- Email: [email protected]
Simulated equilibrium factor studies in radon chamber
Pergamon 0969-8043(95)00334-7
Appl. Radiat, Isot. Vol.47, No. 5/6, pp. 543-550, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-8043/96 $15.00+ 0,00
Simulated Equilibrium Factor Studies in Radon Chamber T I E H - C H I C H U and H O - L I N G LIU Department of Nuclear Science, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C. (Received 11 September 1995; in revised form 10 November 1995)
A series of experiments have been conducted to study the influencesof environmental parameters on the equilibrium factor. Most of them were carried out in a walk-in type chamber. The deposition velocitywas calculated using the Jacobi model. The ranges of the environmental parameters studied in the experiments are humidity 30-90% r.h. and radon concentration 2-40 kBqm-3. The aerosol sources included electric fumigator, mosquito coil, incense, and cigarette with the particle concentration 2000-6500 cm 3 and the attachment rate 10-350 h-'. The results of the experiment show that the equilibrium factor tends to decrease as the radon concentration increases. On the other hand, the equilibrium factor tends to increase as the humidity increases,and so is the increasing attachment rate. Of all the parameters mentioned above, the influence that aerosols have on the equilibrium factor is the predominant factor. The calculated deposition velocity for the unattached fraction of radon daughters tends to increase as the radon concentration increases. However, it tends to decrease as the humidity increases.
Introduction The inhalation of short-lived radon decay products in air has been of public concern in natural radiation; for example, 2~8po, 2~4pb,2~4Biand 2~4poradionuclides are responsible for the greatest amount to be absorbed as an internal dose to the lungs and the respiratory tract. The equilibrium factor F is defined as F = Ce/C o, where Co is the radon activity concentration and Ce is the equilibrium equivalent concentration of radon. The equilibrium factor provides the relationship between radon daughter concentration given in potential at-energy and the radon concentration; hence, the value of the equilibrium factor is influenced by the indoor radon daughter's behavior and environmental conditions. As one of the first formed radon daughters, 218po is positively charged as a single atom, and reacts quickly with gases and water vapor, becoming small particles called "clusters" or as "unattached" radon daughters. Besides the cluster formation, these radionuclides may attach themselves to aerosols in air, which makes the size distribution of 218po bimodal. The group of the larger particle size is called the attached fraction, while that of smaller particle size is called the unattached fraction (cluster). Another important mechanism is the plate-out effect which is the deposition of the radon daughters on walls and other surfaces. This is the main reason why indoor radon and its daughters are not in equilibrium. Since the deposition velocity of unattached radon daughters is about 100 times more than that of those attached,
both the magnitude of the unattached fraction and the deposition diffusion coefficient affect the equilibrium factor (Porstendorfer and Reinlking, 1972). In the present work, the experiment took place in a radon chamber built at the Taiwan Radiation Monitoring Center (TRMC). At fixed temperature and ventilation rates, both environmental parameters, relative humidity and radon concentration were well controlled. Moreover, indoor aerosol sources common to Taiwan such as an electric fumigator, a mosquito coil, incense and cigarettes were used. The possible factors influencing the equilibrium are:
543
1. The aerosol concentration and particle size distribution in the unattached and attached fractions of the radon daughters. 2. Effect of humidity on the diffusion coefficient since it may change the size of the radon daughter aerosol or the charged state. 3, Effect of humidity on the ambient aerosol concentration and its size distribution. 4. Effect of radon concentration on the diffusion coefficient as it may change the charged state of the radon daughters. In the relative humidity or radon concentration controlled experiments, the deposition velocity of the unattached radon daughters can be estimated by fitting the measured results with Jacobi's room model (Jacobi, 1972). Based on the results stated above, the mechanism that the environmental parameters
Tieh-Chi Chu and Ho-Ling Liu
544
influence the deposition velocity and indirectly the equilibrium factor can be discussed.
Experimental This experiment was performed in a walk-in type radon chamber which was designed for testing and calibration of radon measurements at TRMC. Its dimensions are 3.10 m in length, 3.50 m in width, and 3.12 m in height. The total volume is 23 m 3 (Chen et aL, 1993). There is an air conditioner and a heater to control temperature, ranging from 10 to 39°C with an error +0.2°C. There is also a humidifier and a dehumidifier in the chamber; both were used to regulate the relative humidity ranging from 30 to 90% r.h. with an error + 1% r.h. The radon source used was a 226Ra source purchased from Pylon, Canada. It can generate radon concentrations ranging from 1 to 40kBqm -3. In the chamber, the measurement instruments include thermometers, hygrometers, pressure gauges and a laser aerosol spectrometer (PMS model Las-X CRT), the latter was used to measure the particle size distribution of aerosol. The size range of aerosol particle measured by PMS Model Las-X CRT is 0.1-7.5#m and is divided into 16 channels. The resolution is 7 nm and the precision is less than 10%. In addition, a Lucas cell, Pylon, Canada, was used to continuously monitor radon, and a working level monitor and an Alphasmart Smart 770 (Alpha Nuclear, Canada) were used to monitor the variations of radon daughters and the individual radon daughter concentrations. The experimental data were processed with a PC 386 computer. When the radon source was placed inside the chamber, the indoor air must be removed in order to maintain an underpressure of about 758 mmHg for the sake of safety. The regulated flow rate of radon was 3 L min -t, which corresponds to a small air exchange rate. Figure 1 shows the layout of the whole system. The changeable environmental parameters involved in the present work are humidity, radon concentration and aerosol size distribution. In each
song
~{ ~
RADON CHAMBER
h baromel
~~tr~~tt~er
rometer
rodon doughter ter
dehumidif il, r
working level det#¢tor
part of the experiment, one parameter was controlled, and temperature and the other parameters were fixed. The equilibrium factor was continuously measured at hourly intervals for a long period, and a defined attachment rate was set as a fixed parameter. The attachment rate ~,, is dependent on the particle size distribution of the aerosols, and is calculated as follows
2a = ~ ~(di ) AZ(di)
i
where AZ(di) is the aerosol concentration in the channel of corresponding particle-size range di. From a fitting of the measured results with a log-normal distribution, the particle size distribution of aerosol smaller than 0 . 1 p m can be calculated. When di < 0.03 #m,
/~(d~).AZ(a~)
<1%,
and 13(di).AZ(di) can be ignored. Therefore, 2, in Eq. (1) is obtained from the measured data based on the results obtained from 16 channels and those di by fitting when di is 0.03-0.1 #m. 13(di) is the attachment coefficient calculated from:
13(ai ) =
2rid
8 d + dvo 2IoD
(2)
where D is the average diffusion coefficient of unattached radon daughters, assumed to be 6.8 x 10-2em2s-~, and V0 is the mean thermal velocity, which is about 1.72 x l & c m s - ' , while I0 = d/2 + {o, #o = 4.9 x 10 -6 cm which is the average free path of unattached radon daughters, d is the aerosol particle diameter (Porstendorfer and Reinlking, 1972). When D gradually increases, I0 continues to decrease; therefore we believe that the influence of D and I 0 on 13(di), i.e. 2a, is very small and we can adopt the constant values. In the aerosol controlled experiment, the aerosol particles were generated by several common sources available in Taiwan, for example, electric fumigator, mosquito coil, incense, and cigarette. They were placed inside the radon chamber when in use. During the experiment, one of the aerosol sources was always kept in the radon chamber to generate an aerosol in addition to a blank control. Table 1 illustrates the fixed parameter and the controlled parameters in the experiment. The ventilation rate constant was calculated from the fixed exhalation rate 3 L min -~. The equilibrium factor F is attained from F =
Fig. 1. Experimental setup for the measurement of equilibrium factor.
