Paper PRELIMINARY EXPERIMENTS USING A PASSIVE DETECTOR FOR MEASURING INDOOR 220Rn PROGENY CONCENTRATIONS WITH AN AEROSOL CHAMBER Atsuyuki Sorimachi,*† Shinji Tokonami,* Chutima Kranrod,‡ and Tetsuo Ishikawa†‡
Abstract—This paper describes preliminary experiments using a passive detector for integrating measurements of indoor thoron (220Rn) progeny concentrations with an aerosol chamber. A solid state nuclear detector (CR‐39) covered with a thin aluminumvaporized polyethylene plate (Mylar film) was used to detect only alpha particles emitted from 212Po due to 220Rn progeny deposited on the detector surfaces. The initial experiment showed that Mylar film with area density of more than 5 mg cm−2 was suitable to cut off completely alpha particles of 7.7 MeV from 214 Po of 222Rn progeny decay. In the experiment using the passive detector, it was observed that the net track density increased linearly with an increase of time-integrating 220Rn progeny concentration. As a result of dividing deposition rates by atom concentrations, the deposition velocity was given as 0.023 cm s−1 for total 220Rn progeny. The model estimates of deposition velocities were 0.330 cm s−1 for unattached 220Rn progeny and 0.0011 cm s−1 for aerosol-attached 220Rn progeny using LaiNazaroff formulae. These deposition velocities were in the same range with the results reported in the literature. It was also found that the exposure experiments showed little influence of vertical profiles and surface orientations of the passive detector in the chamber on the detection responses, which was in good agreement with that in the model estimates. Furthermore, it was inferred that the main uncertainty of the passive detector was inhomogeneous deposition of 220Rn progeny onto its detection surfaces. Health Phys. 108(6):597–606; 2015 Key Words: aerosols; cancer; 220Rn; 222Rn
* Institute of Radiation Emergency Medicine, Hirosaki University, 66‐1 Hon-cho, Hirosaki, Aomori 036‐8564, Japan; †Present address: Fukushima Medical University, 1 Hikarigaoka, Fukushima 960‐1295, Japan; ‡ National Institute of Radiological Sciences, 4‐9‐1 Anagawa, Inage, Chiba 263‐8555, Japan. The authors declare no conflicts of interest. For correspondence contact: Atsuyuki Sorimachi, Institute of Radiation Emergency Medicine, Hirosaki University, 66‐1 Hon-cho, Hirosaki, Aomori 036‐8564, Japan, or email at [email protected]. (Manuscript accepted 29 September 2014) 0017-9078/15/0 Copyright © 2015 Health Physics Society DOI: 10.1097/HP.0000000000000284
INTRODUCTION AIRBORNE RADON (222Rn) and its progeny are the most important contributors to human exposure from natural radiation sources (UNSCEAR 2000). Radon-222 (222Rn) is globally regarded as a source of disease and is the second highest cause of lung cancer after tobacco smoking. As summarized in the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2006 Report (UNSCEAR 2006), many studies have shown that 222 Rn and thoron (220Rn) are present in dwellings almost everywhere. Although most studies have focused on 222Rn, exposure to 220Rn and its possible health effects have gained increasing attention in recent years. The annual effective dose due to indoor 222Rn exposure can be obtained from the indoor concentration, the equilibrium factor (F) for 222Rn indoors, the time of exposure during a year, and the dose conversion factor for 222 Rn decay products (UNSCEAR 2000). However, for 220 Rn it is difficult to make a similar estimation to that for 222Rn, because only limited information is available on F (Harley et al. 2010; Chen et al. 2012; Janik et al. 2013). The main reasons for this situation are the lack of information on the characteristics of 220Rn and its progeny indoors and adequate metrology for the annual effective dose due to indoor 220Rn exposure. For the measurement technique of 220Rn progeny concentration, Zhou and Iida (2000) and Mishra et al. (2009) have developed passive detectors that were based on measuring the deposition rate of 220Rn progeny using solid-state nuclear detectors. The prototype passive detector developed by Zhou and Iida was improved at the National Institute of Radiological Sciences (NIRS) (Zhou and Tokonami 2005; Tokonami 2010). This passive detector of 220Rn progeny is based on the bare method with a properly adjusted thickness of an energy absorber, and the detector can detect only alpha particles from 212Po, which is in the 220Rn progeny. It is necessary to determine the characteristics of the passive detectors by means of an exposure test before
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practical use to evaluate the reliability of measured values of 220Rn progeny concentrations. However, there are only a few chambers available to carry out calibrations and performance experiments on these passive 220Rn progeny detectors, and consequently, there is little information on the characteristics of the 220Rn progeny passive detectors. Accordingly, it is necessary to establish similar chamber systems. For such purposes, up to now, the authors have set up an aerosol chamber system for calibrations and performance experiments on the 220Rn progeny passive detectors (Sorimachi et al. 2014). This study describes preliminary experiments of the passive 220Rn progeny detector improved by NIRS, such as optimal Mylar films to detect alpha particles from 212Po, detection response, vertical profiles, and surface orientations of the passive detectors in order to determine the characteristics of the passive detectors in the developed aerosol chamber. Then it compares deposition velocities obtained in the exposure experiments with those estimated in the model calculations. MATERIALS AND METHODS A passive detector for measuring 220Rn progeny concentrations Fig. 1 shows a structure of the passive detector for 220 Rn progeny concentration. A piece of commercially available allyl diglycol carbonate (CR‐39), the BARYOTRAK (1 cm 1 cm 0.1 cm, length width thickness; Fukuvi Chemical Industry Co., Ltd.; Fukui, Japan) was used as a detector for alpha particles emitted from 220Rn progeny deposited on the passive detector surfaces. In the passive detector developed by Zhuo and Iida (2000), one CR‐39 detector was used. In the passive detector improved by NIRS (Fig. 1), four CR‐39 detectors were inserted into each hole (1.5 cm in diameter, 0.1 cm in depth) in a stainless steel plate (6cm 6cm 0.1 cm, length width thickness) stacked on another stainless steel plate without the holes in order to investigate measurement precision. The CR‐39
Fig. 1. A schematic diagram of the passive detector for integrating measurements of indoor 220Rn progeny concentrations.
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detectors were covered with thin aluminum-vaporized polyethylene films (Mylar films) as an absorber and transparent polypropylene film (0.8 mg cm−2 in area density). Aerosol chamber system For this study, a 150‐L stainless steel chamber system made by NIRS was used, which consists of an exposure system with a 220Rn flow-through source and 220Rn chamber, an air humidity control system able to adjust the concentration of 220Rn gas generated from the source, a system for monitoring the 220Rn concentration and environmental conditions, and systems for generating and monitoring carnaubawax aerosol particles. These have been described elsewhere (Sorimachi et al. 2009, 2014). The generation of aerosol particles was based on the method of evaporation-condensation at a well defined temperature with the use of a condensation monodisperse aerosol generator (TSI, Inc., Model 3472S; Shoreview, MN, USA) (Sorimachi et al. 2014). The 220Rn concentration in the chamber was measured continuously once every hour using an electrostatic collection instrument, the RAD7 (Durridge Co. Inc.; Billerica, MA, USA). The chamber air temperature and relative humidity were monitored at 1‐h intervals using a hygrometer (ROTRONIC AG; Model HygroPalm‐2; Bassersdorf, Switzerland). The unattached and aerosol-attached 220Rn progeny were collected on a 400‐mesh stainless steel wire screen and a glass-fiber filter (47 mm in diameter), respectively (Sorimachi et al. 2014), at a flow rate of 2 L min−1. The sampling point was located at the height of approximately 35 cm from the bottom. The combination of the 400‐mesh wire screen with air flow rate of 2 L min−1 yielded 50% penetration for particles with diameters of 12 nm, based on theoretical assumptions regarding the screen (Cheng and Yeh 1980). The sampling time for 220Rn progeny was 5 min. After the sampling, alpha particles on the wire screen and filter were counted by using a commercial ZnS(Ag) scintillation detector (Ludlum Measurements, Inc.; Sweetwater, TX, USA). To detect alpha particles efficiently, a plastic ZnS(Ag) scintillator disk was placed directly on the sample. Alpha counts were registered for a period of 6 h at 10‐min intervals. The 220Rn progeny concentrations were calculated on the basis of the two-count method, which was based on two counting intervals (1–60 and 61–360 min) after a 5‐min sampling of the 220Rn progeny and then a 1‐min setting of the metal wire screen and filter on the ZnS(Ag) scintillation detector. Here the influence of alpha particles emitted from the 222Rn progeny on the detection of the 220 Rn progeny can be ignored due to a 222Rn-free atmosphere in the chamber. Accordingly, the 220Rn progeny concentrations were calculated from air concentrations of 212 Pb and 212Bi estimated by the two-count method using eqn (1) (Sorimachi et al. 2014):
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
C ¼ 0:913CPb‐212 þ 0:0866CBi‐212 ;
ð1Þ
where CPb‐212 and CBi‐212 are the concentrations of and 212Bi (Bq m−3), respectively.
