Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 2, pp. 23-26, 2003

Accounts
Counting Efficiency for Rapid Preparation of Known Amounts of 222Rn by
the Air Luminescence Method
Yuko Morita-Murase,* Isao Murakami, and Yoshio Homma
Laboratory for Physical Chemistry, Kyoritsu College of Pharmacy,
1-5-30, Shibakoen, Minato-ku, Tokyo 105 Japan
Received: June 17, 2003; In Final Form: December 19, 2003
The counting efficiency for air luminescence due to 222Rn and its daughters was determined with commerciallyavailable liquid scintillation spectrometers giving much attention to Cherenkov radiation due to β−-particles andinternal conversion electrons from 214Pb and 214Bi. Based on the results, we propose a rapid and reliable method
for preparation of known amounts of 222Rn, which are needed for calibration of detectors measuring 222Rn or anumber of experiments using 222Rn, without elaborate instrumentation or technique.

1. Introduction
The preparation of known amounts of 222Rn is a problem
common to many fields of research dealing with 222Rn. Until
now, known amounts of 222 Rn were prepared by collecting
222
Rn from a standard 2 2 6 Ra source and determining theamounts based on the secular equilibrium between 226Ra and222
Rn. In order to collect accurate amounts of 222Rn by thismethod, it is indispensable to verify experimentally that all or a
fixed fraction of 222Rn from a standard 226Ra source is removed
and collected. In addition, the method requires a long waiting
period (15−30 days) to grow 222Rn from the source. The needfor a simple and reliable method for the preparation of known
amounts of 222Rn has become apparent.
In previous papers,1−3 the present authors determined several
α-emitters including 222 Rn by measuring the luminescence
counts from excited nitrogen4 with a liquid scintillation spectrometer (LSS) and proposed a method for determining 222Rn
using the air luminescence method (ALM). However, thereported counting efficiency of the ALM for 222 Rn and its
daughters (218Po, 214Pb, 214Bi, and 214Po) was determined with a
specially modified LSS which could not be universally
utilized:1 the pulses from the sum circuit of the LSS (Model
6880, Tracor Analytic) were fed into an amplifier and then to a
1024-channel pulse-height analyser. Therefore, the counting
efficiency of air luminescence due to 222Rn and its daughters
was determined with LSSs by use of known amounts of 222Rn
which were standardized by measuring γ-rays from 214Pb witha Ge well detector. Much attention was also given to
Cherenkov counting efficiency due to β−-particles from 214Pb
and 214Bi and the counting efficiency for the short-lived daughters,5 some of which were assumed to be attached to the wallof the counting vial for LSS.6
In the present study, the counting efficiencies of air luminescence due to 222Rn and its daughters were determined forseveral LSSs. Based on the counting efficiencies obtained, weoffer a simple, rapid and reliable method for preparing known
amounts of 222Rn without elaborate instrumentation and technique. Some significant advantages of the proposed method
are that the time required for the preparation of known
amounts of 222Rn is only 3.5 h, and that the 226Ra source, fromwhich 222Rn is collected, does not necessarily have to be a standardized one, because we do not determine 222Rn based on the*Corresponding author. E-mail: murase-yk@kyoritsu-ph.ac.jp.
FAX: +81-3-5400-2685.

secular equilibrium between 226Ra and 222Rn. We can obtainknown amounts of 222Rn simply by measuring 222Rn that is
actually transferred and stored in the counting vial. Therefore,
we need not verify that all of the 222Rn from the 226Ra source
was removed and collected. Disadvantages of the conventional method for preparing known amounts of 222Rn are also
discussed in some detail.
Exactly, the luminescence in this study should be termed“nitrogen luminescence”. However, “air luminescence” wasemployed in conformity with the expression used in theprevious papers.1−3, 7
2. Experimental
222

Rn Sample. One hundred milliliters of 226Ra solution( RaCl2 in 0.1 M HCl) was enclosed in a 200 mL flask with arubber stopper. 222Rn was collected from the air space above
the 226RaCl2 solution with a syringe and added to air in a quartz
tube of 0.7 cm internal diam and 4 cm long, both ends ofwhich were closed with silicone rubber stoppers (Figure 1),
yielding a volume of 1.15 cm3. 222Rn in the tube was allowed
to remain for 3.5 h before measurement. During the time,
226

Figure 1. Schematic diagram of the 222Rn sample and the counting
vial for measuring the counting efficiency of the air luminescencemethod: 1. silicone rubber stopper; 2. counting vial; 3. quartz tube;
4. 222Rn sample.

© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/29/2003

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J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003

218

214

214

Morita-Murase

214

Po, Pb, Bi, and Po came to transient equilibrium withRn.
Measurements. The radioactivity of 222Rn was determinedthrough its daughter 214Pb by measuring γ-rays from 214Pb witha well type Ge detector (Model GCW1521, Canberra Industries).
This system had a 1.9 keV resolution (FWHM) at 1.33 MeV
and relative efficiency of 17.1%.
The full energy peak efficiency of the Ge detector was
obtained by measuring two types of γ-ray standard sources: the
EGP type γ-ray standard sources of 57 Co, 60 Co, and 137 Cs(LMRI, France), and the standard solutions of 131I and 203Hg(AEA Technology Quality and Safety Assurance, Harwell,
UK). The energy and abundance of the γ-rays used were: 57Co
(0.122 MeV, 85.60%), 60Co (1.173 MeV, 99.90% and 1.333
MeV, 99.98%), 137Cs (0.662 MeV, 85.1%), 131I (0.364 MeV,
81.1%), and 203Hg (0.279 MeV, 81.46%).8
The EGP type γ-ray standard source, the point source which
is attached to an end of a plastic rod, was mounted at every
0.25 cm along the axial line of the well of the Ge detector
between positions P1 and P2 (Figure 2) and counted for 5 mineach. By averaging the results obtained for EGP type source,
the full energy peak efficiencies for the γ-emitters, which areassumed to be uniformly distributed in the quartz tube, can becalculated.
The standard solution (131I or 203Hg) was transferred into aquartz tube, which is identical to that for the 222Rn sample, andmounted in the well of the Ge detector and counted for 5 min.
The energy and abundance of γ-ray from 214Pb used for thedetermination were 0.352 MeV and 35.85%, respectively.8
The full energy efficiencies obtained for the γ-ray standard
sources gave a straight line on log-log scale in the energy
range of 0.279 MeV to 1.333 MeV (Figure 3). The equation
fitted to the linear part of the experimental data in Figure 3,
which was calculated by a least squares method, is given by222

liquid scintillation counting vial, which was full of air andsealed with air-tight cap. The background sample was allowed
to remain for ca. 60-days before measurement. Consequently,
222
Rn and its daughters in the background sample, if present,
could be almost completely decayed. The background wasobserved to be constant within statistical error.
After the γ-ray spectrometric determination of 222Rn, thequartz tube containing 222Rn and its daughters was put in theliquid-scintillation-counting vial and, after sealing with asilicone rubber stopper, the counting vial was shaken until thequartz tube in the vial was broken. The sample which contains
the equilibrium mixture of 222 Rn and its daughters is hereinafter referred to as the “standard 222Rn sample”. Table 1 lists
the standard 222Rn samples containing a wide variety of 222Rn
prepared in a similar manner as described above. It has to bestated here that determination of low-level 222Rn samples witha Ge well detector is impractical because it requires a rather
long counting period, although it is a method of choice for alimited number of samples. The air luminescence counts ofthe standard 222Rn samples were measured with LSSs for 30
min (Table 1).
3. Results and Discussion
The counting efficiency (εα) of the standard 222Rn sample
obtained by ALM is given by the relation
εα = [Aair (cps)/ S (Bq)] × 100%,(2)where Y is the full energy peak efficiency expressed in percent
and X is the γ-ray energy expressed in MeV. Thus, the fullenergy peak efficiency for 0.352 MeV γ-ray was estimated tobe (21.2 ± 0.4)%.
The full energy peaks of 214Pb were counted at least 1800
counts to limit the statistical error to less than 2.4%. The deadtime losses were always less than 1%. No peaks which seemto be due to sum effects were observed. Thus, 222Rn in thequartz tube was standardized based on the 214Pb activity.
The background of LSS was determined by measuring awhere Aair is the air luminescence count rate of the standard
Rn sample and S is the total disintegration rate of the standard 222Rn sample, i.e., the total disintegration rate of 222Rn andits daughters.
It is noted that the total disintegration rate of the standard
222
Rn sample is 5 times the activity of 222Rn. Table 1 lists thetotal disintegration rates of the standard 222Rn samples and thecounting efficiencies by the ALM for several LSSs. The
average counting efficiency was calculated based on theweighting factor, which is inversely proportional to the standard deviation of individual εα.
The counting efficiencies are in the range of 21.5% to45.8%, because the counting efficiency by the ALM is a function of the optical properties of the system (e.g., optical
coupling, light collection, reflection), the properties of thephotomultiplier tubes (quantum efficiency, multiplication,
spectral response), and the properties of the electronic circuit.
The air luminescence counts of the standard 222Rn samples

