Applied Radiation and Isotopes 95 (2015) 102–107
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Influence of humidity on radon and thoron exhalation rates from building materials M. Janik a,n, Y. Omori b, H. Yonehara a a b
Research Center for Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan Department of Radiation Physics and Chemistry, School of Medicine, Fukushima Medical University, 1 Hikarigaoka, Fukushima 960-1295, Japan
H I G H L I G H T S
Humidity has significant effect on radon and thoron exhalation rates. The exhalation rate tendency was the same for samples with different porosity and density. Humidity effect should be considered for calculation of the equivalent dose from radon and thoron.
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2014 Received in revised form 27 September 2014 Accepted 10 October 2014 Available online 24 October 2014
The contributions of radon and thoron from building materials to total radon (thoron) entry rates in dwellings range from almost zero to several percent. It is necessary to measure radon and thoron exhalation rates, among other things, to assess the radiological hazard to human health in a living environment. Brick and granite specimens were used to study the changes of these rates as a function of the relative and absolute humidities. Measurement results showed that radon and thoron exhalation rates change to humidity with the same trends as well as effective dose could be changed by the factor of 2 due to this. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon Thoron Exhalation rate Humidity
1. Introduction The earth's crust contains small amounts of the primordial radionuclides 238U and 232Th which decay through a chain of radioactive nuclides until they produce stable isotopes of lead. Most of the decay products are isotopes of solid elements but two are gases: 222Rn (radon) from the decay of uranium and 220Rn (thoron) from thorium. They can migrate to the earth's surface by transport of radon relative to the gas or liquid (molecular diffusion) and with the gas or liquid (convection or groundwater flow) (Cothern and Smith, 1987). The half-lives of radon (3.82 d) and thoron (55 s) are different and this difference is important when assessing their release from the ground and their distribution in the open air above the ground as well as in the room air of buildings. Radon and thoron enter the atmosphere mainly by crossing the soil–air, building material–air or water–air interfaces. In recent decades concern about public exposures due to natural radiation sources has increased. The main contribution to natural radiation comes from terrestrial sources contaminated with naturally n
Corresponding author. E-mail address: [email protected] (M. Janik).
http://dx.doi.org/10.1016/j.apradiso.2014.10.007 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
occurring radioactive materials (NORM), such as uranium, thorium and potassium as well as any of their decay products, such as radium and radon. The indoor radon concentrations mainly depend on radon that has penetrated from the surrounding soil through gaps, cracks, etc., but also on radon exhalation from building materials and radon in domestic water supplies. Recent long-term surveys of indoor thoron and its progeny showed that doses from the thoron series should no longer be considered as negligible. In the report a dosimetric approach was used to calculate the effective dose per unit of equilibrium equivalent concentration (EEC) for thoron progeny and it was almost four times greater than that of radon progeny. In this context the dose contributions of thoron (220Rn) and its decay products can exceed the corresponding radon (222Rn) values, as in the indoor environment thoron is considered more likely to originate from exhalation of building materials rather than from soils due to its shorter half-life. Consequently indoor radon and thoron concentrations should be monitored and building materials should be classified on the basis of their radon and thoron exhalation rates which can then be correlated with their activity concentration index (I) value, based on 232Th, 226Ra and 40 K concentrations.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
In this paper, the influence of relative and absolute humidities on radon and thoron exhalation rates was examined.
Table 1 Physical parameters of specimens. Specimen
Size (cm)
Density (g cm 3)
Concentration (Bq kg 1) 226
232
572 7 4 222 7 2
74 73 3747 5
Ra
103
2. Materials and methods Two materials, brick and granite, with high radium and thorium concentrations respectively, were used for measurements. The physical parameters and concentration of radionuclides, i.e. 226 Ra from the uranium decay chain (radon parent) and 232Th from the thorium decay chain (thoron parent) measured by a HPGe detector system are listed in Table 1.
Th
2.1. Measurement system Brick Granite
21 10 6 20 20 12
1.12 2.30
Fig. 1. Schematic of the system for exhalation rate measurement.