212
Pb
Exposure experiments Exposure experiments were based on five runs shown in Table 1. Experimental conditions in the aerosol chamber were maintained at 31°C and 23% relative humidity (RH) within 1% and 3% relative standard deviations (RSDs), respectively. The mean (± standard deviation) (n = 5) unattached and aerosol-attached 220Rn progeny concentrations were 15.2 (±2.7) Bq m−3 and 269 (±21) Bq m−3, respectively. The unattached fraction ( fp) and F were experimentally determined to be 0.054 (±0.012) and 0.016 (±0.001), respectively. The mean median counter diameter and particle number concentration were 122 nm and 9.2 104 cm−3 during the experiment. In the exposure experiment, the passive detector was introduced from the inlet located at the middle between the center and the end lid of the aerosol chamber because the fan was mounted in the center of the chamber lid. After exposure, the passive detector was left in a plastic bag for at least 3 d until 212Pb (half-life: 10.64 h) decayed by less than 1% relative to the initial concentration. After that, all CR‐39 detectors were taken out of the passive detector and chemically etched with a 6 M NaOH solution at 60°C for 24 h (Tokonami et al. 2002). Alpha tracks were counted manually with a track reading system using a microscope. After the procedures for the alpha track readings, the net track density (NTD), (tracks mm−2), was calculated as follows:
NTD ¼NTr −NBG ;
ð2Þ
where NTr is the alpha track density after exposure to 220Rn progeny (tracks mm−2), and NBG is the background alpha track density (tracks mm−2).
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Experimental determination of deposition velocity The relationship between the 220Rn progeny deposition rate, J (atoms cm−2 s−1), and NTD (tracks cm−2) can be expressed as follows (Zhuo and Iida 2000):
J ¼ NTD= η d η g t ;
where ηd is the branching ratio of 212Bi decayed to 212Po (= 64%); ηg is the geometric efficiency, which depends on the energies of incident alpha particles, incident angles against the absorbers, and the thickness of the absorbers (= 0.063); and t is the exposed period (s). The value of ηg reported from Zhuo and Iida (2000) was used in this study because the thickness of the present absorbers was the same as theirs were. The deposition velocity of the 220Rn progeny, Vd (cm s−1), is defined as follows:
Vd ¼ J =Catom ;
Run
1 2 3 4 5 Average ± SD (RSD)3
3.7 ± 1.7 5.2 ± 2.7 10.7 ± 4.3 12.4 ± 2.6 24.6 ± 2.1
ð4Þ
where Catom is the atom concentration of 220Rn progeny (atoms cm−3), which is calculated in the following equation by changing the unit from Bq m−3 to atoms cm−3 in eqn (1): Catom ¼ 0:913CPb‐212 =λPb‐212 þ 0:0866CBi‐212 =λBi‐212 ;
ð5Þ
where λPb‐212 and λBi‐212 are the decay constants of 212 Pb (= 1.81 10−5 s−1) and of 212Bi (= 1.91 10−4 −1 s ), respectively. The deposition velocity measured in the exposure experiments was obtained for the total 220Rn progeny, because it was difficult to discriminatively find how the unattached and aerosol-attached 220Rn progeny contributed to NTD. Model calculation of deposition velocity The theoretical deposition velocity of 220Rn progeny, Ve (cm s−1), which is termed as “effective deposition velocity,” is expressed in terms of the atom-weighted linear combination of the deposition rates of 212Pb and 212Bi per
Table 1. Mean (± standard deviation) track density, deposition rate, atom concentration during the experiments, and the corresponding deposition velocity of 220Rn progeny in the exposure experiment using the 220Rn-aerosol chamber. Net track densitya, b NTD (tracks mm−2)
ð3Þ
Exposure time t (d)
Deposition rateb J (atoms cm−2 s−1)
Concentrationb Catom (atoms cm−3)
Deposition velocityb Vd (cm s−1)
0.2 0.5 0.9 1.0 2.5
0.52 ± 0.23 0.27 ± 0.14 0.34 ± 0.14 0.35 ± 0.07 0.28 ± 0.02 0.35 ± 0.10 (27.6)
15.1 ± 1.6 15.1 ± 1.6 14.8 ± 1.1 14.3 ± 0.7 17.0 ± 1.7 15.3 ± 1.0 (6.6)
0.034 ± 0.016 0.018 ± 0.010 0.023 ± 0.009 0.025 ± 0.005 0.017 ± 0.002 0.023 ± 0.007 (29.7)
a
All data are derived from Fig. 4a. n = 3–5. c Relative standard deviation (%). b
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equilibrium equivalent (Mishra et al. 2009):
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Vvertical ¼ Vd
220
Rn concentration as follows
0:9134Vd;Pb‐212 CPb‐212 þ 0:0866Vd;Bi‐212 CBi‐212 Ve ¼ ; ð6Þ 0:9134CPb‐212 þ 0:0866CBi‐212 where Vd, Pb‐212 and Vd, Bi‐212 are the deposition velocities for 212Pb and 212Bi, respectively. They can be weighted with respect to the corresponding unattached and aerosolattached 220Rn progeny as follows:
ð7Þ
Vd; i ¼ f p;i Vd u þ ð1 −f p;i ÞVd a ;
where Vdu and Vda are the deposition velocities of unattached and aerosol-attached 220Rn progeny i (index i = 212Pb and 212 Bi), respectively; fp is the unattached fraction; and the corresponding aerosol-attached fraction is (1 − fp). Here the value of fp was experimentally determined as noted before (= 0.054). It is now necessary to provide theoretical estimates of the deposition velocity. Three major processes governing deposition are turbulent transport, Brownian plate-out, and gravitational sedimentation. Lai and Nazaroff (2000) proposed a three-layer model for handling eddy diffusivity. The Lai-Nazaroff formula was used to calculate the deposition velocity in the aerosol chamber as model estimates. An additional advantage of the Lai-Nazaroff formula is that it is based on the use of a single parameter [i.e., friction velocity of the fluid, u* (m s−1)], which was estimated from the characteristics of a fan mounted in the chamber. The u* can be calculated using a fan-turbulence model as follows (Shimada et al. 1989a): Ns DT 2 u* ¼ 0:9 1=3 ; TT 2 H
Vg ¼
Cc ¼ 1 þ
ð10Þ
2λ dp ; 1:141 þ 0:588 exp −0:999 dp 2λ
ð11Þ
where λ is the mean free path of the air (= 6.72 10−8 m at 30 °C and 1 atm). For molecular clusters including the 222Rn progeny, dp in eqn (11) is substituted by d* in the following expression for particles smaller than 1.7 nm (Ramamurthi and Hopke 1989): d * ¼ d 1 þ 3 exp −2:20 107 d ;
ð12Þ
where d is the particle diameter in cm. Vd is the deposition velocity due to turbulent and Brownian diffusion and is given as follows (Lai and Nazaroff 2000):
Vd ¼
u* ; 39 þ 3:64Sc2=3 ða − bÞ
ð13Þ
where " 3 # pffiffiffi −1 8:6−10:92Sc−1=3 10:92Sc−1=3 þ 4:3 1 þ 3 tan a ¼ ln ; pffiffiffi 2 Sc−1 þ 0:0609 310:92Sc−1=3 " 3 # pffiffiffi −1 2r þ −10:92Sc−1=3 10:92Sc−1=3 þ rþ 1 b ¼ ln þ 3 tan ; pffiffiffi 2 Sc−1 þ 7:669 10−4 ðrþ Þ3 310:92Sc−1=3
Vg for face‐down surface; Vface−down ¼ exp Vg =Vd −1 V g for face‐up surface; 1 − exp −Vg =Vd
σ p dp 2 Cc g ; 18μ
where σp is the particle density (= 1,000 kg m−3), dp is the diameter (m), g is the gravitational acceleration (= 9.81 m s−2), and μ is the air dynamic viscosity (= 1.86 10−5 kg m−1 s−1 at 30°C and 1 atm). Cc is the Cunningham correction coefficient and is given as follows:
rþ ¼
Vface−up ¼
ð9Þ
where Vg is the gravitational settling velocity, which is calculated in the following equation:
ð8Þ
where Ns is rotational speed (= 2,300 rotations min−1), DT is the blade length for rotation (= 50 mm), TT is the inner diameter of the chamber (= 565 mm), and H is the height of the chamber (= 570 mm). As a result, the value of u* was calculated to be 0.15 m s−1. Upon combining the effects of gravitational sedimentation and diffusion, the deposition velocities of aerosol-unattached and aerosol-attached 220Rn progeny for different surface orientations are given by Lai and Nazaroff (2000):
f or vertical surface;
dp u* : 2 v
ð14Þ
In eqn (13), Sc is the Schmidt number (v/DB), and v is the kinematic viscosity of air (=1.59 10−5 m2 s−1 at 30 °C and 1 atm). DB is the particle diffusion coefficient in m2 s−1 and calculated as follows:
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
Fig. 2. Experimental procedure for selection of area density of aluminum-vaporized Mylar films to detect only 8.8‐MeV alpha particles emitted from 212Po.