(1)
0.105Cj + 0.516C2 + 0.379C3 C0
(3)
where Co, C~, C2, C3 represent the activity concentration of ~2ZRn, 21*po, 2t4Pb and Z'4Bi respectively. In this experiment, the Lucas cell was coupled to a Pylon
Simulated equilibrium factor studies in radon chamber
545
Table I. The range of regulated and controlled factors in this experiment The range of controlled factor Temperature (rlC) 25 25 25
Relative Radon humidity concentration (%) (Bqm 3) 30-90 2000 50 2000-40000 50 1000
Instruments AB-5 counter for radon concentration measurements. The ~-smart-770 was used to measure the activities of 218po, 2~pb, and 2~4Bi. Hourly data were taken to calculate the equilibrium factor according to Eq. (3).
Ventilation rate (h s) 0.0078 0.0078 0.0078
C~=
Attachment rate (h ~) --I0-350
the superscript a stands for the attached radon daughters. C ~ = 2 , + 2~ + 2, + 2~ Co=-f~Co
Evaluation of deposition velocity The attached and unattached daughter concentrations were calculated using the steady-state Jacobi model (Jacobi, 1972). In Eqns (4)-(9), the superscript u stands for the unattached radon daughters,
Aerosol concentration (cm 3) --2000-6500
c~ =
[a
(4)
c~ 2~ 2~
Co==--f"~Co (5)
22C ~ -I-PI ~.2C~
2~ 22 = i2, + 2 ~ + 2 . + 2 . ] ) ( 2 2 + 2 2 + 2 v + 2 . + 2 ~ ) P121 '~2/~a ] +(21 -.~-~.v-{--,~,~t)(,~l..-~-2vq.-,~a.-.~2~)(,~,2..-~-2v'-~,~a--[--,~)Co=--f~Co
Q=
=
(6)
2 a C~, + ( 1 -- P] )22 C]' 2z+2~+2~ 2~ ).22" (2~ + ).~ + 2. + 2~)(22 + 2v + 2. + 2~)(22 + 2v + 2~ ) P~ 2~ 22 2~
-+ (2, +
L + kS)(k~ + L + 2. + 22)(22 + 2v + 2. + 2~)(22 + 2v + 2~) (I P~ )21 '~2/la 1
(7)
- -
(2] + )w + 2,])()q + ).v +.[. + ).,])(22 + 2v + 2 ] ) ] C°=f~C°
~3 212223 = i2, + 2 v + A . + A ~ ) ( A 2 + 2 v + A . + A ~ ) ( 2 3 + A v + 2 . + 2 . ~ ) PI )-I ~.23"32.~
a__ •~3C~ +
C 3 --
]
Co-.f~Co
(8)
AaC~
23 + ;,~ + ' l ~ (2~ + 2~ + 2~)(2, + L + X. + ~.,9(A2 + L + 2~ + 2,]) 0.2 + L + ~4) (~3 + L + ,~)
(1 - P, )2122 232~ (21 + 2 ~ + 2])(,l, + L + 2. + 2,9(22 + L + 2D(23 + 2~ + 2]) (2]
4
+ l~ + 2~ + 2,~)(22 + 2~ + 2. + 2D(23 + 2~ + 2~ + 2D(23 + 2~ + 2,~)
P, 2j 22 As~.2 ..l ] Co_f . Co. (2, + 2~ + ~.~) (2! + )w + Via+ 2,])(22 + )'v + ~'a + J'~])('~3 + )'v + J'a + "12)(23 +/Iv + )'d)da
(9)
546
Tieh-Chi Chu and Ho-Ling Liu
Substituting the calculated results into Eq. (3)
I
F = 0.105(f~ + f ? ) + 0.516(f~ + f D
?'-
+ 0.379(f~ + f ~ )
Results and Discussion
I. The influence of humidity Under the fixed temperature, the equilibrium factor varies along with humidity as shown in Fig. 2. Figure 3 shows the aerosol concentration and 2, value corresponding to each set of experiments. From Fig. 2, the equilibrium factor increases as the relative humidity increases, but the increase becomes smaller above 70% r.h. From Fig. 3, the aerosol concentration shows no obvious regular pattern, except that it becomes lower when the humidity is 30% r.h.
I
i
I
I
I
i
I
CtS
i
i
I
i
I
i
I
,
I
i
I
250
60OO
'2oo
g 0O
I
I
r ""
T 1
;o o ~r
5O 1000
n 30
40
50 Relative
60
7
Humidity
(%}
90
Fig. 3. Humidity vs aerosol concentration and ~.=.
However, 2, increases apparently as the relative humidity increases. From the calculation of the 3., values, observation can be made that it is due to the increase in particle-size of aerosol as the relative humidity increases. It is also shown that the increase in relative humidity will enhance both the particle size of aerosol and the attachment rate of radon daughters. Thus the equilibrium factor also increases as 2a does. This effect is a concern when it considers the process of humidity change, so it may be somehow different in a non-laboratory environment. Moreover, the influence which humidity has on unattached radon daughters also needs to be taken into consideration. There are two conflicting influences that humidity has on the diffusion coefficient of unattached radon daughters. One is that water molecules benefit clustering; thus, diffusion coefficient should decrease as relative humidity increases. The other is the large proportion of 2tSPo that have a positive charge on initial formation. Because water molecules in air increases the neutralization, the higher r:lative humidity causes a lower-charged proportion of radon daughters giving a higher diffusion coefficient. Both these effects showing conflicting results should be able to coexist, and in fact different experiments have also verified both of these opposite effects (Goldstein and Hopke, 1985; Raes et al., 1985; Raghunath et al., 1979). As shown in Fig. 4, the
25 i
I
(10)
the equilibrium factor F can be found to be the function of 21,22, 23, Pi, 2 , 2~, 2] and 2,~, where 21, 22 and 23 are the decay constants of :~Spo, ~4pb and ~Bi, with the values of 13.6, 1.6, 2.1 h -~, respectively. P~ is the probability of unattached 2~pb resulting from recoil of the attached 2~Spo. Its value is assumed to be 0.5 (Porstendorfer and Reinlking, 1987). 2~ is the ventilation rate constant adjusted to 0.0078 h -l ( = 3 L m i n - ~ / 2 3 m ~) in this experiment. ).a is the attachment rate constant. 2~ and 2~ are the rate constants for deposition of unattached and attached daughters respectively. 2,~ is assumed equal to 2 ~/100, and 2,~ is equal to (S/V)U]. U~ is the deposition velocity for unattached daughters. S is the surface area available for deposition. V is the total volume in this system, and S/V almost equals 55m:/23m 3. Putting all the known quantities into Eq. (10), one can obtain the equilibrium factor F which depends on 2, and U~ only. From Eq. (10), we can get a curve of F via ~.~ for any U~, i.e. for a different U~ there will be a different curve if we plot Fvs 2~. Here we plotted the experimental data (2~, F), and fit them with the curves mentioned above, we can then get the U~. Under normal environmental conditions, U,~ can be derived from the fitting of F and 2~, in order that the variation of deposition velocities with environmental conditions can be observed.