A modified standard method utilized radon and thoron gas monitor (type RAD7) was used to measure radon exhalation (RnExh) and thoron exhalation (TnExh) rates from the materials (Fig. 1). More details about the standard method and its validation can be found elsewhere (Hassan et al., 2011a). Before the measurements the specimens were dried in a temperature controlled furnace (oven) at 110 1C for more than 24 h to ensure complete removal of moisture. Later, specimens were placed one at a time in the 150 dm3 accumulation (measurement) chamber under a controlled atmosphere. The big volume of the chamber (150 dm3) was required for stabilization of the air atmosphere inside. Humidity was generated by a bubbling method which utilized a hot water bath. Before starting the measurement, each specimen was kept at the desired measurement conditions for 3 days to reach a state of equilibrium between it and the chamber environment. Thirty minutes before the start of the measurement the chamber was flushed with room air using a high volume pump (10 L min 1) to remove accumulated radon and thoron. The environment conditions inside the chamber were changed a few percent from the initial state but equilibrium was reached again within one to a few hours (depending on the final conditions) after starting measurements (Fig. 2). The chamber tightness was checked by an appropriate test utilizing CO2. The accumulation chamber was connected to the RAD7 monitor using vinyl tubing, and a gas-drying unit filled with a desiccant was installed between them, to maintain the relative humidity at o 10% within the measurement system as recommended by the manufacturer. The system was a closed loop in which the gas was circulated continuously with the flow rate generated by an external pump with the same flow rate as the RAD7 pump. The flow rate was measured for the correction factor calculation for thoron concentration. The concentrations of radon and thoron released from each specimen inside the chamber were allowed to
Fig. 2. Changes of relative humidity and temperature in the accumulation chamber during measurements.
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M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
build up with time and were measured by the RAD7 monitor in a cycle time of 60 min. Simultaneously, the temperature and the relative humidity inside the accumulation chamber were also recorded. The measurements were performed in an airconditioned room with temperature controlled at 22–23 1C and normal room atmospheric pressure. The RAD7 device was calibrated in the reference thoron atmosphere at the PTB (The Physikalisch-Technische Bundesanstalt, Germany) (Röttger et al., 2010) and in the NIRS radon chamber (Tokonami et al., 2008). When a specimen is put into the accumulation chamber, the radon exhaled from the specimen increases exponentially whereas the thoron concentration can reach stable condition very quickly as presented in Fig. 3 for granite, upper and lower graphs, respectively.
where λeff is the effective decay constant of radon in the measurement chamber. This λeff ¼ λRn þ λl þ λb , where λRn is the physical decay constant of radon and λl and λb are the leakage from the chamber and back diffusion, respectively. For calculation of the thoron exhalation rate it is assumed that the decay constant of the loss processes is small compared with the decay constant of thoron (λTn ). Therefore, if the equilibrium between exhaled and decay thoron is assumed the exhalation rate for thoron can be calculated using the average thoron concentration (CTn) according to the equation: TnExh ¼
C Tn λTn V S
ð5Þ
where λTn is thoron decay constant.
2.2. Calculation To calculate the exhalation rate, first the data from the RAD7 were read into an external PC-system, and then they were approximated by the proper equation to derive the exhalation rate with the units of mBq m2 s 1 by using a computer code written in R language (R Core Team, 2013). The increase of radon activity concentration can be calculated with the following exponential equation: C Rn ¼
Exh S
λRn V eff
ð1 e λRn t Þ
CλV : S ð1 e λt Þ
ð2Þ
In the case of radon due to its longer half-life compared to thoron the back diffusion, diffusion or ventilation loss caused by leakiness of the accumulation (measurement) chamber cannot be neglected. When the function increase is an exponential type like C Rn ¼ C eff ð1 e λeff t Þ
ð3Þ
it is possible to estimate unknown parameters Ceff and λeff using the exponential regression discussed later. This can be modelled in the R and finally Eq. (2) can be rewritten to the following RnExh ¼
C eff λeff V
S ð1 e λeff t Þ
Calculations were made and graphs were prepared using R-language (free software environment for statistical computing and graphics) (R Core Team, 2013) and RStudio (free and open source integrated development environment (IDE) for R) (R Studio Team, 2012). Text was prepared with TeX and utilized MiKTeX and TeXstudio.