DB ¼
kTCc ; 3πμdp
ð15Þ
where k is the Boltzmann constant (= 1.38 10−23 kg m−2 s−2) and T is the temperature (K). RESULTS AND DISCUSSION Selection of Mylar films to detect alpha particles from 212Po In order to selectively detect 8.8‐MeV alpha particles emitted from 212Po, an investigation was made of the optimal area density of the Mylar film to cut off the alpha particles with the second highest energy after 212Po: 7.7‐MeV alpha particles emitted from 214Po. The experimental setup for this is shown in Fig. 2. Radon-222 (222Rn) progeny was collected with a 25‐mm diameter quartz filter at a flow rate of 10 L min−1 for a 5‐min period in an NIRS 222Rn chamber (Ichitsubo et al. 2004; Tokonami et al. 2005). After
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an air sample had been collected on a filter, the filter was left for 30 min so that the 218Po decayed completely. Then the filter was placed on the CR‐39 detector and covered by the Mylar film with a certain thickness (Fig. 2). The filter with the Mylar film cover was left for more than a day for track registration of the alpha particles. After that, the CR‐39 detectors were taken out of the experimental setup, and the subsequent procedures for chemical etchings and alpha track readings were carried out as noted in the previous section. Fig. 3a shows the NTD as a function of the area density of Mylar films to attenuate the 7.7‐MeV alpha particle energy derived from 214Po. Experimental conditions were 20°C and 60% RH in the 222Rn concentration of approximately 5 kBq m−3. NTD in this experiment was calculated by eqn (2) for exposure to 222Rn progeny. As a result, the NTD values decreased rapidly when the area density was about 4 mg cm−2; the NTD values dropped to 0 tracks mm−2 and remained there when the area density was more than 5 mg cm−2. Fig. 3b shows the relationship between the NTD values with the area density of Mylar film of 4.9 to 6.1 mg cm−2 (4.9, 5.3, 5.7, and 6.1 mg cm−2) and limits of detection (LOD) and quantification (LOQ). Filled circles and triangles shown in Fig. 3b were derived from averaging four values of NTD in the area density of 4.9, 5.3, 5.7, and 6.1 mg cm−2 in runs 1 and 2 in Fig. 3a, respectively. The LOD and LOQ were defined by 4.65σ and 14.1σ (σ = blank standard deviation), respectively (Currie 1968). The blank standard deviation was estimated from the track density of the CR‐39 detectors without exposure (σ = 0.0323 tracks mm−2, n = 34), and consequently, the values of LOD and LOQ were calculated to be 0.150 and 0.455 tracks mm−2, respectively. From Fig. 3b, the NTD values were maintained at less than the LOQ as well as LOD. Thus it was found that the area density of more than approximately
Fig. 3. (a) Net track density as a function of area density of Mylar film and (b) relationship between net track density with the area density of Mylar film of 4.9 to 6.1 mg cm−2 and limits of detection (LOD) and quantification (LOQ). The vertical bars in Fig. 3b represent standard deviation (n = 4) derived from the NTD values obtained from the Mylar film area density of 4.9, 5.3, 5.7, and 6.1 mg cm−2. www.health-physics.com
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5 mg cm−2 was suitable to detect only 8.8‐MeV alpha particles emitted from 212Po. To take into account selectivity of Mylar film and efficiency of fabricating the passive detector, the 7.1 mg cm−2 Mylar film was selected. Detection responses of the passive detector In the aerosol chamber that the authors have developed, a homogeneous spatial distribution has been observed using grab sampling measurements, and the value of RSD was within 10% (Sorimachi et al. 2014). In this study, five passive detectors (shown in Fig. 1) were set at the heights of 10 to 50 cm from the bottom of the chamber in order to investigate the detection response of the passive detector to 220Rn progeny. Fig. 4a shows the relationship between the NTD and the time-integrated 220Rn progeny concentration. The passive detectors were exposed to total 220Rn progeny concentrations of 284 Bq m−3 with 9% RSD during the period from 0.2 to 2.5 d. The time-integrated 220Rn progeny concentration ranged from approximately 59 to 788 Bq m−3 d, which corresponded to the period between approximately 1 mo and 1 y on the assumption that the 220Rn progeny concentration indoors was 2 Bq m−3 (Guo et al. 1992). As a result, the values of NTD increased linearly with an increase of the time-integrated 220Rn progeny concentrations (r = 0.957, n = 5). Thus there was the possibility that this relationship was available to estimate the 220Rn progeny concentration in indoor air during a certain period. Using the values of LOD and LOQ for the track density of the CR‐39 detectors as noted in the previous section, LOD and LOQ for 220Rn progeny concentrations measured by the passive detector were estimated to be 0.049 and 0.15 Bq m−3, respectively. These were calculated from the expression presented in Fig. 4a (y = 0.0338x) on the assumption that the exposure period was 90 d.
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From Fig. 4b, the RSDs of NTD also decreased exponentially with an increase of the time-integrated 220Rn progeny concentrations (r = 0.937, n = 5). These values were derived from the NTD values at five different heights in the chamber and were in the range of 8 to 53%. Accordingly, this suggests that the exposure conditions such as 220 Rn concentration and exposure time were important contributors to the uncertainty in NTD for the passive detector in the aerosol chamber. Deposition velocity of 220Rn progeny Table 1 shows a summary of the deposition rate and atom concentration during the experiments and the corresponding deposition velocity for total 220Rn progeny in the aerosol chamber. Using the values of NTD shown in Table 1 and Fig. 4a, the mean deposition rates were calculated by using eqn (3) and 0.35 (±0.10) atoms cm−2 s−1. The mean atom concentrations during the experiment were 15.3 (±1.0) atoms cm−3 by using eqn (5). From the result obtained in eqn (4), the mean deposition velocities of total 220 Rn progeny were 0.023 (±0.007) cm s−1 with 30% RSD. These values were smaller than those carried out in previous chamber experiments (Bigu 1985; Mishra et al. 2009) and in the workplace (Bigu 1985) and, conversely, much larger than those reported in normal indoor measurements (Zhuo and Iida 2000; Mishra et al. 2009) as shown in Fig. 5. Fig. 5 shows the deposition velocities of unattached and aerosol-attached 220Rn progeny, which were estimated by the model calculations shown in Table 2. The model estimates of deposition velocities were calculated in eqns (10) to (15) using the diffusion coefficients of 0.094 cm2 s−1 for unattached 220Rn progeny, reported in the literature (Tokonami 1999), and of 0.050 10−4 cm2 s−1 for aerosolattached 220Rn progeny, estimated on the assumption that
Fig. 4. Relationship between (a) net track density and (b) relative standard deviation (RSD) of net track densities as a function of time-integrated 220 Rn progeny concentration. The track density and time-integrated 220Rn concentration were measured by the passive detector and grab sampling technique, respectively. The vertical bars in Fig. 5a and the horizontal bars in Figs. 5a and b represent standard deviations derived by the passive detector and grab sampling, respectively (n = 3–5). www.health-physics.com
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
the particle diameter was 121 nm. Using eqns (13) and (10), the deposition velocities due to diffusion and the gravitational sedimentation for unattached 220Rn progeny were calculated to be 3.3 10−3 m s−1 and 2.3 10−9 m s−1, respectively, while the corresponding values for aerosolattached 220Rn progeny were 1.1 10−5 m s−1 and 1.1 10−6 m s−1. The results showed that turbulent transport and Brownian plate-out rather than gravitational sedimentation contributed mainly to deposition of 220Rn progeny onto the deposition surfaces of the passive detector. It was reported that the deposition rate was dominated by diffusion deposition for particles smaller than 100 nm and by gravitational sedimentation for particles larger than 500 nm (Cheng 1997; Lai and Nazaroff 2000). Using these values, the deposition velocities for vertical orientation were 0.33 cm s−1 for unattached 220Rn progeny and 0.0011 cm s−1 for aerosol-attached 220Rn progeny in eqns (9) to (13). From Fig. 5, both deposition velocities of unattached and aerosol-attached 220Rn progeny had large variations, ranging from approximately 0.1 to 1 cm s−1 and from 0.001 to 0.1 cm s−1, respectively (Jacobi 1972; Porstendörfer et al. 1978; Knutson et al. 1983; Scott 1983; Toohey et al. 1984; Israeli 1985; Morawska and Jamriska 1996). The deposition velocities for unattached 220Rn progeny obtained in this study seemed to be in the same range of those measured in normal environmental conditions, while the corresponding values for aerosol-attached 220Rn progeny were close to those obtained in controlled environmental conditions.