Figure 2. The position of the EGP type point source in the well of Ge
detector for the determination of the full energy peak efficiency:
1. quartz tube; 2.Ge well detector; P1.the position on the center line of
the well at 0.5 cm from the lower end of the tube; P2. the position on
the center line of the well at 0.5 cm from the upper end of the tube.

Figure 3. Full energy peak efficiencies as a function of γ-ray energy.

log Y = –1.029 log X + 0.8597

(1)

222

Counting Efficiency for Rapid Preparation of Known Amounts of 222Rn

TABLE 1: The Counting Efficiencies of the ALM,

Aloka
LSC-3500

25, for Several LSSs

S /Bq

εα /%

3281.25 ± 2.15
2636.99 ± 1.67
946.11 ± 0.64
665.30 ± 0.54
345.61 ± 0.34
152.45 ± 0.16
51.26 ± 0.10
32.83 ± 0.09
14.77 ± 0.07

42.57 ± 0.15
43.72 ± 0.15
41.41 ± 0.16
42.42 ± 0.20
43.40 ± 0.26
42.96 ± 0.32
43.15 ± 0.57
41.88 ± 0.67
41.92 ± 1.14

LSS

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

LSS
Beckman
LS5801

S/Bq

εα /%

5038.63 ± 9.93
3751.33 ± 8.57
1602.04 ± 3.54
1025.78 ± 1.42
629.96 ± 2.23
76.54 ± 0.39
12.27 ± 0.17
3.47 ± 0.11

44.33 ± 0.12
44.37 ± 0.14
46.14 ± 0.14
46.91 ± 0.09
44.94 ± 0.22
46.25 ± 0.32
46.95 ± 0.92
46.74 ± 2.07
av. 45.70 ± 0.05b

av. 42.67 ± 0.07b
Aloka
LSC-6100a3088.23 ± 3.48
1504.17 ± 2.43
460.71 ± 1.35
45.08 ± 0.43

Beckman
LS600038.66 ± 0.11
37.92 ± 0.11
38.16 ± 0.20
38.46 ± 0.65
av. 38.28 ± 0.07b

1277.35 ± 2.24
592.16 ± 1.53
441.34 ± 1.32
361.20 ± 1.19
243.68 ± 0.98
112.47 ± 0.67
46.00 ± 0.43

45.88 ± 0.11
45.61 ± 0.16
44.09 ± 0.18
46.11 ± 0.21
46.65 ± 0.26
44.61 ± 0.38
44.22 ± 0.58
av. 45.57 ± 0.07b

Packard
Tri-Carb 3170TR/SL(Normal Level Count Mode)

1405.87 ± 2.35
1042.35 ± 2.02
751.60 ± 1.72
291.43 ± 1.07
181.28 ± 0.84
72.52 ± 0.54