I
'
I
I
I
I
T
I
I
~
[
R¢lQtive Humidity
I%1
I
I
t
t
~" 20 >.
ts E
u~
0.~ Q; 01 I 30
i
I 40
i
1 50 Rel(]tiw
~
I 60 Humidity
i
I 70
30
i 80
90
(%)
Fig. 2. The changes of equilibrium factors with humidity.
40
50
60
70
80
90
Fig. 4. The relationship between relative humidity and deposition velocity of unattached radon daughters.
Simulated equilibrium factor studies in radon chamber Table 2. The experimentalresultscontrolledby humidity Aerosol Median particle u~ concentration size 2, (mh i) (cm 1) (pm) (h i)
Relative humidity CC) 30 40 50 60 70 80 90
547
20.0 ± 5.0 14.5+1.5 10.5+1.5 9.5 ± 1.5 7.0±2.0 7 . 0 ± 1.0 9.0 _+ 2.0
2592 ± 488 6324±667 5046±579 5073 ±631 5088±822 4771 ± 112 4467 ± 323
deposition velocity decreases as relative humidity increases which also means the diffusion coefficient decreases as relative humidity increases. It may be because the higher the radon daughter concentration and the lower the proportion of charged radon daughters make the clustering effect dominant. Due to the decrease of deposition velocity when relative humidity increases, it has caused the equilibrium factor to increase as relative humidity increases. Table 2 illustrates the experimental results controlled by humidity.
0.193 0.10 0.22 0.21 0.18 0.29 0.45
15.2 ± 3.5 35.0±5.3 73.3±12.7 8 0 . 2 ± 13.5 91.7±25.5 134.5_+ 10.4 197.9 ± 27.5
0'351. ' 03/, ~• I 0.33
I
'
0.147 ± 0.22 0.342_+0.031 0.552±0.030 0.695±0.018 0.705±0.030 0.740±0.026 0.732 _+ 0.042
I
,
I ] I - - ~- -- Attochment Rote o Equ~libriurn Factor
P--~ . . . .
032
-i.~ E
Equilibrium factor
0.31
P
a"
""+"---.
-.~-+'- .+---'~"
0.30
27 26 25
-o n
24
~
g
23 ~ 22
"
r
I
0
i
l
10000
I
20000
i
,l
30000
t
'~
I
/`00(0)
Rn Concentrotion (Bqm -3)
2. The influence of radon concentration Raabe indicates that in air a small negative ion concentration is in proportion to the radon concentration (Raabe, 1968). Therefore, when radon concentration increases, it should cause the neutralization of charged radon daughters to happen earlier and lower the proportion of charged radon. Neutral radon daughters have a greater diffusion coefficient than the charged ones; therefore the equilibrium factor should be related to the radon concentration. To avoid having to consider effects from aerosol concentrations, we selected two sets of results with similar 2,. A 2, that is required to have more restriction in one set, in which the attachment rate constant is 21.0h -~ and the sample variation is 0.1 h-~; 2, is more lax than in the other set, in which the attachment rate constant is 25.0h i and the sample variation is 0.5 h -~. The results are shown in Figs 5 and 6. In Fig. 5, the influence of 2, can be neglected. Roughly speaking, while the radon concentration is lower, the equilibrium factor is
0.30
I
I
I
]
I
r
+
Fig. 6. The changes of equilibrium factors with radon concentration and their corresponding 2,.
higher and decreases with the increasing concentration, but tends to be stable once radon concentration reaches a certain value. In Fig. 6, not only is the equilibrium factor affected by 2,, but it also has the tendency as mentioned above which can be explained as follows. When radon concentration increases, there are more ionizing electrons in air. That will increase the radon daughters' proportion of neutralization, hence the diffusion coefficient is higher. A higher diffusion coefficient makes the deposition velocity of unattached radon daughter increase and the equilibrium factor decrease. When radon concentration increases to a certain value, radon daughters are nearly all neutralized. Therefore, even though the radon concentration keeps increasing, it won't decrease the equilibrium factor. Figure 7 shows
t6.0
I
0.2 c.
15.5
0,28 "g
0.27
E
0.26
T
0.251
q -~
~5.0
~
14.0
13.5
0.24 13.0 023 Q2~
I
I 10000
=
I 20000
=
I 30000
=
I 40000
Rn Concentration ( B q m ' 3 )
Fig. 5. The changes of equilibrium factors with radon concentration.
12.5 0
I
I , tO000
I
I 20000
I
30000
i
I t,OCO0
Rn Concentrotion (Bqm -3)
Fig. 7. The relationship between radon concentration and deposition velocity of unattached radon daughters.
Tieh-Chi Chu and Ho-Ling Liu
548
Table 3. The experimentalresults controlled by radonconcentration
Radon
Group 1 I I 1 2 2 2 2 2 2 2
concentration (Bq m-3) 35565 -+320 13021 +284 8138-+ 107 2519-+25 39013-+239 36811 +- 171 30323+_116 16000-+279 9520 -+5 7578-+68 4730 -+9
~.+ (h-I) 20.9 + 0.5 21.1 +_0.4 21.0-+0.3 20.9-+0.6 25.4-+0,3 24.7_+0,6 24.2-+0,6 25.2+_0.9 25.3 -+0.2 25.4-+0.2 24.5 _+0.3
the relation between the radon concentration and the deposition velocity of unattached radon daughters. The result shows that the effect is more d o m i n a n t if the radon concentration is below l0 kBqm -3. It can be found that as radon concentration is above 10kBqm -3, deposition velocity increases as radon concentration increases. The result agrees with our inference mentioned above. Table 3 shows all the experimental results controlled by radon concentration.