ð1Þ
where V eff ¼ V chamber V specimen is volume of the free space in the accumulation chamber, S is specimen area and λRn is radon decay constant. The exhalation rate ðExhÞ of a given amount of material may thus be determined from the equation: Exh ¼
2.3. Software
ð4Þ
3. Results The effect of the humidity in the environmental atmosphere on exhalation rates for the two different materials – brick (upper graphs) and granite (lower graphs) can be seen in Figs. 4 and 5 for radon and thoron, respectively. The RnExh was higher for the brick specimen than for the granite one but the opposite was seen for TnExh. In the examined range of relative (RH: 7–88%) and absolute (AH: 1–24 g m 3) humidities the radon and thoron exhalation rate increased and then above a certain level decreased or reached a plateau. The plotted fits of the polynomial model to RnExh and TnExh with the R2 parameter as 0.80 (RnExh brick), 0.71 (RnExh granite) and 0.82 (TnExh brick) and 0.84 (TnExh granite). Obtained results were compared with other radon and thoron exhalation measurement results. Since experimental conditions were different among studies, with the main differences being the volume of the accumulation chamber, measurement method and measured parameters, the features of the variation were analyzed. It was concluded that the present trend of changes was consistent with other results (Cothern and Smith, 1987; Stranden et al., 1984).
Fig. 3. Radon (upper graph) and thoron (lower graph) concentrations as a function of time inside the accumulation chamber due to radon and thoron exhalation from the granite specimen.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
105
Fig. 4. Relationship between radon exhalation rate (brick, upper graph and granite, lower graph) and absolute humidity.
Fig. 5. Relationship between thoron exhalation rate (brick, upper graph and granite, lower graph) and absolute humidity.
On the other hand, the exponential increase of radon and thoron exhalation rates with increasing absolute humidity for granite only was observed by Hassan et al. (2011b). The difference in tendency between their work and current work lies in the different range of AH: 1–19 g cm 3 for the former and 1–24 g cm 3 for the latter. As described in the literature, the effect of moisture on exhalation rate can be explained from a microstructural viewpoint (Stranden et al., 1984) and from a behavioral viewpoint, that is the deposition of aerosols on the surfaces (Yu et al., 1999, 2000). In the radon emanation process most of the radon is generated by alpha recoil (transfer from solids into pore spaces) from the inner grain regions and from grain surfaces. The radon recoils into internal pores (nanopores) and then diffuses out to the intergranular pores. Moisture in pore spaces between grains can increase the direct-recoil fraction relative to air-filled pores because of the shorter recoil distance in water. However, if the nanopores within individual grains are filled with water, the rates of radon diffusion from within the grains are reduced. Experiments carried out by several groups (Chau et al., 2005; Cozmuta et al., 2003; Faheem and Matiullah, 2008; Stranden et al., 1984) confirmed the hypothesis of the presence and role of nanopores in radon transfer from solids. In addition, when relative humidity of the ambient atmosphere is high the higher water content in the air will lead to greater deposition of aerosols on the internal building surfaces, which will
lower the 222Rn and 220Rn emanation rates and reduce the gas concentrations. It should be noted that the mass of the granite specimen did not change with increasing humidity for the range from 7 to 88% (R2 ¼ 0:78) in contrast to the mass of the brick specimen which increased linearity with R2 ¼ 0:95 (Fig. 6). This phenomenon could be explained by the porosity (it was low for granite and high for brick) and density (high for granite and low for brick) of the measured materials.
4. Assessment The following scenario was considered to assess the contribution of humidity of tested materials to dose from indoor radon and thoron. Under steady conditions, the radon or thoron level in a room due to exhalation from an indoor material approaches a steady-state radon or thoron concentration, C in Bq m 3 that can be expressed by following equation: C¼
Exh Aw ðλ þ λw Þ V w
ð6Þ
where Exh is radon or thoron exhalation rate of the material (Bq m 2 s 1), A is the area of the material exhaling radon or
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M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
Fig. 6. Changes of specimen weight versus relative humidity in the accumulation chamber.