21.02 ± 0.06
21.51 ± 0.07
21.93 ± 0.08
21.69 ± 0.14
21.65 ± 0.18
21.43 ± 0.27

Beckman
LS6500av. 21.43 ± 0.04b

1277.35 ± 2.24
592.16 ± 1.53
441.34 ± 1.32
361.20 ± 1.19
243.68 ± 0.98
112.47 ± 0.67
46.00 ± 0.43

43.87 ± 0.11
43.43 ± 0.16
42.23 ± 0.18
43.69 ± 0.20
44.60 ± 0.25
42.75 ± 0.36
42.96 ± 0.58
av. 43.52 ± 0.07bab

window setting: 0.05–1000 keV
weighted average

TABLE 2: The Counting Efficiency of the ALM,
Measured with the LLCM of the Tri-Carb 3170TR/SL
LSS
Packard
Tri-Carb 3170TR/SL(Low Level Count Mode)

,

S /Bq

εα /%

1405.87 ± 2.35
1042.35 ± 2.02
751.60 ± 1.72
291.43 ± 1.07
181.28 ± 0.84
72.52 ± 0.54

3.32 ± 0.02
3.34 ± 0.02
3.35 ± 0.03
3.47 ± 0.05
3.38 ± 0.06
3.61 ± 0.10
av. 3.35 ± 0.01aaweighted average

were also measured with the low level counting mode (LLCM)
of the Tri-Carb LSS (3170TR/SL) (Table 2). The LLCM of
the instrument excludes most of the pulses which are notproduced in the liquid scintillator (LS). The use of the LLCM
for measuring the air luminescence counts is impractical,
because air luminescence counts are not produced in the LS.
However, it must be emphasized that when the standard 222Rn
samples were measured with the normal counting mode of the
instrument, rather practical counting efficiency for the air
luminescence was obtained (Table 1).
As has been reported in the previous papers,1−3 εα measured
in this way is the sum of the following three counting efficiencies:
εα = εu + εa + εc,(3)where εu is the counting efficiency for α-emitters which are
uniformly distributed in the air in the counting vial and do notattach to the wall of the counting vial (unattached daughters),
εa is the counting efficiency for α-emitters which are attachedto the wall of the counting vial, and ε c is the Cherenkov
counting efficiency due to β−-particles from 214Pb and 214Bi in

the counting vial.
Determination of εu, εa, and εc for an Aloka LSS have beenreported. 6 The results can be summarized as follows: The
value of εu was determined by measuring the air luminescence
counts due to α-particles from a standardized 210 Po source(which was electroplated on a silver disc) at different positionsin the counting vial with the LSS.
εu = (the air luminescence count rate/the α-emission rate)
× 100%.
(4)
In spite of the different positions of the source in the counting
vial, the measured counting efficiencies showed good agreement with each other, and εu could reasonably be taken as theaverage of those values, i.e., (67.5 ± 0.4)%. On the other hand,a part of the short-lived daughters of 222Rn seem to be attachedto the wall of the counting vial and are supposed to be countedwith 2π geometry. Therefore, εa is ca. (1 / 2) × (67.5 ± 0.4) =(33.8 ± 0.2)%. It is noted that the β−-emitters do not produce
an air luminescence count and that 222Rn, which has a relatively long half-life (3.82 d), is assumed to be unattached to thewall of the counting vial. Beta-particles from 214Pb (av. Eβ- =0.220 MeV)9 and 214Bi (av. Eβ- = 0.641 MeV)9 in the counting
vial gave off Cherenkov radiation by interacting with the wallof the counting vial, and gave additional count rates. In orderto determine εc, we prepared and standardized a variety of β−emitters and radionuclides which emit internal conversion
electrons, and measured the Cherenkov radiation with the
Aloka LSS. The values of εc, which are plotted as functions of
the average energy of β−-particles and the internal conversion
electrons, gave a straight line on the log-log scale. Based on
the results, the values of εc for 214Pb and 214Bi were found to be(3.7 ± 0.2)% and (32.9 ± 1.3)%, respectively.5 Cherenkov
radiation may also be produced if the energy of photoelectron
and Compton electrons due to γ-rays is greater than thethreshold energy of a medium, e.g., the glass wall of the