3. The influence of a specific aerosol We used different aerosol sources which are very common in Taiwan, including mosquito coils, electric fumigators, incense, and cigarettes to investigate their influence on the equilibrium factor. The results are shown in Fig. 8. When 2a is less than about 100 h - 1, the equilibrium factor increases with the increasing
1.01
i
I
Equilibrium factor 0.253+-0.003 0.250+0.006 0.251_+0.004 0.266-+0.006 0.265-+0.004 0.274+0.004 0.254-+0.004 0.262-+0,007 0.276+ 0.001 0.273-+0.004 0.286-+0.003
u,]
(mh-I) 14.0+ 0.5 14.1 _+0.4 14.1 _+0.4 13.1 +_0.5 15.0-+0.5 14.1 -+0.4 15.0-+0.5 15.0+_0.5 14.3 -+0.3 14.4-+0.6 13.4-+0.6
2a. It can be derived from U,~<
I
I
i
I
i
i
0.90.8o °°° ~ ~ °
0.7-
<~
0
V
~
V
L
o
o
u
0
°~, oq ~
0.6-
0.5-_
.~
+
w
o/,0.3
q
o :blank set ~, : electric fumigator . : mosquito coil
F-d -f
0.2-
. ',incense
q
v :cigarette
0.1O.C
q
I
I 100
J
I
J
200 Attachment
I 300
400
Rate (h "1)
Fig. 8. The changes of equilibrium factor from various aerosol sources with different attachment rates.
Simulated equilibrium factor studies in radon chamber
1.0 '
I
I
r
I
I
I
I
I
l
I
549 I
I
i
I
0.9
O.8
J
0
~o
o
0.7
o v
v to ID
ii
E L--
.
v
v
O.6
vv
v @~
-
v
O.5 Lk,.
O.4 Ua
w
t.d
v
O.3
&
6
~,~
a
. :blank set * :electric fumigator <>:mosquito coil o incense cigarette
f
AO
oo
0.2 0.1 O9
1
IOO0
i
I
2000
t
I 3000
I
I 4000
Aerosol
I
I 5000
i
Concentration
I 6000
t
I 7000
i
I
8000
( c m -3)
Fig. 9. The relationship between equilibrium factor and various aerosol concentration. thus be in proportion to aerosol concentration &=
Ztot.
fl(d)Z(d) dd
,l,, In Fig. 9, we have replaced 2, with aerosol concentration, and found poor connection between aerosol concentration and equilibrium factor. This is due to the different kinds of aerosol particle size distribution. Table 4 lists the result of each experiment; the greatest median particle size is that of the mosquito coil; following are those of incenses and cigarettes. But there was no obvious difference between the set with the electric fumigator and the blank set. The blank set was the experiment conducted without adding any other aerosol in the normal radon chamber. The aerosol concentration in the normal radon chamber is changeable and the experiment was carried out at a higher aerosol concentration than in the blank set. Therefore, the aerosol concentration is not obviously lower than the other sets. From the
measured equilibrium factor we can prove that 2,, not the aerosol concentration, is the main parameter affecting the equilibrium factor. In addition to aerosol concentration, the aerosol particle size is another important factor to be taken into account. Figure 8 shows that though this experiment used different aerosol sources, it can predict that the data should be very close if we use the Jacobi model to simulate the relation between equilibrium factor and 2,. That means that different kinds of aerosol do not have much influence on unattached radon daughters' clustering, and the difference between each deposition velocity is not so great. However, the incense seems to correspond to the smaller deposition velocity. This could be because its constituents of combustion products are favorable to the unattached radon daughters' clustering, which makes the average diffusion coefficient of unattached radon daughters decrease. In Table 4, the deposition velocity of the electric fumigator and the cigarettes are lower than that of the blank set, but the difference is smaller. The reason is perhaps the same as that in the case of the incense.
Table 4. The experimental results for various types of aerosols Specific aerosol source
Blank Electric Fumigator Mosquito Coil Incense Cigarette
Aerosol concentration (cm ))
Median particle size (#m)
2. (h ')
Equilibrium factor
(mh ])
3793 _+ 854 3510 + 518
0.094 0.083
21.6 + 5.1 17.6 + 2.8
0.299 _+ 0.036 0.298 _+ 0.018
11.7_+0.8 10.3 _+0.7
3805 + 133 2808 + 457 6153 _+ 1038
0.74 0.40 0.22
319.7 _+ 24.7 138.6 + 12.2 130.0 _+ 13.6
0.665 + 0.022 0.745 _+ 0.020 0.650 + 0.021
13.5 _+ 1.5 10.3 _+ 1.0 10.3 _+ 0.9
Tieh-Chi Chu and Ho-Ling Liu
550
Conclusion
RefeFences
As far as the radiation dose is concerned, the major contribution comes from the inhalation of short-lived radon daughters. This experiment studies the relation between equilibrium factor and environmental parameters in order to understand radon daughters' behavior in air. In the present work, this experiment was accomplished in a radon chamber with 23 m 3 at TRMC. The parameters involved include humidity, radon concentration and aerosols. The result shows that the equilibrium factor increases as the humidity increases, but decreases when radon concentration increases. When the attachment rate calculated from the aerosol distribution increases, the equilibrium factor increases as well. Among these parameters, aerosols have the greatest influence on equilibrium factor. The correspondent deposition velocity can be obtained from the fitting of results of each experiment by the Jacobi model. On analyzing, we found that the deposition velocity decreases as the humidity increases but increases as the radon concentration increases. Different aerosols also cause different deposition velocity. Among these environmental parameters, humidity has the greatest influence.
Chen C. J., Tung C. W. and Lin Y. M. (1993) Specification of a radon chamber in Taiwan: The 3rd symposium on radiation monitoring technology, pp. 2.1-2.10. Kaoshiung, Taiwan, R.O.C. Goldstein S. D. and Hopke P. K. (1985) Environmental neutralization of Polonium 218. Environ. Sci. Technol. 19, 146-150, Jacobi W. (1972) Activity and potential ~-energy of 222Rn and 22°Rn-daughtersin different air atmosphere. Health Phys. 22, 441-450. Nero A. V., Sextro R. G., Doyle S. M., Moed B. A,, Nazaroff W. W., Revzan K. L. and Schwehr M. B. (1985) Characterizing the sources, range, and environmental influencesof radon 222 and its decay products. Sei. Total Environ. 45, 234-244. Porstendorfer J. and Reinlking A. (1992) Indoor behaviour and characteristics of radon progeny. Radiat. Protect. Dosim. 45(1/4), 303-311. Porstendorfer J., Reinlking A. and Becket K. H. (1987) Free fraction, attachment rates, and plate-out rates of radon daughters in house. In Radon and its Decay Products: Occurrence, Properties, and Health Effect (Hopke P. K., Ed.), pp. 285-300. ACS Symposium Series 331, Washington, D.C. Raabe O. G. (1968) Measurement of the diffusioncoefficient of RaA. Nature 217, 1143-1145. Raes F., Janssens A. and Vanmarcke H. (1985) A closer look at the behaviour of radioactive decay products in air. Sci. Total Environ. 45, 205-218. Raghunath B. and Kotrappa P. (1979) Diffusion coefficient of decay products of radon and thoron. J. Aerosol. Sci. 10, 133-138. Schery S. D., Wang R., Eack K. and Whittlestone S. (1992) New models for radon progeny near the earth's surface. Radiat. Protect. Dosim. 45(I/4), 343-347.
Acknowledgements--The authors are grateful to Professor
P. S. Weng for useful suggestions and also express their hearty thanks to Mr Y. M. Lin, Dr C. J. Chen and Mr C. J. Liu of Taiwan Radiation Monitoring Center, Republic of China, for their assistance in this work.
