2
hours to 12 hours. This is of significance while calibrating passive integrating radon
monitors. This should be small compared to the total duration of measurements. For
example, for a 48-hour calibration with the flow rate in the range of 0.4 L min the
TTRSS is about 5 hours and will not affect the calibration significantly. However,
calibration over extended period is desirable. Anther parameter of interest is the mean
residence time (MRT) for radon. This time is calculated by dividing the volume in liters
by the flow rate in L min. This time (in hours) is given in column 3. Column 4 gives the
radon decay correction for the MRT. The NIST predicted radon concentration should be
multiplied by this coefficient for calculating decay corrected radon concentration. For
example, the calculated radon concentration given in column 2 (Table 1) is multiplied by
the decay correction factor (column 4 of Table 2) and introduced in column 4 of Table 1.
As can be seen this correction is negligible for flow rates of 0.4 L min and higher.
However, the corrections become significant at low flow rates. Tables 3 and 4 give the
results for EP electret passive radon monitors at two different flow rates; 0.281 and 0.602
L min, at two different duration of exposure. The detailed methods of analysis of these
detectors are given in a related publication (2).
Results are excellent indicating that these monitors have been calibrated well.
The arrangement for the passage of radon-laden air through the chamber appears to
provide sufficient mixing, as demonstrated by the results.
When air is taken from ambient environment, the background can be varying and may
introduce some uncertainty. For the continuous radon monitors of pulsed ion type,
nitrogen can be used as carrier gas to do away with this uncertainty. This can not be done
for E-PERMs, because the calibration of E-PERMs can be different for different carrier
gases.
CONCLUSIONS
The NIST sources provide an easy method of calibrating radon monitors. The steady
flow method offers many advantages over the accumulation mode (2). First, the
requirements to the sealing of the exposure chamber with radon counters are not as high
as in the case of accumulation method because of slight positive pressure inside the
exposure chamber. Second, it is not necessary to know the exact air volume of the
chamber. A 24-hour run is recommended for calibrating continuous monitors, so as to
provide multiple data points to calculate the steady state radon concentration. Because of
the relatively large exposure chamber (40 liters), the flow rates should be larger than 0.5
L min1 to minimize the correction due to decay of radon. For calibration of integrating
passive radon monitors, the recommended flow rate is above 0.5-L min1 and the exposure
duration of three days or above. Outside these limits, corrections are required.
A 40-liter exposure chamber is large enough for several instruments at a time. It is
important to have a good pump with a flow controller as well as a primary flow calibrator
with a precision of about 1 % to take full advantage of the high precision of the NIST
certified radon emanation fraction of about 2%. Reasonably priced commercially
available components meet these needs. The more active NIST capsules (with 5000 Bq
activity of 226Ra) would be more convenient for steady flow calibrations. If
manufacturers and the primary users of radon detector use this method many ambiguities
present in radon data will be resolved.
ACKNOWLEDGEMENT
This work is carried as a part of the ongoing CRADA (Cooperative Research and
Development Agreement) Program No.CN-1818 between Rad Elec Inc. and National
Institute of Standards and Technology. NOTE: This paper is in the process of publication
in the Journal of Radiation Protection Dosimetry (2005).
REFERENCES
1. Collé, R. Kotrappa, P and Hutchinson, J.M.R “calibration of Electret-Based Radon
Monitors Using NIST Polyethylene-Encapsulated 222Rn Emanation (PERE)
Standards” Journal of Research of the National Institute of Standards and Technology
Vol. 100, 629-639, 1995
2. Kotrappa, P and Stieff, L.R. “Application of NIST 222Rn Emanation Standards for
Calibrating 222Rn Monitors” Radiation Protection Dosimetry Vol. 55, 211-218, 1994
3. Bios International Corporation, 10 Park Place, NJ 07405 USA
4. Genitron Instruments GmbH,D-60488,Heerstrausse 149, Frankfurt, Main/ Germany
5. Femto Tech Inc. 325, Industry Drive, Carlisle, OH 45005 USA
6. Rad Elec Inc. 5714-C, Industry lane, Frederick, MD 21704 USA
Table 1
Steady state radon concentration as recorded by Alpha Guard® and Femto-Tech®
continuous radon monitors compared to calculated NIST reference radon concentration.
NIST Standard: 1036 Bq of 226Ra; Emanation coefficient: 0.8737
Flow rate varied from 0.1 L min to 1 L min.