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J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003

counting vial and glass components of the detection system.
Therefore, we also measured the Cherenkov counting efficiencies due to γ-rays from 214Pb and 214Bi, and these were estimated to be (0.003 ± 0.001)% and (0.088 ± 0.003)%,
respectively.5 The Cherenkov counting efficiencies describedabove were corrected for those due to γ-rays, details of whichwere reported elsewhere.5
Based on the values of εα, εu, εa, and εc, the average ratio of
the number of the daughters of 222Rn which are attached to thewall to the total number of the daughters was determined to be(38.7 ± 2.8)%. The ratio obtained in this study is importantbecause it gives information about adsorption of short-lived
daughters of 222Rn in a small container such as a counting vial.
More extensive experiments would be desirable to determine
the concentration of the short-lived daughters in various
containers and indoor air, which will be of value for the determination of the equilibrium factor, the ratio of the equilibrium
equivalent concentration and the 222Rn gas concentration.
It must be emphasized that only ε α is required in actual
preparation and determination of 222Rn, and that environmental
222
Rn concentrations obtained by this method are in good
agreement with those obtained by other independent methods
using the PICO-RADTM cartridge and track detector.10
Over the years it has been established, in far too many
reports to list here, that known amounts of 222 Rn can beprepared by collecting 222Rn from a standard 226Ra source anddetermined based on the secular equilibrium between 226Ra and222
Rn. However, it seems relatively certain that the conventional method for preparing 222Rn has several disadvantages:(1) In most cases, a long waiting period (15−30 days) is
required to grow 222Rn from a standard 226Ra source.
(2) In most reports, it was not confirmed experimentally that
all of the 222Rn from the standard 226Ra source is removed
and transferred to the intended equipment, although it is ofgreat importance for the preparation of an accurate amountof 222Rn. This seems mainly due to the fact that, at the
instant immediately after the removal of 222Rn, measurements of trace amounts of 222Rn which might be left in thestandard 226Ra source would be difficult.
(3) Arbitrary amounts of 222Rn, which are independent of theactivity of the standard 226Ra source, cannot be obtained.
(4) Most of the methods for the preparation of 222Rn require
specialized equipment for handling 222Rn. The preparation
method should be simple to permit measurement byroutine counting instruments.
(5) The instrument for the determination of 222Rn suffers fromthe ready deposition of the daughter products on its innersurface, which produces a background “memory effect”
and makes accurate measurements of low-level 222Rn difficult.
On the other hand, the advantages of the ALM are, we
think, readily apparent. These include:(1) The time required for the preparation of known amounts ofMorita-Murase
222

Rn is only 3.5 h.
(2) The 226Ra solution, from which 222Rn is collected, does notnecessarily have to be a standard one, because we do not
determine 222Rn based on the secular equilibrium between
226
Ra and 222Rn. Known amounts of 222Rn can be obtained
simply by measuring the air luminescence count of 222Rn
that is actually transferred and stored in the counting vial.
Consequently, we need not confirm that all of the 222Rn
from the 226Ra solution was transferred.
(3) Arbitrary amounts of 222 Rn necessary for a variety ofexperiments can be obtained by adding standard 222 Rn
samples of various activities.
(4) The ALM is simple to permit measurement by a commercially available LSS and does not require special equipment for handling 222Rn.
(5) The ALM is free from the memory effect, because thedeposition of the daughter nuclides of 222Rn on the LSS isprevented.
In closing, it should be pointed out that the obtained
counting efficiency, εα, is based on an experimental techniqueand does not depend on any theoretical calculations. It can be
determined in the same manner as described above and applied
to rapid and reliable determination of known amounts of 222Rn.
Acknowledgements. The authors wish to express theirappreciation to Ms. Etsuko Furuta of Ochanomizu University,and Dr. Yukio Yoshizawa and Dr. Makoto Takiue of The Jikei
University School of Medicine for the use of their apparatuses
and for their help.
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139 (1979).
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