Appl. Radiat, Isot. Vol.47, No. 5/6, pp. 543-550, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-8043/96 $15.00+ 0,00
Simulated Equilibrium Factor Studies in Radon Chamber T I E H - C H I C H U and H O - L I N G LIU Department of Nuclear Science, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C. (Received 11 September 1995; in revised form 10 November 1995)
A series of experiments have been conducted to study the influencesof environmental parameters on the equilibrium factor. Most of them were carried out in a walk-in type chamber. The deposition velocitywas calculated using the Jacobi model. The ranges of the environmental parameters studied in the experiments are humidity 30-90% r.h. and radon concentration 2-40 kBqm-3. The aerosol sources included electric fumigator, mosquito coil, incense, and cigarette with the particle concentration 2000-6500 cm 3 and the attachment rate 10-350 h-'. The results of the experiment show that the equilibrium factor tends to decrease as the radon concentration increases. On the other hand, the equilibrium factor tends to increase as the humidity increases,and so is the increasing attachment rate. Of all the parameters mentioned above, the influence that aerosols have on the equilibrium factor is the predominant factor. The calculated deposition velocity for the unattached fraction of radon daughters tends to increase as the radon concentration increases. However, it tends to decrease as the humidity increases.
Introduction The inhalation of short-lived radon decay products in air has been of public concern in natural radiation; for example, 2~8po, 2~4pb,2~4Biand 2~4poradionuclides are responsible for the greatest amount to be absorbed as an internal dose to the lungs and the respiratory tract. The equilibrium factor F is defined as F = Ce/C o, where Co is the radon activity concentration and Ce is the equilibrium equivalent concentration of radon. The equilibrium factor provides the relationship between radon daughter concentration given in potential at-energy and the radon concentration; hence, the value of the equilibrium factor is influenced by the indoor radon daughter's behavior and environmental conditions. As one of the first formed radon daughters, 218po is positively charged as a single atom, and reacts quickly with gases and water vapor, becoming small particles called "clusters" or as "unattached" radon daughters. Besides the cluster formation, these radionuclides may attach themselves to aerosols in air, which makes the size distribution of 218po bimodal. The group of the larger particle size is called the attached fraction, while that of smaller particle size is called the unattached fraction (cluster). Another important mechanism is the plate-out effect which is the deposition of the radon daughters on walls and other surfaces. This is the main reason why indoor radon and its daughters are not in equilibrium. Since the deposition velocity of unattached radon daughters is about 100 times more than that of those attached,
both the magnitude of the unattached fraction and the deposition diffusion coefficient affect the equilibrium factor (Porstendorfer and Reinlking, 1972). In the present work, the experiment took place in a radon chamber built at the Taiwan Radiation Monitoring Center (TRMC). At fixed temperature and ventilation rates, both environmental parameters, relative humidity and radon concentration were well controlled. Moreover, indoor aerosol sources common to Taiwan such as an electric fumigator, a mosquito coil, incense and cigarettes were used. The possible factors influencing the equilibrium are:
543
1. The aerosol concentration and particle size distribution in the unattached and attached fractions of the radon daughters. 2. Effect of humidity on the diffusion coefficient since it may change the size of the radon daughter aerosol or the charged state. 3, Effect of humidity on the ambient aerosol concentration and its size distribution. 4. Effect of radon concentration on the diffusion coefficient as it may change the charged state of the radon daughters. In the relative humidity or radon concentration controlled experiments, the deposition velocity of the unattached radon daughters can be estimated by fitting the measured results with Jacobi's room model (Jacobi, 1972). Based on the results stated above, the mechanism that the environmental parameters
Tieh-Chi Chu and Ho-Ling Liu
544
influence the deposition velocity and indirectly the equilibrium factor can be discussed.
Experimental This experiment was performed in a walk-in type radon chamber which was designed for testing and calibration of radon measurements at TRMC. Its dimensions are 3.10 m in length, 3.50 m in width, and 3.12 m in height. The total volume is 23 m 3 (Chen et aL, 1993). There is an air conditioner and a heater to control temperature, ranging from 10 to 39°C with an error +0.2°C. There is also a humidifier and a dehumidifier in the chamber; both were used to regulate the relative humidity ranging from 30 to 90% r.h. with an error + 1% r.h. The radon source used was a 226Ra source purchased from Pylon, Canada. It can generate radon concentrations ranging from 1 to 40kBqm -3. In the chamber, the measurement instruments include thermometers, hygrometers, pressure gauges and a laser aerosol spectrometer (PMS model Las-X CRT), the latter was used to measure the particle size distribution of aerosol. The size range of aerosol particle measured by PMS Model Las-X CRT is 0.1-7.5#m and is divided into 16 channels. The resolution is 7 nm and the precision is less than 10%. In addition, a Lucas cell, Pylon, Canada, was used to continuously monitor radon, and a working level monitor and an Alphasmart Smart 770 (Alpha Nuclear, Canada) were used to monitor the variations of radon daughters and the individual radon daughter concentrations. The experimental data were processed with a PC 386 computer. When the radon source was placed inside the chamber, the indoor air must be removed in order to maintain an underpressure of about 758 mmHg for the sake of safety. The regulated flow rate of radon was 3 L min -t, which corresponds to a small air exchange rate. Figure 1 shows the layout of the whole system. The changeable environmental parameters involved in the present work are humidity, radon concentration and aerosol size distribution. In each
song
~{ ~
RADON CHAMBER
h baromel
~~tr~~tt~er
rometer
rodon doughter ter
dehumidif il, r
working level det#¢tor
part of the experiment, one parameter was controlled, and temperature and the other parameters were fixed. The equilibrium factor was continuously measured at hourly intervals for a long period, and a defined attachment rate was set as a fixed parameter. The attachment rate ~,, is dependent on the particle size distribution of the aerosols, and is calculated as follows
2a = ~ ~(di ) AZ(di)
i
where AZ(di) is the aerosol concentration in the channel of corresponding particle-size range di. From a fitting of the measured results with a log-normal distribution, the particle size distribution of aerosol smaller than 0 . 1 p m can be calculated. When di < 0.03 #m,
/~(d~).AZ(a~)
<1%,
and 13(di).AZ(di) can be ignored. Therefore, 2, in Eq. (1) is obtained from the measured data based on the results obtained from 16 channels and those di by fitting when di is 0.03-0.1 #m. 13(di) is the attachment coefficient calculated from:
13(ai ) =
2rid
8 d + dvo 2IoD
(2)
where D is the average diffusion coefficient of unattached radon daughters, assumed to be 6.8 x 10-2em2s-~, and V0 is the mean thermal velocity, which is about 1.72 x l & c m s - ' , while I0 = d/2 + {o, #o = 4.9 x 10 -6 cm which is the average free path of unattached radon daughters, d is the aerosol particle diameter (Porstendorfer and Reinlking, 1972). When D gradually increases, I0 continues to decrease; therefore we believe that the influence of D and I 0 on 13(di), i.e. 2a, is very small and we can adopt the constant values. In the aerosol controlled experiment, the aerosol particles were generated by several common sources available in Taiwan, for example, electric fumigator, mosquito coil, incense, and cigarette. They were placed inside the radon chamber when in use. During the experiment, one of the aerosol sources was always kept in the radon chamber to generate an aerosol in addition to a blank control. Table 1 illustrates the fixed parameter and the controlled parameters in the experiment. The ventilation rate constant was calculated from the fixed exhalation rate 3 L min -~. The equilibrium factor F is attained from F =
Fig. 1. Experimental setup for the measurement of equilibrium factor.
