Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
SIZE DISTRIBUTION, EQUILIBRIUM RATIO AND UNATTACHED FRACTION OF
RADON DECAY PRODUCTS UNDER TYPICAL INDOOR DOMESTIC CONDITIONS
C. Huet1, G. Tymen1 and D. Boulaud2
1
Laboratoire de Recherches Appliquées Atmosphère-Hydrosphère, Faculté des Sciences,
B.P. 809, 29285 Brest Cedex, France.
2
Laboratoire de Physique et Métrologie des Aérosols, Institut de Protection et de Sûreté Nucléaire,
CEA Saclay, 91191 Gif sur Yvette Cedex, France.
In order to characterise the behaviour of radon decay products under domestic conditions, long-term
measurements were carried out from May 1997 to April 1998 in a typical dwelling located in Brittany
(France). In particular, the unattached fraction and equilibrium factor were continuously measured.
Moreover, the size distributions of unattached and attached radon daughters were investigated by
using specific instruments implemented in the laboratory. All these experiments were carried out
under different typical aerosol conditions. The results evidenced the strong influence exerted by the
characteristics (concentration, size) of ambient aerosol on these different parameters.
Key words: radon daughters, unattached fraction, equilibrium factor, radon daughter size distribution
INTRODUCTION
Radon-222, a rare natural radioactive gas, is present at different concentrations in soils and building
materials. After emanation, it gives birth to the following short-lived radon decay products: Po-218,
Pb-214, Bi-214 and Po-214. Due to their high diffusity, these solid particles can, then, attach to
other particles present in the ambient air. It is well known that inhalation of radon daughters is
responsible for about 40% of the total radiation dose received by populations. Their effect on the
health depends on their behavior in indoor atmosphere. One can consider two main groups of
radioactive particles: the first one called “unattached fraction” of size within 0.5 and several
nanometers, while the second one corresponds to radon decay products in a larger size mode (10 1000 nm) characterizing the “attached fraction”. Unattached fraction, equilibrium factor and size
distribution are the main influent parameters involved in the lung-dose calculation.
Within the framework of the European program RARAD (1996-1999), experiments were carried
out in a dwelling situated in Brittany during one year. Unattached fraction and equilibrium factor
were continuously measured under different aerosol conditions. Moreover, spot measurements of
unattached and attached size distributions of radon daughters were also performed.
MATERIAL AND METHODS
Continuous measurements of unattached fraction and equilibrium factor
In order to continuously measure the unattached fraction and equilibrium ratio, we developed an
original device consisting of two parallel sampling lines: an annular diffusion channel (ADC) and
an open filter. As a matter of fact the diffusional behavior of the ADC geometry has been studied by
analytical computation (Kerouanton et al., 1996, Tymen et al., 1999), it has then been shown that
the annular diffusion channel had almost the same collection efficiency as a flat rectangular channel
of equivalent dimensions usually considered as the most selective geometry. The particles that are
not deposited by diffusion on the ADC walls are collected on the downstream filter facing an alpha
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Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
detector (PIPS CAM 900). Thus, only attached particles are collected. On the other hand, the open
filter being also equipped with an alpha detector collects all the particles. Total and attached activity
concentrations were determined by alpha spectroscopy using the method of Tremblay et al. (1979).
Measurements were self-acting and made every three hours.
Size distribution of unattached an attached radon daughters
A granular bed diffusion battery was built in order to determine the size distribution of unattached
Po-218 and Pb-214. It was composed of five granular beds, filled with glass or stainless steel balls,
and a reference channel in parallel. Cut-off diameters ranged within 0.85 to 4.8 nm. The particles
which were not deposited in the granular beds were collected on the downstream filters.
The SDI-2001 used to collect the attached radioactive aerosol consists of an Andersen impactor
followed by five granular beds in parallel (Tymen et al., 1992). Cut-off diameters ranged from 14 to
4 500 nm.
After sampling periods of 5 to 10 minutes, activity concentrations of Po-218, Pb-214 and Bi-214
were determined by gross alpha counting using a modified TELS method (Hartley and Hartley,
1989). Then, the kernel matrixes of our sampling devices and the activity on each filter being
known, size distributions were retrieved using Twomey (1975) and EVE (Paatero, 1990)
algorithms.
RESULTS
Experimental conditions
Throughout the year of experimentation temperature and hygrometry remained quite stable with
respective values around 20°C and 50%. Particle concentration was measured using a CNC TSI
3025, whereas the particle size distribution was determined thanks to a CNC TSI 3022 associated to
a diffusion battery TSI 3040. Without sources, particle concentration was systematically below 5
000 cm-3, which is low. The mean particle concentration was 1 200 cm-3. It was greatly increased
and reached 1 000 000 cm-3 in the presence of aerosols sources such as cigar and cooking smokes,
or burning candles and fumigating sticks. Radon activity concentration was measured by electroprecipitation of the Po218+and ranged between 240 and 2 800 Bq.m-3 with a mean value of 1 400
Bq.m-3. The ventilation rate was estimated to 0.25 h-1 according to the method described by
Porstendörfer et al. (1980).