Table 2 Radon and thoron concentrations in Bq m 3 (annual effective dose in mSv y 1 in parenthesis) calculated for different (minimal and maximum) absolute humidity (AH), exhalation rates (Exh) and air exchange rates (ACH). AH (g cm 3)
Material
Radon 1.35 20 1.35 22
Granite Granite Brick Brick
Thoron 1.35 22 1.35 20
Brick Brick Granite Granite
TnExh (mBq s 1 m 2)
0.70 1.33 3.09 7.07 100 640 2515 5900
thoron (m2), V is the air volume of the room (m 3) i.e. the room volume minus the volume occupied by the room contents, λ is the radon or thoron decay constant (s 1), and λw is the air removal rate due to air exchange rate (s 1). In this scenario granite and brick were used as raw (without painting) decorative materials for walls and flooring. The room size was assumed to be 5 (length) 4 (width) 2.5 (height) m. The area of the measured materials that exhaled the radon or thoron was 17 m2 (about 20% of the total wall, ceiling and floor area). The concentration depends also on air exchange rate (ACH) of the room. In a typical dwelling the ACH ranges from 0.20 to 1.20 h 1 (Brelih and Seppnen, 2011) with 0.5 as average. These boundary conditions were used for the calculation and the results are presented in Table 2. The difference in behavior of radon and thoron with respect to the air exhalation rate is observed. The higher ACH reduce radon concentration whereas there is no effect on thoron. Latest investigations (Kavasi et al., 2012) in the NIRS thoron experimental room showed that unmixed condition clearly causes inhomogeneous thoron concentration but increase the concentration of thoron progeny. On the other hand a mixed condition was reported to cause homogeneity of thoron gas with one order magnitude lower concentration than for unmixed air. This was attributed mainly to relatively long-life thoron decay product, i.e. 212Pb (t 1=2 ¼ 10:6 h) which can migrate from the source and finally decay to the important, from a dosimetric point of view, alpha-emitter 212Bi (t 1=2 ¼ 60:6 min). The contribution of radon at the average ACH ¼ 0.5 and the highest measured value of RnExh ¼0.7 mBq s 1 m 2 is relatively
ACH ¼ 0 (h 1)
ACH ¼ 0.2
113 (2.86) 215 (5.43) 500 (12.62) 1146 (28.88) 3 17 69 161
(0.02) (0.10) (0.38) (0.90)
ACH ¼ 0.5
ACH ¼ 1.2
4 8 18 42
(0.10) (0.20) (0.46) (1.05)
2 3 7 17
(0.04) (0.08) (0.19) (0.43)
1 1 3 7
(0.02) (0.03) (0.08) (0.18)
3 17 68 160
(0.02) (0.10) (0.38) (0.90)
3 17 68 159
(0.02) (0.10) (0.38) (0.89)
3 17 67 157
(0.01) (0.10) (0.37) (0.88)
small, only 17 Bq m 3. For the same ACH and the highest TnExh the thoron concentration on the material surface is ten times higher (159 Bq m 3) than that of radon. The annual effective dose due to indoor exposure was calculated based on formulas given by UNSCEAR (2000) report: E ¼ C F T EEC
ð7Þ
where C is the average radon or thoron concentration (Bq m 3), F is the equilibrium factor, i.e. typically 0.4 for radon indoor and 0.02 for thoron indoor, T is home occupancy time, i.e. 7000 h, and EEC is the recommended value to convert radon or thoron equilibrium-equivalent concentration to population effective dose, i.e. 9 nSv (Bq h m 3) 1 for radon and 40 nSv (Bq h m 3) 1 for thoron. For radon and based on the highest concentration calculation and ACH¼ 0.5, the population dose due to indoor radon exposure is determined to be 0.43 mSv whereas the population dose due to indoor thoron exposure is 0.89 mSv. On the other hand for the lowest radon and thoron concentrations it is 0.04 and 0.02 mSv, respectively. In the worst case when there was no ventilation (ACH ¼0) the indoor radon concentration will rise significantly above 100 Bq m 3 with the annual dose being about 3 mSv if the examined area is covered with a material having a radon exhalation rate of 0.7 mBq s 1 m 2 (minimal RnExh). In the thoron case the dose does not exceed 0.90 mSv for maximum measured TnExh but is constant and does not depends on ACH.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
The total population dose due to indoor radon and thoron exposure, with the assumption of ACH ¼ 0.5, is found to be 0.42 mSv for the lowest humidity (AH¼ 1.35 g cm 3) up to 0.97 mSv for the highest humidity (AH 20 g cm 3 ) for granite while the corresponding values are 0.21 and 0.53 mSv for brick. These results show that effective dose can change by the factor 2 due to humidity. Further investigations will be necessary to find relations between radon and thoron exhalation to other environmental parameters, e.g., temperature and pressure as well as their gradients. 5. Conclusion The main purpose of this study was to find a relationship between the ambient atmosphere, especially its humidity and radon and thoron exhalation rates. Two different materials with high radium (brick) and high thorium (granite) concentrations were chosen for measurements. The findings are summarized below. (1) The obtained results showed that for both materials radon and thoron exhalation rates increased and that above a certain level they decreased or reached a plateau; these trends were consistent with results of previous studies. (2) The results showed the same trends of exhalations for both materials with different structural characteristics (porosity and density) regardless of their weight. (3) It was concluded that effective dose could be changed by the factor of 2 due to humidity. (4) The present results indicated that humidity should be taken into consideration to calculate the equivalent dose from radon and thoron.