(1)
0.105Cj + 0.516C2 + 0.379C3 C0
(3)
where Co, C~, C2, C3 represent the activity concentration of ~2ZRn, 21*po, 2t4Pb and Z'4Bi respectively. In this experiment, the Lucas cell was coupled to a Pylon
Simulated equilibrium factor studies in radon chamber
545
Table I. The range of regulated and controlled factors in this experiment The range of controlled factor Temperature (rlC) 25 25 25
Relative Radon humidity concentration (%) (Bqm 3) 30-90 2000 50 2000-40000 50 1000
Instruments AB-5 counter for radon concentration measurements. The ~-smart-770 was used to measure the activities of 218po, 2~pb, and 2~4Bi. Hourly data were taken to calculate the equilibrium factor according to Eq. (3).
Ventilation rate (h s) 0.0078 0.0078 0.0078
C~=
Attachment rate (h ~) --I0-350
the superscript a stands for the attached radon daughters. C ~ = 2 , + 2~ + 2, + 2~ Co=-f~Co
Evaluation of deposition velocity The attached and unattached daughter concentrations were calculated using the steady-state Jacobi model (Jacobi, 1972). In Eqns (4)-(9), the superscript u stands for the unattached radon daughters,
Aerosol concentration (cm 3) --2000-6500
c~ =
[a
(4)
c~ 2~ 2~
Co==--f"~Co (5)
22C ~ -I-PI ~.2C~
2~ 22 = i2, + 2 ~ + 2 . + 2 . ] ) ( 2 2 + 2 2 + 2 v + 2 . + 2 ~ ) P121 '~2/~a ] +(21 -.~-~.v-{--,~,~t)(,~l..-~-2vq.-,~a.-.~2~)(,~,2..-~-2v'-~,~a--[--,~)Co=--f~Co
Q=
=
(6)
2 a C~, + ( 1 -- P] )22 C]' 2z+2~+2~ 2~ ).22" (2~ + ).~ + 2. + 2~)(22 + 2v + 2. + 2~)(22 + 2v + 2~ ) P~ 2~ 22 2~
-+ (2, +
L + kS)(k~ + L + 2. + 22)(22 + 2v + 2. + 2~)(22 + 2v + 2~) (I P~ )21 '~2/la 1
(7)
- -
(2] + )w + 2,])()q + ).v +.[. + ).,])(22 + 2v + 2 ] ) ] C°=f~C°
~3 212223 = i2, + 2 v + A . + A ~ ) ( A 2 + 2 v + A . + A ~ ) ( 2 3 + A v + 2 . + 2 . ~ ) PI )-I ~.23"32.~
a__ •~3C~ +
C 3 --
]
Co-.f~Co
(8)
AaC~
23 + ;,~ + ' l ~ (2~ + 2~ + 2~)(2, + L + X. + ~.,9(A2 + L + 2~ + 2,]) 0.2 + L + ~4) (~3 + L + ,~)
(1 - P, )2122 232~ (21 + 2 ~ + 2])(,l, + L + 2. + 2,9(22 + L + 2D(23 + 2~ + 2]) (2]
4
+ l~ + 2~ + 2,~)(22 + 2~ + 2. + 2D(23 + 2~ + 2~ + 2D(23 + 2~ + 2,~)
P, 2j 22 As~.2 ..l ] Co_f . Co. (2, + 2~ + ~.~) (2! + )w + Via+ 2,])(22 + )'v + ~'a + J'~])('~3 + )'v + J'a + "12)(23 +/Iv + )'d)da
(9)
546
Tieh-Chi Chu and Ho-Ling Liu
Substituting the calculated results into Eq. (3)
I
F = 0.105(f~ + f ? ) + 0.516(f~ + f D
?'-
+ 0.379(f~ + f ~ )
Results and Discussion
I. The influence of humidity Under the fixed temperature, the equilibrium factor varies along with humidity as shown in Fig. 2. Figure 3 shows the aerosol concentration and 2, value corresponding to each set of experiments. From Fig. 2, the equilibrium factor increases as the relative humidity increases, but the increase becomes smaller above 70% r.h. From Fig. 3, the aerosol concentration shows no obvious regular pattern, except that it becomes lower when the humidity is 30% r.h.
I
i
I
I
I
i
I
CtS
i
i
I
i
I
i
I
,
I
i
I
250
60OO
'2oo
g 0O
I
I
r ""
T 1
;o o ~r
5O 1000
n 30
40
50 Relative
60
7
Humidity
(%}
90
Fig. 3. Humidity vs aerosol concentration and ~.=.
However, 2, increases apparently as the relative humidity increases. From the calculation of the 3., values, observation can be made that it is due to the increase in particle-size of aerosol as the relative humidity increases. It is also shown that the increase in relative humidity will enhance both the particle size of aerosol and the attachment rate of radon daughters. Thus the equilibrium factor also increases as 2a does. This effect is a concern when it considers the process of humidity change, so it may be somehow different in a non-laboratory environment. Moreover, the influence which humidity has on unattached radon daughters also needs to be taken into consideration. There are two conflicting influences that humidity has on the diffusion coefficient of unattached radon daughters. One is that water molecules benefit clustering; thus, diffusion coefficient should decrease as relative humidity increases. The other is the large proportion of 2tSPo that have a positive charge on initial formation. Because water molecules in air increases the neutralization, the higher r:lative humidity causes a lower-charged proportion of radon daughters giving a higher diffusion coefficient. Both these effects showing conflicting results should be able to coexist, and in fact different experiments have also verified both of these opposite effects (Goldstein and Hopke, 1985; Raes et al., 1985; Raghunath et al., 1979). As shown in Fig. 4, the
25 i
I
(10)
the equilibrium factor F can be found to be the function of 21,22, 23, Pi, 2 , 2~, 2] and 2,~, where 21, 22 and 23 are the decay constants of :~Spo, ~4pb and ~Bi, with the values of 13.6, 1.6, 2.1 h -~, respectively. P~ is the probability of unattached 2~pb resulting from recoil of the attached 2~Spo. Its value is assumed to be 0.5 (Porstendorfer and Reinlking, 1987). 2~ is the ventilation rate constant adjusted to 0.0078 h -l ( = 3 L m i n - ~ / 2 3 m ~) in this experiment. ).a is the attachment rate constant. 2~ and 2~ are the rate constants for deposition of unattached and attached daughters respectively. 2,~ is assumed equal to 2 ~/100, and 2,~ is equal to (S/V)U]. U~ is the deposition velocity for unattached daughters. S is the surface area available for deposition. V is the total volume in this system, and S/V almost equals 55m:/23m 3. Putting all the known quantities into Eq. (10), one can obtain the equilibrium factor F which depends on 2, and U~ only. From Eq. (10), we can get a curve of F via ~.~ for any U~, i.e. for a different U~ there will be a different curve if we plot Fvs 2~. Here we plotted the experimental data (2~, F), and fit them with the curves mentioned above, we can then get the U~. Under normal environmental conditions, U,~ can be derived from the fitting of F and 2~, in order that the variation of deposition velocities with environmental conditions can be observed.