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Comparison of measured deposition velocities with models An investigation was made into how vertical profiles and surface orientations (face-up and vertical directions, inward and outward directions to the center of the chamber) of passive detectors inserted in the chamber influence the detection responses. The model estimates of deposition velocities for three surface orientations (face-down, face-up and vertical) were derived from eqn (9) and were used for these investigations. The passive detectors were set at heights of 0 cm and 10–50 cm from the bottom of the chamber for face-up and vertical orientations, respectively, to take into account the position of the fan in the chamber. Table 2 shows the measured deposition velocities for two orientations. For passive detectors at the vertical orientation, the variation in deposition velocity of the 220Rn progeny was within 15% RSD. Similar results were obtained by using a grab sampling technique (Sorimachi et al. 2014). The deposition velocity of 0.013 (±0.005) cm s−1 for faceup orientation was lower than that of 0.023 (±0.003) cm s−1 for vertical orientation and the difference in deposition velocity was 54%. This discrepancy might be due to influence of wind direction and turbulence condition on and above the bottom. In this experiment on the outward directions to the center of chamber, the passive detections were set at three heights of 20, 30, and 40 cm from the bottom. As a result, the deposition velocity values were 0.025 (±0.002) cm s−1
Fig. 5. Summary of the deposition velocities of 222Rn and 220Rn progenies indoors as measured in this study and as reported in the literature. The horizontal bars represent experimental errors. “Model,” “House” and “Chamber” mean model calculations, indoor measurements in houses, and chamber experiments, respectively. “u-DP”, “a-DP”, and “t-DP” are unattached, aerosol-attached and total 220Rn progeny, respectively. RnDP and TnDP are 222Rn and 220Rn progeny, respectively. www.health-physics.com
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Table 2. Deposition velocities of 220Rn progeny onto the passive detectors obtained in exposure experiments and model calculations for surface orientations and vertical profiles in the 220Rn-aerosol chamber. Deposition velocity (cm s−1) Surface orientation (H in cm)a Face-down Face-up (0) Vertical (10) Vertical (20) Vertical (30) Vertical (40) Vertical (50) Vertical (10–50) Vertical (20–40)c
Modele Experiment Vd b NMd 0.013 ± 0.005 (4) 0.019 ± 0.005 (4) 0.027 ± 0.019 (5) 0.024 ± 0.006 (5) 0.025 ± 0.009 (5) 0.022 ± 0.015 (4) 0.023 ± 0.003 (23)
Vd,
Vd,
Ve
Pb-212
Bi-212
Vd u
Vd a
0.019 0.019
0.019 0.019
0.027 0.028
0.33 0.33
0.0011 0.0012
0.019
0.019
0.027
0.33
0.0011
0.020 ± 0.004 (3)
a
Height of the passive detectors above the bottom of the chamber. Average ± standard deviation (n). c The detection of passive detector surfaces was parallel to the direction of the chamber wall. d Not measured. e Ve is effective deposition velocity calculated in eqn (6). Vd, Pb-212 and Vd, Bi-212 are the deposition velocities for 212Pb and 212Bi, respectively, weighted with respect to the unattached and aerosol-attached 220Rn progeny, which were calculated in eqn (7). Vd u and Vd a are the deposition velocities of unattached and aerosol-attached 220Rn progeny, respectively, while in the calculation in eqn (9) the corresponding diffusion coefficients was the values reported in the literature (Tokonami 1999) for unattached 220Rn progeny and estimated on the assumption that the particle diameter was 121 nm for aerosol-attached 220 Rn progeny. b
for the inward direction and 0.020 (±0.004) cm s−1 for the outward direction. Accordingly, agreement among them was within 24%. The model estimates of deposition velocities (Ve) were calculated in eqn (6) by using the deposition velocities for 212 Pb and 212Bi, weighted with respect to the unattached and aerosol-attached 220Rn progeny in eqn (7). Table 2 shows a comparison between the measured deposition velocities and the model estimates for face-up and vertical orientations. The deposition velocities for the vertical orientation showed good agreement with 20%. For the face-up orientation, the difference in deposition velocity was 36%. Thus taking into account the measured values and model estimates, these findings suggest little influence of the vertical profiles and surface orientations of passive detectors on the detection responses within approximately 50% RSD. Uncertainty analysis In this study, all uncertainties discussed in this section refer to one standard deviation. The estimates of uncertainties in net track densities obtained from each passive detector with four CR‐39 detectors are based on uncertainties in all quantities contributing to the values. The uncertainty
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in the laboratory arises from uncertainties in chamber conditions, collection, and subsequent analysis procedures for the amounts of 220Rn progeny by the passive detector. It has been reported that the main sources of uncertainty in the repeated results of measurements are as follows (Mellander and Enflo 1992): (1) mean exposure concentration in the experimental room during exposure, (2) CR‐39 detector sensitivity, (3) read-out, (4) counting statistics, and (5) assumed background. In addition to mean 220Rn progeny concentration, spatial variation in airborne 220Rn progeny concentrations may contribute to the uncertainty in track density because the passive detectors were set at several positions in the chamber space in carrying out the calibration and performance experiments. As noted in the previous section, the spatial variation in deposition velocity onto the passive detector was 14% RSD in terms of the vertical profiles from 10 to 50 cm from the bottom. The mean variation in 220Rn progeny concentration measured by the grab sampling technique was 8.6% RSD during each exposure experiment. The standard deviation in the sheet sensitivity from the same lot (sheet) was estimated to be, on average, 5.7% in the range of 3.7 to 8.4% obtained from five experiments in which five CR‐39 detectors randomly selected from the same lot were radiated by a standard 241Am alpha source (approximately 500 Bq) (Ito 2014). For the read-out, the uncertainty was assumed to be negligible because the reader for counting the tracks was same through all experiments. The uncertainty in the counting statistics is assumed to be the square root of the total number of acceptable tracks per the area of the CR‐39 detector (1 cm2) using the data obtained in five runs in Fig. 4a, and consequently, was estimated to be, on average, 3.5% relative to the total number. The uncertainty in the tracks for the background detector is difficult to estimate due to the log-normal distribution. If the standard deviation of the background detector (blank standard deviation) was used (σ = 0.0323 tracks mm−2, n = 34), the uncertainty was 0.0323 (tracks mm−2)/0.0338 (kBq m−3 d)/(tracks mm−2) = 1.0 kBq m−3 d, and consequently, was calculated to be 1.6% relative to the lowest time-integrated 220Rn progeny concentration indicated in Fig. 4a (59 kBq m−3 d). Thus the uncertainty for the passive detector was estimated to be the square root of the quadratic sum of component uncertainty, 17%. Component uncertainty of the passive detector is shown in Fig. 6. The overall uncertainty was derived from the individual RSD obtained from four CR‐39 detectors inserted into each passive detector. Fig. 7 shows the relationship between the individual RSD of the passive detector and the NTD. All data were derived from the experimental results shown in Fig. 4a. It was found that the obtained RSD had a large variation and was independent of the NTD. Consequently, the overall uncertainty was 66(±17)%
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
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in all exposure experiments. However, there was a large discrepancy between the uncertainties shown in the previous paragraph (17%) and in Fig. 7 (66%). This may be due to inhomogeneity of 220Rn progeny deposition on the detection surfaces, which was estimated to be 64%. An apparent reason for this discrepancy has not been found. The inhomogeneity of 220Rn progeny deposition may be caused by the complicated deposition mechanisms of unattached and aerosol-attached 220Rn progeny that coexisted in the chamber at a turbulence condition and/or subsequent bounce-off of 220Rn progeny from the detection surfaces due to a dry condition (less than 30% RH). It has been also reported that the presence of an electric field enhances Brownian and turbulent diffusive deposition of the aerosol particles (Shimada et al. 1989b). Thus, if there is a difference in surface static charge between the passive detectors, a variation is likely to arise in deposition of 220Rn progeny on the detection surfaces. Accordingly, further studies are necessary to elucidate these mechanisms. CONCLUSION The preliminary experiments were described for the passive detector to integrate measurements of indoor 220Rn concentrations using the aerosol chamber. As a result, the following results were obtained. 1. The optimal density area of Mylar film to detect only alpha particles emitted from the 220Rn progeny was found for the passive detector; 2. It was observed that the net track density increased linearly with an increase in time-integrating 220Rn progeny concentration, suggesting that this relationship is possibly suitable to estimate indoor 220Rn concentrations from the tracks of the passive detector obtained in the exposure experiments; 3. The deposition velocities of 220Rn progeny obtained in the experiments were in the same range as results
Fig. 7. Relationship between individual RSD and net track density. The values of RSD were obtained from four CR-39 detectors inserted into the passive detector. The vertical and horizontal bars represent standard deviations (n = 4 and n = 3–5), respectively.
reported in the literature despite different exposure conditions; 4. It was also found that the exposure experiments showed little influence of the vertical profiles and surface orientations on the detection responses of the passive detector, which was in good agreement with that in the model estimates. This finding can give one a better understanding to carry out the calibration and performance experiments of the passive detector; and 5. It was inferred that the main uncertainty of the passive detector was inhomogeneity of 220Rn progeny deposited on its detection surfaces.
Acknowledgments—This work was supported by the grant-in-aid “Construction of Natural Radiation Exposure Study Network” from the Special Coordination Funds for Promoting Science and Technology of Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES
Fig. 6. Estimated component uncertainty of the passive detector used in this study.
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Currie LA. Limits for qualitative detection and quantitative determination. Anal Chem 40:586–593; 1968. Guo Q, Shimo M, Ikebe Y, Minato S. The study of thoron and radon progeny concentrations in dwellings in Japan. Radiat Protect Dosim 45:357–359; 1992. Harley NH, Chittaporn P, Medora R, Merrill R. Measurement of the indoor and outdoor 220Rn (thoron) equilibrium factor: application to lung dose. Radiat Protect Dosim 141: 357–362; 2010. Ichitsubo H, Yamada Y, Shimo M, Koizumi A. Development of a radon-aerosol chamber at NIRS—general design and aerosol performance. J Aerosol Sci 35:217–232; 2004. Israeli M. Deposition rates of Rn progeny in houses. Health Phys 49:1069–1083; 1985. Ito Y. Performance test of passive-type radon-thoron discriminative monitor. Hirosaki University School of Health Sciences; 2014 (in Japanese). Thesis. Jacobi W. Activity and potential alpha-energy of 222Radon-and 220 Radon-daughters in different air atmospheres. Health Phys 22:441–450; 1972. Janik M, Tokonami S, Kranrod C, Sorimachi A, Ishikawa T, Hosoda M, McLaughlin J, Chang B-U, Kim Y-J. Comparative analysis of radon, thoron and thoron progeny concentration measurements. J Radiat Res 54:597–610; 2013. Knutson EO, George AC, Frey JJ, Koh BR. Radon daughter plateout—II prediction model. Health Phys 45:445–452; 1983. Lai ACK, Nazaroff WW. Modeling indoor particle deposition from turbulent flow onto smooth surfaces. J Aerosol Sci 31: 463–476; 2000. Mellander H, Enflo A. The alpha track method used in the Swedish radon epidemiological study. Radiat Protect Dosim 45:65–71; 1992. Mishra R, Mayya YS, Kushwaha HS. Measurement of 220 Rn/222Rn progeny deposition velocities on surfaces and their comparison with theoretical models. J Aerosol Sci 40: 1–15; 2009. Morawska L, Jamriska M. Deposition of radon progeny on indoor surfaces. J Aerosol Sci 27:305–312; 1996. Porstendörfer J, Wicke A, Schraub A. The influence of exhalation, ventilation and deposition processes upon their concentration of radon (222Rn), thoron (220Rn) and their decay products in room air. Health Phys 34:465–473; 1978. Ramamurthi M, Hopke PK. On improving the validity of wire screen ‘unattached’ fraction Rn daughter measurements. Health Phys 56:189–194; 1989.