Unattached fraction and equilibrium factor
The results of unattached fraction and equilibrium factor measurements with the aged aerosol and
various aerosol sources are summarized in Table 1. Under ambient conditions the unattached
fraction ranged within 0.08 and 0.67 with a mean value of 0.31 calculated from a set of 1 000 data.
The mean unattached fractions of Po-218 (fPo) and Pb-214 (fPb) were equal to 0.69 and 0.23,
respectively. Only 8% of the Bi-214 was found unattached. The equilibrium factor mean value was
of 0.16 (with variations between 0.04 and 0.45). This low value can be explained by the low particle
concentration. As a matter of fact, the attachment process seldom occurs under low particle
concentration, so, radon daughters are mainly free and thus plate out on surfaces leading to an
important disequilibrium between radon and its daughters. Figure 1 illustrates a typical example of
the variations of unattached fraction and equilibrium factor with particle concentration. One should
note the increase of unattached fraction with decreasing particle concentration.
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Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
With aerosol sources, the unattached fraction became negligible, i.e. below 5%, whereas the
equilibrium factor was highly increased and sometimes reached values around 0.75. No unattached
Pb-214 and no unattached Bi-214 were found; there was, sometimes, unattached Po-218. The time
variation of unattached fraction and equilibrium factor with cigar smoke and fumigating sticks is
illustrated on Figure 2.
Nanometer size distribution of Po-218 and Pb-214
The nanometer size distributions of Po-218 and Pb-214 were found unimodal. The diameters
obtained for the Po-218 with EVE and Twomey methods ranged from 0.5 to 1.25 nm, whereas
geometric standard deviation (GSD) varied between 1.2 and 1.4 and 1.15 and 1.3, respectively. The
EVE and Twomey algorithms gave mean diameters of 0.8 and 0.95, respectively. A typical example
of the nanometer Po-218 size distribution is presented on Figure 3.
The results obtained for Pb-214 and Po-218 were comparable. As a matter of fact, the EVE
algorithm gave diameters within 0.55 and 1.25 nm with a GSD between 1.2 and 1.35 whereas
Twomey gave diameters in the range 0.55-1.5 nm with GSD between 1.15 and 1.4. A typical
example of nanometer Pb-214 size distribution is displayed on Figure 4.
However, the size determination of these clusters is still a problem: sometimes, the existence of
four discrete modes is reported in the literature depending on the air composition (Reineking et al.,
1998). It is likely that that the modes below 1 nm deal with neutral clusters, whereas the modes
above 1 nm concern charged clusters.
Attached size distributions
The attached size distributions of radon daughters were also investigated under different aerosol
conditions. No difference was observed between the radon daughters, so only the results concerning
the potential alpha energy concentration (PAEC) will be presented hereafter. With the aged aerosol,
the attached size distribution was unimodal with an accumulation mode around 190 nm and a GSD
of 1.64. As more than 70% of experiments gave results with accumulation mode between 180 and
200 nm, that is only a difference of 20 nm the aged aerosol could be considered as stable.
The size distributions obtained with cigar smoke and fumigating sticks were also unimodal. With
the former, accumulation mode diameters ranged from 200 to 260 nm with a mean diameter of 222
nm and a GSD of 1.6. With the latter, accumulation modes varied between 240 and 280 nm with a
mean diameter of 255 nm and a GSD of 1.57. These results confirmed those previously obtained
during an inter-comparison campaign carried out in the same dwelling (Huet et al., 1998). Figure 5
illustrates an attached size distribution of PAEC obtained with cigar smoke.
Some changes were observed with cooking smoke and burning candles. The former produced either
unimodal or bimodal distributions. When the distributions were unimodal, the accumulation modes
were between 160 and 205 nm with a mean diameter of 195 nm and a GSD of 1.6. In 63% of the
cases, bimodal distributions were retrieved. The nucleation mode was around 35 nm with a GSD of
1.76 and accounted for 11% of the attached activity. Eighty nine percent of the attached activity fell
within an accumulation mode around 200 nm with a GSD of 1.72.
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Radon in the Living Environment,
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085
CONCLUSION
In order to characterize the radon decay products under typical living conditions measurements
were carried out over one year. They provided us with good quality information on unattached
fraction, equilibrium factor and radon daughters size distribution. We noticed that the nature, i.e.
concentration, size, of ambient aerosol exerted an important influence on parameters such as
unattached fraction, equilibrium factor and radon daughters size distribution. This study provided us
with a large database useful for a better characterization of uncertainties in dose-exposure
calculations.
In the future, a more precise and better characterization of the charged fraction of radon aerosol
will be required to get a better understanding of the different cluster modes mentioned in the
literature.
ACKNOWLEDGMENTS
This research was supported by the CEC contract FI4PCT950025. We wish to thank Dr. M. P.
Friocourt for assistance in the writing of English manuscript.
REFERENCES
[1]
Hartley B.M., Hartley A; M.. A new method for the determination of the activity of short half-life
descendants of radon. Radiological Protection 1989; 9: 165-177.
[2]
Huet C., Tymen G., Reineking A., Wendt J. Inter-comparison measurements of the activity size
distribution of aerosol attached short-lived radon decay products in a dwelling located in Brittany. J.