I
'
I
I
I
I
T
I
I
~
[
R¢lQtive Humidity
I%1
I
I
t
t
~" 20 >.
ts E
u~
0.~ Q; 01 I 30
i
I 40
i
1 50 Rel(]tiw
~
I 60 Humidity
i
I 70
30
i 80
90
(%)
Fig. 2. The changes of equilibrium factors with humidity.
40
50
60
70
80
90
Fig. 4. The relationship between relative humidity and deposition velocity of unattached radon daughters.
Simulated equilibrium factor studies in radon chamber Table 2. The experimentalresultscontrolledby humidity Aerosol Median particle u~ concentration size 2, (mh i) (cm 1) (pm) (h i)
Relative humidity CC) 30 40 50 60 70 80 90
547
20.0 ± 5.0 14.5+1.5 10.5+1.5 9.5 ± 1.5 7.0±2.0 7 . 0 ± 1.0 9.0 _+ 2.0
2592 ± 488 6324±667 5046±579 5073 ±631 5088±822 4771 ± 112 4467 ± 323
deposition velocity decreases as relative humidity increases which also means the diffusion coefficient decreases as relative humidity increases. It may be because the higher the radon daughter concentration and the lower the proportion of charged radon daughters make the clustering effect dominant. Due to the decrease of deposition velocity when relative humidity increases, it has caused the equilibrium factor to increase as relative humidity increases. Table 2 illustrates the experimental results controlled by humidity.
0.193 0.10 0.22 0.21 0.18 0.29 0.45
15.2 ± 3.5 35.0±5.3 73.3±12.7 8 0 . 2 ± 13.5 91.7±25.5 134.5_+ 10.4 197.9 ± 27.5
0'351. ' 03/, ~• I 0.33
I
'
0.147 ± 0.22 0.342_+0.031 0.552±0.030 0.695±0.018 0.705±0.030 0.740±0.026 0.732 _+ 0.042
I
,
I ] I - - ~- -- Attochment Rote o Equ~libriurn Factor
P--~ . . . .
032
-i.~ E
Equilibrium factor
0.31
P
a"
""+"---.
-.~-+'- .+---'~"
0.30
27 26 25
-o n
24
~
g
23 ~ 22
"
r
I
0
i
l
10000
I
20000
i
,l
30000
t
'~
I
/`00(0)
Rn Concentrotion (Bqm -3)
2. The influence of radon concentration Raabe indicates that in air a small negative ion concentration is in proportion to the radon concentration (Raabe, 1968). Therefore, when radon concentration increases, it should cause the neutralization of charged radon daughters to happen earlier and lower the proportion of charged radon. Neutral radon daughters have a greater diffusion coefficient than the charged ones; therefore the equilibrium factor should be related to the radon concentration. To avoid having to consider effects from aerosol concentrations, we selected two sets of results with similar 2,. A 2, that is required to have more restriction in one set, in which the attachment rate constant is 21.0h -~ and the sample variation is 0.1 h-~; 2, is more lax than in the other set, in which the attachment rate constant is 25.0h i and the sample variation is 0.5 h -~. The results are shown in Figs 5 and 6. In Fig. 5, the influence of 2, can be neglected. Roughly speaking, while the radon concentration is lower, the equilibrium factor is
0.30
I
I
I
]
I
r
+
Fig. 6. The changes of equilibrium factors with radon concentration and their corresponding 2,.
higher and decreases with the increasing concentration, but tends to be stable once radon concentration reaches a certain value. In Fig. 6, not only is the equilibrium factor affected by 2,, but it also has the tendency as mentioned above which can be explained as follows. When radon concentration increases, there are more ionizing electrons in air. That will increase the radon daughters' proportion of neutralization, hence the diffusion coefficient is higher. A higher diffusion coefficient makes the deposition velocity of unattached radon daughter increase and the equilibrium factor decrease. When radon concentration increases to a certain value, radon daughters are nearly all neutralized. Therefore, even though the radon concentration keeps increasing, it won't decrease the equilibrium factor. Figure 7 shows
t6.0
I
0.2 c.
15.5
0,28 "g
0.27
E
0.26
T
0.251
q -~
~5.0
~
14.0
13.5
0.24 13.0 023 Q2~
I
I 10000
=
I 20000
=
I 30000
=
I 40000
Rn Concentration ( B q m ' 3 )
Fig. 5. The changes of equilibrium factors with radon concentration.
12.5 0
I
I , tO000
I
I 20000
I
30000
i
I t,OCO0
Rn Concentrotion (Bqm -3)
Fig. 7. The relationship between radon concentration and deposition velocity of unattached radon daughters.
Tieh-Chi Chu and Ho-Ling Liu
548
Table 3. The experimentalresults controlled by radonconcentration
Radon
Group 1 I I 1 2 2 2 2 2 2 2
concentration (Bq m-3) 35565 -+320 13021 +284 8138-+ 107 2519-+25 39013-+239 36811 +- 171 30323+_116 16000-+279 9520 -+5 7578-+68 4730 -+9
~.+ (h-I) 20.9 + 0.5 21.1 +_0.4 21.0-+0.3 20.9-+0.6 25.4-+0,3 24.7_+0,6 24.2-+0,6 25.2+_0.9 25.3 -+0.2 25.4-+0.2 24.5 _+0.3
the relation between the radon concentration and the deposition velocity of unattached radon daughters. The result shows that the effect is more d o m i n a n t if the radon concentration is below l0 kBqm -3. It can be found that as radon concentration is above 10kBqm -3, deposition velocity increases as radon concentration increases. The result agrees with our inference mentioned above. Table 3 shows all the experimental results controlled by radon concentration.