References Brelih, N., Seppnen, O., 2011. Ventilation rates and IAQ in European standards and national regulations. In: The Proceedings of the 32nd AIVC Conference and 1st TightVent Conference in Brussels, pp. 12–13. Chau, N.D., Chrusciel, E., Prokolski, L., 2005. Factors controlling measurements of radon mass exhalation rate. J. Environ. Radioact. 82 (3), 363–369.
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Cothern, C.R., Smith, J.E., 1987. Environmental Radon. Springer. Cozmuta, I., Van der Graaf, E., DeMeijer, R., 2003. Moisture dependence of radon transport in concrete: measurements and modeling. Health Phys. 85 (4), 438–456. Faheem, M., Matiullah, 2008. Radon exhalation and its dependence on moisture content from samples of soil and building materials. Radiat. Meas. 43 (8), 1458–1462. Hassan, N.M., Hosoda, M., Iwaoka, K., Sorimachi, A., Janik, M., Kranrod, C., Sahoo, S.K., Ishikawa, T., Yonehara, H., Fukushi, M., et al., 2011a. Simultaneous measurement of radon and thoron released from building materials used in Japan. Progr. Nucl. Sci. Technol. 1, 404–407. Hassan, N.M., Tokonami, S., Fukushi, M., 2011b. A simple technique for studying the dependence of radon and thoron exhalation rate from building materials on absolute humidity. J. Radioanal. Nucl. Chem. 287 (1), 185–191. Kavasi, N., Janik, M., Prasad, G., Omori, Y., Ishikawa, T., Yonehara, H., 2012. Thoron experimental room at the National Institute of Radiological Sciences (NIRS). Jpn. Radiat. Prot. Dosim. 152, 150–153. R Core Team, 2013. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria, URL 〈http://www.R-pro ject.org/〉. R Studio Team, 2012. RStudio. RStudio, Boston, MA, URL 〈http://www.rstudio.org/〉. Röttger, A., Honig, A., Dersch, R., Ott, O., Arnold, D., 2010. A primary standard for activity concentration of 220Rn (thoron) in air. Appl. Radiat. Isot. 68 (7), 1292–1296. Stranden, E., Kolstad, A., Lind, B., 1984. The influence of moisture and temperature on radon exhalation. Radiat. Prot. Dosim. 7 (1–4), 55–58. Tokonami, S., Ishikawa, T., Sorimachi, A., Takahashi, H., Miyahara, N., 2008. The Japanese radon and thoron reference chambers. In: The Natural Radiation Environment: 8th International Symposium (NRE VIII). vol. 1034. AIP Publishing, pp. 202–205. UNSCEAR, 2000. United Nations Scientific Committee on the Effects of Atomic, Radiation Sources and Effects of Ionizing Radiation: Sources, vol. 1., United Nations Publications. Yu, K., Cheung, T., Guan, Z., Mui, B., Ng, Y., 2000. 222Rn, 220Rn and their progeny concentrations in offices in Hong Kong. J. Environ. Radioact. 48 (Apr (2)), 211–221. Yu, K., Cheung, T., Guan, Z., Young, E., Mui, B., Wong, Y., 1999. Concentrations of 222 Rn, 220Rn and their progeny in residences in Hong Kong. J. Environ. Radioact. 45 (Nov (3)), 291–308.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
Influence of humidity on radon and thoron exhalation rates from building materials M. Janik a,n, Y. Omori b, H. Yonehara a a b
Research Center for Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan Department of Radiation Physics and Chemistry, School of Medicine, Fukushima Medical University, 1 Hikarigaoka, Fukushima 960-1295, Japan
H I G H L I G H T S
Humidity has significant effect on radon and thoron exhalation rates. The exhalation rate tendency was the same for samples with different porosity and density. Humidity effect should be considered for calculation of the equivalent dose from radon and thoron.