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Scott AG. Radon daughter deposition velocities estimated from field measurements. Health Phys 45:481–485; 1983. Shimada M, Okuyama K, Kousaka Y. Influence of particle inertia on aerosol deposition in a stirred turbulent flow field. J Aerosol Sci 20:419–429; 1989a. Shimada M, Okuyama K, Kousaka Y, Okuyama Y, Seinfeld JH. Enhancement of Brownian and turbulent diffusive deposition of charged aerosol particles in the presence of an electric field. J Colloid Interface Sci 128:1579–168; 1989b. Sorimachi A, Sahoo SK, Tokonami S. Generation and control of thoron emanated from lantern mantles. Rev Sci Instrum 80: 015104‐1–015104‐4; 2009. Sorimachi A, Ishikawa T, Tokonami S. Development of an aerosol chamber for calibration of 220Rn progeny detectors. Rev Sci Instrum 85:095104‐1–095104‐7; 2014. Tokonami S. Determination of the diffusion coefficient of unattached radon progeny with a graded screen array at the EML radon/aerosol chamber. Radiat Protect Dosim 81: 285–290; 1999. Tokonami S. Why is 220Rn (thoron) measurement important? Radiat Protect Dosim 141:335–339; 2010. Tokonami S, Sun Q, Yonehara H, Yamada Y. A simple measurement technique of the equilibrium equivalent thoron concentration with a CR‐39 detector. Jpn J Health Phys 37:59–63; 2002. Tokonami S, Ishimori Y, Ishikawa T, Yamasaki K, Yamada Y. Intercomparison exercise of measurement techniques for radon, radon decay products and their particle size distributions at NIRS. Jpn J Health Phys 40:183–190; 2005. Toohey RE, Essling MA, Rundo J, Hengde W. Measurements of the deposition rates of radon daughters on indoor surfaces. Radiat Protect Dosim 7:143–146; 1984. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. 2000 Report to the General Assembly. New York: United Nations; 2000. United Nations Scientific Committee on the Effects of Atomic Radiation. Effects of ionizing radiation. 2006 Report to the General Assembly. New York: United Nations; 2006. Zhuo W, Iida T. Estimation of thoron progeny concentrations in dwellings with their deposition rate measurements. Jpn J Health Phys 35:365–370; 2000. Zhuo W, Tokonami S. Convenient methods for evaluation of indoor thoron progeny concentrations. Int Cong Ser 1276: 219–220; 2005.
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■■
Abstract—This paper describes preliminary experiments using a passive detector for integrating measurements of indoor thoron (220Rn) progeny concentrations with an aerosol chamber. A solid state nuclear detector (CR‐39) covered with a thin aluminumvaporized polyethylene plate (Mylar film) was used to detect only alpha particles emitted from 212Po due to 220Rn progeny deposited on the detector surfaces. The initial experiment showed that Mylar film with area density of more than 5 mg cm−2 was suitable to cut off completely alpha particles of 7.7 MeV from 214 Po of 222Rn progeny decay. In the experiment using the passive detector, it was observed that the net track density increased linearly with an increase of time-integrating 220Rn progeny concentration. As a result of dividing deposition rates by atom concentrations, the deposition velocity was given as 0.023 cm s−1 for total 220Rn progeny. The model estimates of deposition velocities were 0.330 cm s−1 for unattached 220Rn progeny and 0.0011 cm s−1 for aerosol-attached 220Rn progeny using LaiNazaroff formulae. These deposition velocities were in the same range with the results reported in the literature. It was also found that the exposure experiments showed little influence of vertical profiles and surface orientations of the passive detector in the chamber on the detection responses, which was in good agreement with that in the model estimates. Furthermore, it was inferred that the main uncertainty of the passive detector was inhomogeneous deposition of 220Rn progeny onto its detection surfaces. Health Phys. 108(6):597–606; 2015 Key Words: aerosols; cancer; 220Rn; 222Rn
* Institute of Radiation Emergency Medicine, Hirosaki University, 66‐1 Hon-cho, Hirosaki, Aomori 036‐8564, Japan; †Present address: Fukushima Medical University, 1 Hikarigaoka, Fukushima 960‐1295, Japan; ‡ National Institute of Radiological Sciences, 4‐9‐1 Anagawa, Inage, Chiba 263‐8555, Japan. The authors declare no conflicts of interest. For correspondence contact: Atsuyuki Sorimachi, Institute of Radiation Emergency Medicine, Hirosaki University, 66‐1 Hon-cho, Hirosaki, Aomori 036‐8564, Japan, or email at [email protected]. (Manuscript accepted 29 September 2014) 0017-9078/15/0 Copyright © 2015 Health Physics Society DOI: 10.1097/HP.0000000000000284
INTRODUCTION AIRBORNE RADON (222Rn) and its progeny are the most important contributors to human exposure from natural radiation sources (UNSCEAR 2000). Radon-222 (222Rn) is globally regarded as a source of disease and is the second highest cause of lung cancer after tobacco smoking. As summarized in the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2006 Report (UNSCEAR 2006), many studies have shown that 222 Rn and thoron (220Rn) are present in dwellings almost everywhere. Although most studies have focused on 222Rn, exposure to 220Rn and its possible health effects have gained increasing attention in recent years. The annual effective dose due to indoor 222Rn exposure can be obtained from the indoor concentration, the equilibrium factor (F) for 222Rn indoors, the time of exposure during a year, and the dose conversion factor for 222 Rn decay products (UNSCEAR 2000). However, for 220 Rn it is difficult to make a similar estimation to that for 222Rn, because only limited information is available on F (Harley et al. 2010; Chen et al. 2012; Janik et al. 2013). The main reasons for this situation are the lack of information on the characteristics of 220Rn and its progeny indoors and adequate metrology for the annual effective dose due to indoor 220Rn exposure. For the measurement technique of 220Rn progeny concentration, Zhou and Iida (2000) and Mishra et al. (2009) have developed passive detectors that were based on measuring the deposition rate of 220Rn progeny using solid-state nuclear detectors. The prototype passive detector developed by Zhou and Iida was improved at the National Institute of Radiological Sciences (NIRS) (Zhou and Tokonami 2005; Tokonami 2010). This passive detector of 220Rn progeny is based on the bare method with a properly adjusted thickness of an energy absorber, and the detector can detect only alpha particles from 212Po, which is in the 220Rn progeny. It is necessary to determine the characteristics of the passive detectors by means of an exposure test before
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practical use to evaluate the reliability of measured values of 220Rn progeny concentrations. However, there are only a few chambers available to carry out calibrations and performance experiments on these passive 220Rn progeny detectors, and consequently, there is little information on the characteristics of the 220Rn progeny passive detectors. Accordingly, it is necessary to establish similar chamber systems. For such purposes, up to now, the authors have set up an aerosol chamber system for calibrations and performance experiments on the 220Rn progeny passive detectors (Sorimachi et al. 2014). This study describes preliminary experiments of the passive 220Rn progeny detector improved by NIRS, such as optimal Mylar films to detect alpha particles from 212Po, detection response, vertical profiles, and surface orientations of the passive detectors in order to determine the characteristics of the passive detectors in the developed aerosol chamber. Then it compares deposition velocities obtained in the exposure experiments with those estimated in the model calculations. MATERIALS AND METHODS A passive detector for measuring 220Rn progeny concentrations Fig. 1 shows a structure of the passive detector for 220 Rn progeny concentration. A piece of commercially available allyl diglycol carbonate (CR‐39), the BARYOTRAK (1 cm 1 cm 0.1 cm, length width thickness; Fukuvi Chemical Industry Co., Ltd.; Fukui, Japan) was used as a detector for alpha particles emitted from 220Rn progeny deposited on the passive detector surfaces. In the passive detector developed by Zhuo and Iida (2000), one CR‐39 detector was used. In the passive detector improved by NIRS (Fig. 1), four CR‐39 detectors were inserted into each hole (1.5 cm in diameter, 0.1 cm in depth) in a stainless steel plate (6cm 6cm 0.1 cm, length width thickness) stacked on another stainless steel plate without the holes in order to investigate measurement precision. The CR‐39
Fig. 1. A schematic diagram of the passive detector for integrating measurements of indoor 220Rn progeny concentrations.