Flow Rate
L min
0.104
0.203
0.392
0.607
0.796
0.961
NIST
Bq m3
1097
562
291
188
143
119
NIST DC
Bq m-3
1046
549
287
186
142
119
AG-BG
Bq m-3
1013
576
280
190
144
118
(AG-BG)/
NIST DC
0.969
1.050
0.977
1.023
1.009
0.993
FT-BG
Bq m-3
985
606
289
191
141
126
(FT-BG)/
(NIST DC)
0.942
1.104
1.007
1.027
0.993
1.059
Table 2
Time taken to reach steady state (TTRSS) at different flow rates in a 40-liter radon
exposure chamber, Mean residence time (MRT) and mean radon decay correction
(MDC)
Flow Rate
L min
0.104
0.203
0.392
0.607
0.796
0.961
TTRSS
Hours
12
7
5
4
3
2
MRT
Hours
6.3
3.3
1.7
1.1
0.8
0.7
MDC
Coefficient
0.953
0.975
0.987
0.9924
0.995
1.000
Table 3
Calibration verification for E-PERM® Electret Ion Chambers. These are passive
integrating radon monitors. Flow Rate: 0.602 L min; NIST Standard: 1036 Bq of 226Ra;
Emanation coefficient: 0.8737
Electret #
SBE230
SBE756
SBD828
SBD757
SBE949
SBD469
SBD550
SBD460
SBD464
SBD447
Exposure
period
Days
2.8333
-do-do-do-do-do-do-do-do-do-
Start
Charge
volts
319
353
486
393
457
405
442
405
410
410
End
Charge
Volts
285
320
450
358
423
372
408
372
374
376
Average
EPBG
Radon
Bq m3
212
202
213
213
202
199
203
199
219
205
207
NIST
Radon
Bq m3
204
-do-do-do-do-do-do-do-do-do-
(EP-BG)/
NIST
Ratio
1.04
0.99
1.04
1.04
0.99
0.98
1.00
0.98
1.07
1.01
1.01
Table 4
Calibration verification for E-PERM® Electret Ion Chambers. These are passive
integrating radon monitors. Flow Rate: 0.281 L min; NIST Standard: 1036 Bq of 226Ra;
Emanation coefficient: 0.8737
NIST
(EPBG)
Electret # Exposure
Start
End
EPBG
Radon
/NIST
period
charge
Charge
Radon
3
3
Bq m
Ratio
Days
volts
Volts
Bq m
SBD828
1.8750
449
405
421
420
1.00
SBD688
-do373
329
431
-do1.03
SBD757
-do357
312
444
-do1.06
SBD757
-do285
242
433
-do1.03
SY2130
-do372
330
411
-do0.98
SBD986
-do422
379
414
-do0.99
SBE949
-do435
391
423
-do1.01
SBD201
-do417
371
446
-do1.06
SBD686
-do319
277
418
-do1.00
SBE756
-do550
505
418
-do1.00
Average
430
1.01
Flow-meter with control
Flow Calibrator
NIST
Source
Pump
In
Out
Air from outside
Filter
Exposure Chamber (40 L)
Figure 1. Schematic of the Experimental Arrangement
Air
Flow rate: 0.203 L/ min, Steady state radon
concentration after subtracting the background:
576 Bq /m3, NIST predicted radon concentration,
after decay correction: 549 Bq/m3
200
23
21
19
17
15
13
11
Hours
9
0
1
25
22
19
16
13
10
7
4
0
400
7
500
600
5
1000
800
3
Radon Bq/m3
1500
1
Radon Bq/m3
Flow rate: 0.104 L/min, Steady state radon
concentration after subtracting the background:
1013 Bq/m3, NIST predicted radon
concentration, after decay correction: 1046
Bq/m3
Hours
Flow rate: 0.392 L/min, Steady state radon concentration
after subtracting the background: 280 Bq/m3, NIST
predicted radon concentration, after decay correction: 287
Bq/m3.
Flow rate: 0.607 L /min, Steady state radon concentration
after subtracting the background: 190 Bq/m3, NIST
predicted radon concentration, after decay correction: 186
Bq/m3
400
200
23
21
19
17
15
200
150
100
50
21
19
17
15
13
11
9
7
1
5
0
Hours
3
13
11
9
7
5
3
1
0
Radon Bq/m3
Radon
Bq/m3
250
Hours
Flow rate: 0.961 L /min, Steady state radon concentration after
subtracting the background: 118 Bq /m3, NIST predicted radon
concentration, after decay correction: 119 Bq/m3
Hours
23
21
19
17
15
13
11
9
7
5
3
Radon Bq/m3
23
21
19
17
180
160
140
120
100
80
60
40
20
0
1
Hours
15
13
11
9
7
5
3
300
200
100
0
1
Radon Bq/m3
Flow rate: 0.796 L/min, Steady state radon concentration
after subtracting the background: 144 Bq/m3, NIST
predicted radon concentration, after decay correction:
142 Bq/m3.