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2014 Received in revised form 27 September 2014 Accepted 10 October 2014 Available online 24 October 2014
The contributions of radon and thoron from building materials to total radon (thoron) entry rates in dwellings range from almost zero to several percent. It is necessary to measure radon and thoron exhalation rates, among other things, to assess the radiological hazard to human health in a living environment. Brick and granite specimens were used to study the changes of these rates as a function of the relative and absolute humidities. Measurement results showed that radon and thoron exhalation rates change to humidity with the same trends as well as effective dose could be changed by the factor of 2 due to this. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon Thoron Exhalation rate Humidity
1. Introduction The earth's crust contains small amounts of the primordial radionuclides 238U and 232Th which decay through a chain of radioactive nuclides until they produce stable isotopes of lead. Most of the decay products are isotopes of solid elements but two are gases: 222Rn (radon) from the decay of uranium and 220Rn (thoron) from thorium. They can migrate to the earth's surface by transport of radon relative to the gas or liquid (molecular diffusion) and with the gas or liquid (convection or groundwater flow) (Cothern and Smith, 1987). The half-lives of radon (3.82 d) and thoron (55 s) are different and this difference is important when assessing their release from the ground and their distribution in the open air above the ground as well as in the room air of buildings. Radon and thoron enter the atmosphere mainly by crossing the soil–air, building material–air or water–air interfaces. In recent decades concern about public exposures due to natural radiation sources has increased. The main contribution to natural radiation comes from terrestrial sources contaminated with naturally n
Corresponding author. E-mail address: [email protected] (M. Janik).
http://dx.doi.org/10.1016/j.apradiso.2014.10.007 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
occurring radioactive materials (NORM), such as uranium, thorium and potassium as well as any of their decay products, such as radium and radon. The indoor radon concentrations mainly depend on radon that has penetrated from the surrounding soil through gaps, cracks, etc., but also on radon exhalation from building materials and radon in domestic water supplies. Recent long-term surveys of indoor thoron and its progeny showed that doses from the thoron series should no longer be considered as negligible. In the report a dosimetric approach was used to calculate the effective dose per unit of equilibrium equivalent concentration (EEC) for thoron progeny and it was almost four times greater than that of radon progeny. In this context the dose contributions of thoron (220Rn) and its decay products can exceed the corresponding radon (222Rn) values, as in the indoor environment thoron is considered more likely to originate from exhalation of building materials rather than from soils due to its shorter half-life. Consequently indoor radon and thoron concentrations should be monitored and building materials should be classified on the basis of their radon and thoron exhalation rates which can then be correlated with their activity concentration index (I) value, based on 232Th, 226Ra and 40 K concentrations.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
In this paper, the influence of relative and absolute humidities on radon and thoron exhalation rates was examined.
Table 1 Physical parameters of specimens. Specimen
Size (cm)
Density (g cm 3)
Concentration (Bq kg 1) 226
232
572 7 4 222 7 2
74 73 3747 5
Ra
103
2. Materials and methods Two materials, brick and granite, with high radium and thorium concentrations respectively, were used for measurements. The physical parameters and concentration of radionuclides, i.e. 226 Ra from the uranium decay chain (radon parent) and 232Th from the thorium decay chain (thoron parent) measured by a HPGe detector system are listed in Table 1.
Th
2.1. Measurement system Brick Granite
21 10 6 20 20 12
1.12 2.30
Fig. 1. Schematic of the system for exhalation rate measurement.