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detectors were covered with thin aluminum-vaporized polyethylene films (Mylar films) as an absorber and transparent polypropylene film (0.8 mg cm−2 in area density). Aerosol chamber system For this study, a 150‐L stainless steel chamber system made by NIRS was used, which consists of an exposure system with a 220Rn flow-through source and 220Rn chamber, an air humidity control system able to adjust the concentration of 220Rn gas generated from the source, a system for monitoring the 220Rn concentration and environmental conditions, and systems for generating and monitoring carnaubawax aerosol particles. These have been described elsewhere (Sorimachi et al. 2009, 2014). The generation of aerosol particles was based on the method of evaporation-condensation at a well defined temperature with the use of a condensation monodisperse aerosol generator (TSI, Inc., Model 3472S; Shoreview, MN, USA) (Sorimachi et al. 2014). The 220Rn concentration in the chamber was measured continuously once every hour using an electrostatic collection instrument, the RAD7 (Durridge Co. Inc.; Billerica, MA, USA). The chamber air temperature and relative humidity were monitored at 1‐h intervals using a hygrometer (ROTRONIC AG; Model HygroPalm‐2; Bassersdorf, Switzerland). The unattached and aerosol-attached 220Rn progeny were collected on a 400‐mesh stainless steel wire screen and a glass-fiber filter (47 mm in diameter), respectively (Sorimachi et al. 2014), at a flow rate of 2 L min−1. The sampling point was located at the height of approximately 35 cm from the bottom. The combination of the 400‐mesh wire screen with air flow rate of 2 L min−1 yielded 50% penetration for particles with diameters of 12 nm, based on theoretical assumptions regarding the screen (Cheng and Yeh 1980). The sampling time for 220Rn progeny was 5 min. After the sampling, alpha particles on the wire screen and filter were counted by using a commercial ZnS(Ag) scintillation detector (Ludlum Measurements, Inc.; Sweetwater, TX, USA). To detect alpha particles efficiently, a plastic ZnS(Ag) scintillator disk was placed directly on the sample. Alpha counts were registered for a period of 6 h at 10‐min intervals. The 220Rn progeny concentrations were calculated on the basis of the two-count method, which was based on two counting intervals (1–60 and 61–360 min) after a 5‐min sampling of the 220Rn progeny and then a 1‐min setting of the metal wire screen and filter on the ZnS(Ag) scintillation detector. Here the influence of alpha particles emitted from the 222Rn progeny on the detection of the 220 Rn progeny can be ignored due to a 222Rn-free atmosphere in the chamber. Accordingly, the 220Rn progeny concentrations were calculated from air concentrations of 212 Pb and 212Bi estimated by the two-count method using eqn (1) (Sorimachi et al. 2014):
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
C ¼ 0:913CPb‐212 þ 0:0866CBi‐212 ;
ð1Þ
where CPb‐212 and CBi‐212 are the concentrations of and 212Bi (Bq m−3), respectively.
212
Pb
Exposure experiments Exposure experiments were based on five runs shown in Table 1. Experimental conditions in the aerosol chamber were maintained at 31°C and 23% relative humidity (RH) within 1% and 3% relative standard deviations (RSDs), respectively. The mean (± standard deviation) (n = 5) unattached and aerosol-attached 220Rn progeny concentrations were 15.2 (±2.7) Bq m−3 and 269 (±21) Bq m−3, respectively. The unattached fraction ( fp) and F were experimentally determined to be 0.054 (±0.012) and 0.016 (±0.001), respectively. The mean median counter diameter and particle number concentration were 122 nm and 9.2 104 cm−3 during the experiment. In the exposure experiment, the passive detector was introduced from the inlet located at the middle between the center and the end lid of the aerosol chamber because the fan was mounted in the center of the chamber lid. After exposure, the passive detector was left in a plastic bag for at least 3 d until 212Pb (half-life: 10.64 h) decayed by less than 1% relative to the initial concentration. After that, all CR‐39 detectors were taken out of the passive detector and chemically etched with a 6 M NaOH solution at 60°C for 24 h (Tokonami et al. 2002). Alpha tracks were counted manually with a track reading system using a microscope. After the procedures for the alpha track readings, the net track density (NTD), (tracks mm−2), was calculated as follows:
NTD ¼NTr −NBG ;
ð2Þ
where NTr is the alpha track density after exposure to 220Rn progeny (tracks mm−2), and NBG is the background alpha track density (tracks mm−2).
599
Experimental determination of deposition velocity The relationship between the 220Rn progeny deposition rate, J (atoms cm−2 s−1), and NTD (tracks cm−2) can be expressed as follows (Zhuo and Iida 2000):
J ¼ NTD= η d η g t ;
where ηd is the branching ratio of 212Bi decayed to 212Po (= 64%); ηg is the geometric efficiency, which depends on the energies of incident alpha particles, incident angles against the absorbers, and the thickness of the absorbers (= 0.063); and t is the exposed period (s). The value of ηg reported from Zhuo and Iida (2000) was used in this study because the thickness of the present absorbers was the same as theirs were. The deposition velocity of the 220Rn progeny, Vd (cm s−1), is defined as follows:
Vd ¼ J =Catom ;
Run
1 2 3 4 5 Average ± SD (RSD)3
3.7 ± 1.7 5.2 ± 2.7 10.7 ± 4.3 12.4 ± 2.6 24.6 ± 2.1
ð4Þ
where Catom is the atom concentration of 220Rn progeny (atoms cm−3), which is calculated in the following equation by changing the unit from Bq m−3 to atoms cm−3 in eqn (1): Catom ¼ 0:913CPb‐212 =λPb‐212 þ 0:0866CBi‐212 =λBi‐212 ;
ð5Þ
where λPb‐212 and λBi‐212 are the decay constants of 212 Pb (= 1.81 10−5 s−1) and of 212Bi (= 1.91 10−4 −1 s ), respectively. The deposition velocity measured in the exposure experiments was obtained for the total 220Rn progeny, because it was difficult to discriminatively find how the unattached and aerosol-attached 220Rn progeny contributed to NTD. Model calculation of deposition velocity The theoretical deposition velocity of 220Rn progeny, Ve (cm s−1), which is termed as “effective deposition velocity,” is expressed in terms of the atom-weighted linear combination of the deposition rates of 212Pb and 212Bi per
Table 1. Mean (± standard deviation) track density, deposition rate, atom concentration during the experiments, and the corresponding deposition velocity of 220Rn progeny in the exposure experiment using the 220Rn-aerosol chamber. Net track densitya, b NTD (tracks mm−2)
ð3Þ
Exposure time t (d)
Deposition rateb J (atoms cm−2 s−1)
Concentrationb Catom (atoms cm−3)
Deposition velocityb Vd (cm s−1)
0.2 0.5 0.9 1.0 2.5
0.52 ± 0.23 0.27 ± 0.14 0.34 ± 0.14 0.35 ± 0.07 0.28 ± 0.02 0.35 ± 0.10 (27.6)
15.1 ± 1.6 15.1 ± 1.6 14.8 ± 1.1 14.3 ± 0.7 17.0 ± 1.7 15.3 ± 1.0 (6.6)
0.034 ± 0.016 0.018 ± 0.010 0.023 ± 0.009 0.025 ± 0.005 0.017 ± 0.002 0.023 ± 0.007 (29.7)
a
All data are derived from Fig. 4a. n = 3–5. c Relative standard deviation (%). b
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equilibrium equivalent (Mishra et al. 2009):
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Vvertical ¼ Vd
220
Rn concentration as follows
0:9134Vd;Pb‐212 CPb‐212 þ 0:0866Vd;Bi‐212 CBi‐212 Ve ¼ ; ð6Þ 0:9134CPb‐212 þ 0:0866CBi‐212 where Vd, Pb‐212 and Vd, Bi‐212 are the deposition velocities for 212Pb and 212Bi, respectively. They can be weighted with respect to the corresponding unattached and aerosolattached 220Rn progeny as follows:
ð7Þ
Vd; i ¼ f p;i Vd u þ ð1 −f p;i ÞVd a ;
where Vdu and Vda are the deposition velocities of unattached and aerosol-attached 220Rn progeny i (index i = 212Pb and 212 Bi), respectively; fp is the unattached fraction; and the corresponding aerosol-attached fraction is (1 − fp). Here the value of fp was experimentally determined as noted before (= 0.054). It is now necessary to provide theoretical estimates of the deposition velocity. Three major processes governing deposition are turbulent transport, Brownian plate-out, and gravitational sedimentation. Lai and Nazaroff (2000) proposed a three-layer model for handling eddy diffusivity. The Lai-Nazaroff formula was used to calculate the deposition velocity in the aerosol chamber as model estimates. An additional advantage of the Lai-Nazaroff formula is that it is based on the use of a single parameter [i.e., friction velocity of the fluid, u* (m s−1)], which was estimated from the characteristics of a fan mounted in the chamber. The u* can be calculated using a fan-turbulence model as follows (Shimada et al. 1989a): Ns DT 2 u* ¼ 0:9 1=3 ; TT 2 H
Vg ¼
Cc ¼ 1 þ
ð10Þ
2λ dp ; 1:141 þ 0:588 exp −0:999 dp 2λ
ð11Þ
where λ is the mean free path of the air (= 6.72 10−8 m at 30 °C and 1 atm). For molecular clusters including the 222Rn progeny, dp in eqn (11) is substituted by d* in the following expression for particles smaller than 1.7 nm (Ramamurthi and Hopke 1989): d * ¼ d 1 þ 3 exp −2:20 107 d ;
ð12Þ
where d is the particle diameter in cm. Vd is the deposition velocity due to turbulent and Brownian diffusion and is given as follows (Lai and Nazaroff 2000):
Vd ¼
u* ; 39 þ 3:64Sc2=3 ða − bÞ
ð13Þ
where " 3 # pffiffiffi −1 8:6−10:92Sc−1=3 10:92Sc−1=3 þ 4:3 1 þ 3 tan a ¼ ln ; pffiffiffi 2 Sc−1 þ 0:0609 310:92Sc−1=3 " 3 # pffiffiffi −1 2r þ −10:92Sc−1=3 10:92Sc−1=3 þ rþ 1 b ¼ ln þ 3 tan ; pffiffiffi 2 Sc−1 þ 7:669 10−4 ðrþ Þ3 310:92Sc−1=3
Vg for face‐down surface; Vface−down ¼ exp Vg =Vd −1 V g for face‐up surface; 1 − exp −Vg =Vd
σ p dp 2 Cc g ; 18μ
where σp is the particle density (= 1,000 kg m−3), dp is the diameter (m), g is the gravitational acceleration (= 9.81 m s−2), and μ is the air dynamic viscosity (= 1.86 10−5 kg m−1 s−1 at 30°C and 1 atm). Cc is the Cunningham correction coefficient and is given as follows:
rþ ¼
Vface−up ¼
ð9Þ
where Vg is the gravitational settling velocity, which is calculated in the following equation:
ð8Þ
where Ns is rotational speed (= 2,300 rotations min−1), DT is the blade length for rotation (= 50 mm), TT is the inner diameter of the chamber (= 565 mm), and H is the height of the chamber (= 570 mm). As a result, the value of u* was calculated to be 0.15 m s−1. Upon combining the effects of gravitational sedimentation and diffusion, the deposition velocities of aerosol-unattached and aerosol-attached 220Rn progeny for different surface orientations are given by Lai and Nazaroff (2000):
f or vertical surface;
dp u* : 2 v
ð14Þ
In eqn (13), Sc is the Schmidt number (v/DB), and v is the kinematic viscosity of air (=1.59 10−5 m2 s−1 at 30 °C and 1 atm). DB is the particle diffusion coefficient in m2 s−1 and calculated as follows:
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
Fig. 2. Experimental procedure for selection of area density of aluminum-vaporized Mylar films to detect only 8.8‐MeV alpha particles emitted from 212Po.