3. The influence of a specific aerosol We used different aerosol sources which are very common in Taiwan, including mosquito coils, electric fumigators, incense, and cigarettes to investigate their influence on the equilibrium factor. The results are shown in Fig. 8. When 2a is less than about 100 h - 1, the equilibrium factor increases with the increasing
1.01
i
I
Equilibrium factor 0.253+-0.003 0.250+0.006 0.251_+0.004 0.266-+0.006 0.265-+0.004 0.274+0.004 0.254-+0.004 0.262-+0,007 0.276+ 0.001 0.273-+0.004 0.286-+0.003
u,]
(mh-I) 14.0+ 0.5 14.1 _+0.4 14.1 _+0.4 13.1 +_0.5 15.0-+0.5 14.1 -+0.4 15.0-+0.5 15.0+_0.5 14.3 -+0.3 14.4-+0.6 13.4-+0.6
2a. It can be derived from U,~<
I
I
i
I
i
i
0.90.8o °°° ~ ~ °
0.7-
<~
0
V
~
V
L
o
o
u
0
°~, oq ~
0.6-
0.5-_
.~
+
w
o/,0.3
q
o :blank set ~, : electric fumigator . : mosquito coil
F-d -f
0.2-
. ',incense
q
v :cigarette
0.1O.C
q
I
I 100
J
I
J
200 Attachment
I 300
400
Rate (h "1)
Fig. 8. The changes of equilibrium factor from various aerosol sources with different attachment rates.
Simulated equilibrium factor studies in radon chamber
1.0 '
I
I
r
I
I
I
I
I
l
I
549 I
I
i
I
0.9
O.8
J
0
~o
o
0.7
o v
v to ID
ii
E L--
.
v
v
O.6
vv
v @~
-
v
O.5 Lk,.
O.4 Ua
w
t.d
v
O.3
&
6
~,~
a
. :blank set * :electric fumigator <>:mosquito coil o incense cigarette
f
AO
oo
0.2 0.1 O9
1
IOO0
i
I
2000
t
I 3000
I
I 4000
Aerosol
I
I 5000
i
Concentration
I 6000
t
I 7000
i
I
8000
( c m -3)
Fig. 9. The relationship between equilibrium factor and various aerosol concentration. thus be in proportion to aerosol concentration &=
Ztot.
fl(d)Z(d) dd
,l,, In Fig. 9, we have replaced 2, with aerosol concentration, and found poor connection between aerosol concentration and equilibrium factor. This is due to the different kinds of aerosol particle size distribution. Table 4 lists the result of each experiment; the greatest median particle size is that of the mosquito coil; following are those of incenses and cigarettes. But there was no obvious difference between the set with the electric fumigator and the blank set. The blank set was the experiment conducted without adding any other aerosol in the normal radon chamber. The aerosol concentration in the normal radon chamber is changeable and the experiment was carried out at a higher aerosol concentration than in the blank set. Therefore, the aerosol concentration is not obviously lower than the other sets. From the
measured equilibrium factor we can prove that 2,, not the aerosol concentration, is the main parameter affecting the equilibrium factor. In addition to aerosol concentration, the aerosol particle size is another important factor to be taken into account. Figure 8 shows that though this experiment used different aerosol sources, it can predict that the data should be very close if we use the Jacobi model to simulate the relation between equilibrium factor and 2,. That means that different kinds of aerosol do not have much influence on unattached radon daughters' clustering, and the difference between each deposition velocity is not so great. However, the incense seems to correspond to the smaller deposition velocity. This could be because its constituents of combustion products are favorable to the unattached radon daughters' clustering, which makes the average diffusion coefficient of unattached radon daughters decrease. In Table 4, the deposition velocity of the electric fumigator and the cigarettes are lower than that of the blank set, but the difference is smaller. The reason is perhaps the same as that in the case of the incense.
Table 4. The experimental results for various types of aerosols Specific aerosol source
Blank Electric Fumigator Mosquito Coil Incense Cigarette
Aerosol concentration (cm ))
Median particle size (#m)
2. (h ')
Equilibrium factor
(mh ])
3793 _+ 854 3510 + 518
0.094 0.083
21.6 + 5.1 17.6 + 2.8
0.299 _+ 0.036 0.298 _+ 0.018
11.7_+0.8 10.3 _+0.7
3805 + 133 2808 + 457 6153 _+ 1038
0.74 0.40 0.22
319.7 _+ 24.7 138.6 + 12.2 130.0 _+ 13.6
0.665 + 0.022 0.745 _+ 0.020 0.650 + 0.021
13.5 _+ 1.5 10.3 _+ 1.0 10.3 _+ 0.9
Tieh-Chi Chu and Ho-Ling Liu
550
Conclusion
RefeFences
As far as the radiation dose is concerned, the major contribution comes from the inhalation of short-lived radon daughters. This experiment studies the relation between equilibrium factor and environmental parameters in order to understand radon daughters' behavior in air. In the present work, this experiment was accomplished in a radon chamber with 23 m 3 at TRMC. The parameters involved include humidity, radon concentration and aerosols. The result shows that the equilibrium factor increases as the humidity increases, but decreases when radon concentration increases. When the attachment rate calculated from the aerosol distribution increases, the equilibrium factor increases as well. Among these parameters, aerosols have the greatest influence on equilibrium factor. The correspondent deposition velocity can be obtained from the fitting of results of each experiment by the Jacobi model. On analyzing, we found that the deposition velocity decreases as the humidity increases but increases as the radon concentration increases. Different aerosols also cause different deposition velocity. Among these environmental parameters, humidity has the greatest influence.
Chen C. J., Tung C. W. and Lin Y. M. (1993) Specification of a radon chamber in Taiwan: The 3rd symposium on radiation monitoring technology, pp. 2.1-2.10. Kaoshiung, Taiwan, R.O.C. Goldstein S. D. and Hopke P. K. (1985) Environmental neutralization of Polonium 218. Environ. Sci. Technol. 19, 146-150, Jacobi W. (1972) Activity and potential ~-energy of 222Rn and 22°Rn-daughtersin different air atmosphere. Health Phys. 22, 441-450. Nero A. V., Sextro R. G., Doyle S. M., Moed B. A,, Nazaroff W. W., Revzan K. L. and Schwehr M. B. (1985) Characterizing the sources, range, and environmental influencesof radon 222 and its decay products. Sei. Total Environ. 45, 234-244. Porstendorfer J. and Reinlking A. (1992) Indoor behaviour and characteristics of radon progeny. Radiat. Protect. Dosim. 45(1/4), 303-311. Porstendorfer J., Reinlking A. and Becket K. H. (1987) Free fraction, attachment rates, and plate-out rates of radon daughters in house. In Radon and its Decay Products: Occurrence, Properties, and Health Effect (Hopke P. K., Ed.), pp. 285-300. ACS Symposium Series 331, Washington, D.C. Raabe O. G. (1968) Measurement of the diffusioncoefficient of RaA. Nature 217, 1143-1145. Raes F., Janssens A. and Vanmarcke H. (1985) A closer look at the behaviour of radioactive decay products in air. Sci. Total Environ. 45, 205-218. Raghunath B. and Kotrappa P. (1979) Diffusion coefficient of decay products of radon and thoron. J. Aerosol. Sci. 10, 133-138. Schery S. D., Wang R., Eack K. and Whittlestone S. (1992) New models for radon progeny near the earth's surface. Radiat. Protect. Dosim. 45(I/4), 343-347.
Acknowledgements--The authors are grateful to Professor
P. S. Weng for useful suggestions and also express their hearty thanks to Mr Y. M. Lin, Dr C. J. Chen and Mr C. J. Liu of Taiwan Radiation Monitoring Center, Republic of China, for their assistance in this work.