Aerosol Sci. 1998; 29: S1311-S1312.
[3]
Kerouanton D., Tymen G., Boulaud D. Small particle diffusion penetration of an annular duct
compared to other geometries. J. Aerosol Sci. 1996; 27: 345-349.
[4]
Paatero P. The Extreme Value Estimation deconvolution method with application to aerosol research,
University of Helsinki, Report series in Physics 1990, HU-P-250.
[5]
Porstendörfer J., Wicke A., Schraub A. Methods for continuous registration of radon, thoron, and their
decay products indoors and outdoors, in Natural Radiation Environment III, published by Technical
Information Center/US Department of Energy 1980; 2: 1293-1307.
[6]
Reineking A., Portendörfer J., Dankelmann V., Wendt J. The size distribution of the unattached shortlived radon decay products. Radioaktivität in Mensch und Umwelt, 28 September-2 October 1998,
Lindau, Germany, 503-508.
[7]
Tremblay R. J., Leclerc A., Mathieu C., Pepin R., Townsend M. G. Measurement of radon progeny
concentration in air by *-particle spectrometric counting during and after air sampling. Health Physics
1979; 36: 401-411.
[8]
Twomey S.. Comparison of constrained linear inversion and iterative nonlinear algorithm applied to
the indirect estimation of particle size distribution. J. Comput. Phys. 1975; 18: 188-200.
[9]
Tymen G., Robe M. C., Rannou A. Measurements of aerosol and radon daughters in five radon
houses. Rad. Prot. Dosim. 1992; 45: 319-322.
[10] Tymen G., Kerouanton D., Huet C., Boulaud D. An annular diffusion channel equipped with a track
detector film for long-term measurements of activity concentration and size distribution of nanometer
218Po particles. J. Aerosol Sci. 1999; 30: 205-216.
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19-23 April 1999, Athens, Greece
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Table 1: Particle concentration, unattached fraction of PAEC, unattached fraction of radon
daughters and equilibrium factor under different aerosol conditions.
Z
fp
fPo
fPb
×103 part/cm3
Aged
1.2
0.31
0.69
0.23
(0.5-5)
(0.67-0.08) (0.86-0.37)
(0-0.61)
Cooking
250
0.046
0.18
0.017
(80-600)
(0.01-0.09)
(0.03-0.3)
(0-0.09)
Fumigating
100
0.02
0.069
0.017
sticks
(60-450)
(0.01-0.05) (0.047-0.17) (0-0.075)
Candles
390
0.032
0.10
0.023
(100-1 000) (0.022-0.047) (0.01-0.16) (0.014-0.06)
Cigar
80
0.024
0.10
0.001
(60-300)
(0.012-0.039) (0.07-0.15) (0-0.007)
745
fBi
F
0.08
(0-0.37)
0
(0-0.02)
0
0.16
(0.04-0.45)
0.27
(0.15-0.4)
0.49
(0.3-0.59)
0.31
(0.26-0.35)
0.56
(0.26-0.74)
0
0.001
(0-0.007)
Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
1
0.9
fp
F
Particles
fp ,F
0.8
0.7
3000
Z
3
(part/cm )
2500
2000
0.6
1500
0.5
0.4
1000
0.3
0.2
500
0.1
0
0
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00:00
12:00
00:00
12:00
00:00
12:00
00:00
12:00
00:00
12:00
00:00
Figure 1: Variations of unattached fraction of PAEC and equilibrium factor with the particle
concentration (aged aerosol).
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Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
1
0.9
0.8
Unattached fraction
Equilibrium factor
Particles
F, fp
3
part/cm
100000
Cigar
0.7
Fumigating sticks
10000
0.6
0.5
0.4
1000
0.3
0.2
0.1
100
0
23/03/98 23/03/98 24/03/98 24/03/98 25/03/98 25/03/98 26/03/98 26/03/98
10:34
22:34
10:34
22:34
10:34
22:34
10:34
22:34
Figure 2: Variations of unattached fraction, equilibrium factor and particle concentration with
aerosol sources.
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Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
2.5
dA/AdlogD
Twomey AMD=0.85 nm,σ =1.25
EVE AMD=0.8 nm, σ =1.25
2
Central solution EVE
Confidence interval EVE
Twomey solution
1.5
1
0.5
D (µm)
0
0.0001
0.001
Figure 3: Typical example of nanometer Po-218 size distribution.
748
0.01
Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
2.5
dA/AdlogD
2
Twomey AM D=0.85 nm, σ=1.25
EVE AM D=0.75 nm, σ=1.2
1.5
Central solution EVE
Confidence interval EVE
Twomey solution
1
0.5
D (µm)
0
0.0001
0.001
Figure 4: Typical example of nanometer Pb-214 size distribution.
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Radon in the Living Environment,
19-23 April 1999, Athens, Greece
085
5
4.5
dA/AdlogD
Central solution EVE
4
Confidence interval EVE
3.5
3
2.5
2
1.5
1
0.5
0
0.01
0.1
1
D (µm) 10
Figure 5: Example of attached size distribution obtained of PAEC obtained with cigar smoke.
750