DB ¼
kTCc ; 3πμdp
ð15Þ
where k is the Boltzmann constant (= 1.38 10−23 kg m−2 s−2) and T is the temperature (K). RESULTS AND DISCUSSION Selection of Mylar films to detect alpha particles from 212Po In order to selectively detect 8.8‐MeV alpha particles emitted from 212Po, an investigation was made of the optimal area density of the Mylar film to cut off the alpha particles with the second highest energy after 212Po: 7.7‐MeV alpha particles emitted from 214Po. The experimental setup for this is shown in Fig. 2. Radon-222 (222Rn) progeny was collected with a 25‐mm diameter quartz filter at a flow rate of 10 L min−1 for a 5‐min period in an NIRS 222Rn chamber (Ichitsubo et al. 2004; Tokonami et al. 2005). After
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an air sample had been collected on a filter, the filter was left for 30 min so that the 218Po decayed completely. Then the filter was placed on the CR‐39 detector and covered by the Mylar film with a certain thickness (Fig. 2). The filter with the Mylar film cover was left for more than a day for track registration of the alpha particles. After that, the CR‐39 detectors were taken out of the experimental setup, and the subsequent procedures for chemical etchings and alpha track readings were carried out as noted in the previous section. Fig. 3a shows the NTD as a function of the area density of Mylar films to attenuate the 7.7‐MeV alpha particle energy derived from 214Po. Experimental conditions were 20°C and 60% RH in the 222Rn concentration of approximately 5 kBq m−3. NTD in this experiment was calculated by eqn (2) for exposure to 222Rn progeny. As a result, the NTD values decreased rapidly when the area density was about 4 mg cm−2; the NTD values dropped to 0 tracks mm−2 and remained there when the area density was more than 5 mg cm−2. Fig. 3b shows the relationship between the NTD values with the area density of Mylar film of 4.9 to 6.1 mg cm−2 (4.9, 5.3, 5.7, and 6.1 mg cm−2) and limits of detection (LOD) and quantification (LOQ). Filled circles and triangles shown in Fig. 3b were derived from averaging four values of NTD in the area density of 4.9, 5.3, 5.7, and 6.1 mg cm−2 in runs 1 and 2 in Fig. 3a, respectively. The LOD and LOQ were defined by 4.65σ and 14.1σ (σ = blank standard deviation), respectively (Currie 1968). The blank standard deviation was estimated from the track density of the CR‐39 detectors without exposure (σ = 0.0323 tracks mm−2, n = 34), and consequently, the values of LOD and LOQ were calculated to be 0.150 and 0.455 tracks mm−2, respectively. From Fig. 3b, the NTD values were maintained at less than the LOQ as well as LOD. Thus it was found that the area density of more than approximately
Fig. 3. (a) Net track density as a function of area density of Mylar film and (b) relationship between net track density with the area density of Mylar film of 4.9 to 6.1 mg cm−2 and limits of detection (LOD) and quantification (LOQ). The vertical bars in Fig. 3b represent standard deviation (n = 4) derived from the NTD values obtained from the Mylar film area density of 4.9, 5.3, 5.7, and 6.1 mg cm−2. www.health-physics.com
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5 mg cm−2 was suitable to detect only 8.8‐MeV alpha particles emitted from 212Po. To take into account selectivity of Mylar film and efficiency of fabricating the passive detector, the 7.1 mg cm−2 Mylar film was selected. Detection responses of the passive detector In the aerosol chamber that the authors have developed, a homogeneous spatial distribution has been observed using grab sampling measurements, and the value of RSD was within 10% (Sorimachi et al. 2014). In this study, five passive detectors (shown in Fig. 1) were set at the heights of 10 to 50 cm from the bottom of the chamber in order to investigate the detection response of the passive detector to 220Rn progeny. Fig. 4a shows the relationship between the NTD and the time-integrated 220Rn progeny concentration. The passive detectors were exposed to total 220Rn progeny concentrations of 284 Bq m−3 with 9% RSD during the period from 0.2 to 2.5 d. The time-integrated 220Rn progeny concentration ranged from approximately 59 to 788 Bq m−3 d, which corresponded to the period between approximately 1 mo and 1 y on the assumption that the 220Rn progeny concentration indoors was 2 Bq m−3 (Guo et al. 1992). As a result, the values of NTD increased linearly with an increase of the time-integrated 220Rn progeny concentrations (r = 0.957, n = 5). Thus there was the possibility that this relationship was available to estimate the 220Rn progeny concentration in indoor air during a certain period. Using the values of LOD and LOQ for the track density of the CR‐39 detectors as noted in the previous section, LOD and LOQ for 220Rn progeny concentrations measured by the passive detector were estimated to be 0.049 and 0.15 Bq m−3, respectively. These were calculated from the expression presented in Fig. 4a (y = 0.0338x) on the assumption that the exposure period was 90 d.
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From Fig. 4b, the RSDs of NTD also decreased exponentially with an increase of the time-integrated 220Rn progeny concentrations (r = 0.937, n = 5). These values were derived from the NTD values at five different heights in the chamber and were in the range of 8 to 53%. Accordingly, this suggests that the exposure conditions such as 220 Rn concentration and exposure time were important contributors to the uncertainty in NTD for the passive detector in the aerosol chamber. Deposition velocity of 220Rn progeny Table 1 shows a summary of the deposition rate and atom concentration during the experiments and the corresponding deposition velocity for total 220Rn progeny in the aerosol chamber. Using the values of NTD shown in Table 1 and Fig. 4a, the mean deposition rates were calculated by using eqn (3) and 0.35 (±0.10) atoms cm−2 s−1. The mean atom concentrations during the experiment were 15.3 (±1.0) atoms cm−3 by using eqn (5). From the result obtained in eqn (4), the mean deposition velocities of total 220 Rn progeny were 0.023 (±0.007) cm s−1 with 30% RSD. These values were smaller than those carried out in previous chamber experiments (Bigu 1985; Mishra et al. 2009) and in the workplace (Bigu 1985) and, conversely, much larger than those reported in normal indoor measurements (Zhuo and Iida 2000; Mishra et al. 2009) as shown in Fig. 5. Fig. 5 shows the deposition velocities of unattached and aerosol-attached 220Rn progeny, which were estimated by the model calculations shown in Table 2. The model estimates of deposition velocities were calculated in eqns (10) to (15) using the diffusion coefficients of 0.094 cm2 s−1 for unattached 220Rn progeny, reported in the literature (Tokonami 1999), and of 0.050 10−4 cm2 s−1 for aerosolattached 220Rn progeny, estimated on the assumption that
Fig. 4. Relationship between (a) net track density and (b) relative standard deviation (RSD) of net track densities as a function of time-integrated 220 Rn progeny concentration. The track density and time-integrated 220Rn concentration were measured by the passive detector and grab sampling technique, respectively. The vertical bars in Fig. 5a and the horizontal bars in Figs. 5a and b represent standard deviations derived by the passive detector and grab sampling, respectively (n = 3–5). www.health-physics.com
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
the particle diameter was 121 nm. Using eqns (13) and (10), the deposition velocities due to diffusion and the gravitational sedimentation for unattached 220Rn progeny were calculated to be 3.3 10−3 m s−1 and 2.3 10−9 m s−1, respectively, while the corresponding values for aerosolattached 220Rn progeny were 1.1 10−5 m s−1 and 1.1 10−6 m s−1. The results showed that turbulent transport and Brownian plate-out rather than gravitational sedimentation contributed mainly to deposition of 220Rn progeny onto the deposition surfaces of the passive detector. It was reported that the deposition rate was dominated by diffusion deposition for particles smaller than 100 nm and by gravitational sedimentation for particles larger than 500 nm (Cheng 1997; Lai and Nazaroff 2000). Using these values, the deposition velocities for vertical orientation were 0.33 cm s−1 for unattached 220Rn progeny and 0.0011 cm s−1 for aerosol-attached 220Rn progeny in eqns (9) to (13). From Fig. 5, both deposition velocities of unattached and aerosol-attached 220Rn progeny had large variations, ranging from approximately 0.1 to 1 cm s−1 and from 0.001 to 0.1 cm s−1, respectively (Jacobi 1972; Porstendörfer et al. 1978; Knutson et al. 1983; Scott 1983; Toohey et al. 1984; Israeli 1985; Morawska and Jamriska 1996). The deposition velocities for unattached 220Rn progeny obtained in this study seemed to be in the same range of those measured in normal environmental conditions, while the corresponding values for aerosol-attached 220Rn progeny were close to those obtained in controlled environmental conditions.
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Comparison of measured deposition velocities with models An investigation was made into how vertical profiles and surface orientations (face-up and vertical directions, inward and outward directions to the center of the chamber) of passive detectors inserted in the chamber influence the detection responses. The model estimates of deposition velocities for three surface orientations (face-down, face-up and vertical) were derived from eqn (9) and were used for these investigations. The passive detectors were set at heights of 0 cm and 10–50 cm from the bottom of the chamber for face-up and vertical orientations, respectively, to take into account the position of the fan in the chamber. Table 2 shows the measured deposition velocities for two orientations. For passive detectors at the vertical orientation, the variation in deposition velocity of the 220Rn progeny was within 15% RSD. Similar results were obtained by using a grab sampling technique (Sorimachi et al. 2014). The deposition velocity of 0.013 (±0.005) cm s−1 for faceup orientation was lower than that of 0.023 (±0.003) cm s−1 for vertical orientation and the difference in deposition velocity was 54%. This discrepancy might be due to influence of wind direction and turbulence condition on and above the bottom. In this experiment on the outward directions to the center of chamber, the passive detections were set at three heights of 20, 30, and 40 cm from the bottom. As a result, the deposition velocity values were 0.025 (±0.002) cm s−1
Fig. 5. Summary of the deposition velocities of 222Rn and 220Rn progenies indoors as measured in this study and as reported in the literature. The horizontal bars represent experimental errors. “Model,” “House” and “Chamber” mean model calculations, indoor measurements in houses, and chamber experiments, respectively. “u-DP”, “a-DP”, and “t-DP” are unattached, aerosol-attached and total 220Rn progeny, respectively. RnDP and TnDP are 222Rn and 220Rn progeny, respectively. www.health-physics.com
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Table 2. Deposition velocities of 220Rn progeny onto the passive detectors obtained in exposure experiments and model calculations for surface orientations and vertical profiles in the 220Rn-aerosol chamber. Deposition velocity (cm s−1) Surface orientation (H in cm)a Face-down Face-up (0) Vertical (10) Vertical (20) Vertical (30) Vertical (40) Vertical (50) Vertical (10–50) Vertical (20–40)c
Modele Experiment Vd b NMd 0.013 ± 0.005 (4) 0.019 ± 0.005 (4) 0.027 ± 0.019 (5) 0.024 ± 0.006 (5) 0.025 ± 0.009 (5) 0.022 ± 0.015 (4) 0.023 ± 0.003 (23)
Vd,
Vd,
Ve
Pb-212
Bi-212
Vd u
Vd a
0.019 0.019
0.019 0.019
0.027 0.028
0.33 0.33
0.0011 0.0012
0.019
0.019
0.027
0.33
0.0011
0.020 ± 0.004 (3)
a
Height of the passive detectors above the bottom of the chamber. Average ± standard deviation (n). c The detection of passive detector surfaces was parallel to the direction of the chamber wall. d Not measured. e Ve is effective deposition velocity calculated in eqn (6). Vd, Pb-212 and Vd, Bi-212 are the deposition velocities for 212Pb and 212Bi, respectively, weighted with respect to the unattached and aerosol-attached 220Rn progeny, which were calculated in eqn (7). Vd u and Vd a are the deposition velocities of unattached and aerosol-attached 220Rn progeny, respectively, while in the calculation in eqn (9) the corresponding diffusion coefficients was the values reported in the literature (Tokonami 1999) for unattached 220Rn progeny and estimated on the assumption that the particle diameter was 121 nm for aerosol-attached 220 Rn progeny. b
for the inward direction and 0.020 (±0.004) cm s−1 for the outward direction. Accordingly, agreement among them was within 24%. The model estimates of deposition velocities (Ve) were calculated in eqn (6) by using the deposition velocities for 212 Pb and 212Bi, weighted with respect to the unattached and aerosol-attached 220Rn progeny in eqn (7). Table 2 shows a comparison between the measured deposition velocities and the model estimates for face-up and vertical orientations. The deposition velocities for the vertical orientation showed good agreement with 20%. For the face-up orientation, the difference in deposition velocity was 36%. Thus taking into account the measured values and model estimates, these findings suggest little influence of the vertical profiles and surface orientations of passive detectors on the detection responses within approximately 50% RSD. Uncertainty analysis In this study, all uncertainties discussed in this section refer to one standard deviation. The estimates of uncertainties in net track densities obtained from each passive detector with four CR‐39 detectors are based on uncertainties in all quantities contributing to the values. The uncertainty
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in the laboratory arises from uncertainties in chamber conditions, collection, and subsequent analysis procedures for the amounts of 220Rn progeny by the passive detector. It has been reported that the main sources of uncertainty in the repeated results of measurements are as follows (Mellander and Enflo 1992): (1) mean exposure concentration in the experimental room during exposure, (2) CR‐39 detector sensitivity, (3) read-out, (4) counting statistics, and (5) assumed background. In addition to mean 220Rn progeny concentration, spatial variation in airborne 220Rn progeny concentrations may contribute to the uncertainty in track density because the passive detectors were set at several positions in the chamber space in carrying out the calibration and performance experiments. As noted in the previous section, the spatial variation in deposition velocity onto the passive detector was 14% RSD in terms of the vertical profiles from 10 to 50 cm from the bottom. The mean variation in 220Rn progeny concentration measured by the grab sampling technique was 8.6% RSD during each exposure experiment. The standard deviation in the sheet sensitivity from the same lot (sheet) was estimated to be, on average, 5.7% in the range of 3.7 to 8.4% obtained from five experiments in which five CR‐39 detectors randomly selected from the same lot were radiated by a standard 241Am alpha source (approximately 500 Bq) (Ito 2014). For the read-out, the uncertainty was assumed to be negligible because the reader for counting the tracks was same through all experiments. The uncertainty in the counting statistics is assumed to be the square root of the total number of acceptable tracks per the area of the CR‐39 detector (1 cm2) using the data obtained in five runs in Fig. 4a, and consequently, was estimated to be, on average, 3.5% relative to the total number. The uncertainty in the tracks for the background detector is difficult to estimate due to the log-normal distribution. If the standard deviation of the background detector (blank standard deviation) was used (σ = 0.0323 tracks mm−2, n = 34), the uncertainty was 0.0323 (tracks mm−2)/0.0338 (kBq m−3 d)/(tracks mm−2) = 1.0 kBq m−3 d, and consequently, was calculated to be 1.6% relative to the lowest time-integrated 220Rn progeny concentration indicated in Fig. 4a (59 kBq m−3 d). Thus the uncertainty for the passive detector was estimated to be the square root of the quadratic sum of component uncertainty, 17%. Component uncertainty of the passive detector is shown in Fig. 6. The overall uncertainty was derived from the individual RSD obtained from four CR‐39 detectors inserted into each passive detector. Fig. 7 shows the relationship between the individual RSD of the passive detector and the NTD. All data were derived from the experimental results shown in Fig. 4a. It was found that the obtained RSD had a large variation and was independent of the NTD. Consequently, the overall uncertainty was 66(±17)%
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Preliminary experiments using passive detector for measuring indoor 220Rn c SORIMACHI ET AL.
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in all exposure experiments. However, there was a large discrepancy between the uncertainties shown in the previous paragraph (17%) and in Fig. 7 (66%). This may be due to inhomogeneity of 220Rn progeny deposition on the detection surfaces, which was estimated to be 64%. An apparent reason for this discrepancy has not been found. The inhomogeneity of 220Rn progeny deposition may be caused by the complicated deposition mechanisms of unattached and aerosol-attached 220Rn progeny that coexisted in the chamber at a turbulence condition and/or subsequent bounce-off of 220Rn progeny from the detection surfaces due to a dry condition (less than 30% RH). It has been also reported that the presence of an electric field enhances Brownian and turbulent diffusive deposition of the aerosol particles (Shimada et al. 1989b). Thus, if there is a difference in surface static charge between the passive detectors, a variation is likely to arise in deposition of 220Rn progeny on the detection surfaces. Accordingly, further studies are necessary to elucidate these mechanisms. CONCLUSION The preliminary experiments were described for the passive detector to integrate measurements of indoor 220Rn concentrations using the aerosol chamber. As a result, the following results were obtained. 1. The optimal density area of Mylar film to detect only alpha particles emitted from the 220Rn progeny was found for the passive detector; 2. It was observed that the net track density increased linearly with an increase in time-integrating 220Rn progeny concentration, suggesting that this relationship is possibly suitable to estimate indoor 220Rn concentrations from the tracks of the passive detector obtained in the exposure experiments; 3. The deposition velocities of 220Rn progeny obtained in the experiments were in the same range as results
Fig. 7. Relationship between individual RSD and net track density. The values of RSD were obtained from four CR-39 detectors inserted into the passive detector. The vertical and horizontal bars represent standard deviations (n = 4 and n = 3–5), respectively.
reported in the literature despite different exposure conditions; 4. It was also found that the exposure experiments showed little influence of the vertical profiles and surface orientations on the detection responses of the passive detector, which was in good agreement with that in the model estimates. This finding can give one a better understanding to carry out the calibration and performance experiments of the passive detector; and 5. It was inferred that the main uncertainty of the passive detector was inhomogeneity of 220Rn progeny deposited on its detection surfaces.
Acknowledgments—This work was supported by the grant-in-aid “Construction of Natural Radiation Exposure Study Network” from the Special Coordination Funds for Promoting Science and Technology of Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Fig. 6. Estimated component uncertainty of the passive detector used in this study.
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