A modified standard method utilized radon and thoron gas monitor (type RAD7) was used to measure radon exhalation (RnExh) and thoron exhalation (TnExh) rates from the materials (Fig. 1). More details about the standard method and its validation can be found elsewhere (Hassan et al., 2011a). Before the measurements the specimens were dried in a temperature controlled furnace (oven) at 110 1C for more than 24 h to ensure complete removal of moisture. Later, specimens were placed one at a time in the 150 dm3 accumulation (measurement) chamber under a controlled atmosphere. The big volume of the chamber (150 dm3) was required for stabilization of the air atmosphere inside. Humidity was generated by a bubbling method which utilized a hot water bath. Before starting the measurement, each specimen was kept at the desired measurement conditions for 3 days to reach a state of equilibrium between it and the chamber environment. Thirty minutes before the start of the measurement the chamber was flushed with room air using a high volume pump (10 L min 1) to remove accumulated radon and thoron. The environment conditions inside the chamber were changed a few percent from the initial state but equilibrium was reached again within one to a few hours (depending on the final conditions) after starting measurements (Fig. 2). The chamber tightness was checked by an appropriate test utilizing CO2. The accumulation chamber was connected to the RAD7 monitor using vinyl tubing, and a gas-drying unit filled with a desiccant was installed between them, to maintain the relative humidity at o 10% within the measurement system as recommended by the manufacturer. The system was a closed loop in which the gas was circulated continuously with the flow rate generated by an external pump with the same flow rate as the RAD7 pump. The flow rate was measured for the correction factor calculation for thoron concentration. The concentrations of radon and thoron released from each specimen inside the chamber were allowed to
Fig. 2. Changes of relative humidity and temperature in the accumulation chamber during measurements.
104
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
build up with time and were measured by the RAD7 monitor in a cycle time of 60 min. Simultaneously, the temperature and the relative humidity inside the accumulation chamber were also recorded. The measurements were performed in an airconditioned room with temperature controlled at 22–23 1C and normal room atmospheric pressure. The RAD7 device was calibrated in the reference thoron atmosphere at the PTB (The Physikalisch-Technische Bundesanstalt, Germany) (Röttger et al., 2010) and in the NIRS radon chamber (Tokonami et al., 2008). When a specimen is put into the accumulation chamber, the radon exhaled from the specimen increases exponentially whereas the thoron concentration can reach stable condition very quickly as presented in Fig. 3 for granite, upper and lower graphs, respectively.
where λeff is the effective decay constant of radon in the measurement chamber. This λeff ¼ λRn þ λl þ λb , where λRn is the physical decay constant of radon and λl and λb are the leakage from the chamber and back diffusion, respectively. For calculation of the thoron exhalation rate it is assumed that the decay constant of the loss processes is small compared with the decay constant of thoron (λTn ). Therefore, if the equilibrium between exhaled and decay thoron is assumed the exhalation rate for thoron can be calculated using the average thoron concentration (CTn) according to the equation: TnExh ¼
C Tn λTn V S
ð5Þ
where λTn is thoron decay constant.
2.2. Calculation To calculate the exhalation rate, first the data from the RAD7 were read into an external PC-system, and then they were approximated by the proper equation to derive the exhalation rate with the units of mBq m2 s 1 by using a computer code written in R language (R Core Team, 2013). The increase of radon activity concentration can be calculated with the following exponential equation: C Rn ¼
Exh S
λRn V eff
ð1 e λRn t Þ
CλV : S ð1 e λt Þ
ð2Þ
In the case of radon due to its longer half-life compared to thoron the back diffusion, diffusion or ventilation loss caused by leakiness of the accumulation (measurement) chamber cannot be neglected. When the function increase is an exponential type like C Rn ¼ C eff ð1 e λeff t Þ
ð3Þ
it is possible to estimate unknown parameters Ceff and λeff using the exponential regression discussed later. This can be modelled in the R and finally Eq. (2) can be rewritten to the following RnExh ¼
C eff λeff V
S ð1 e λeff t Þ
Calculations were made and graphs were prepared using R-language (free software environment for statistical computing and graphics) (R Core Team, 2013) and RStudio (free and open source integrated development environment (IDE) for R) (R Studio Team, 2012). Text was prepared with TeX and utilized MiKTeX and TeXstudio.
ð1Þ
where V eff ¼ V chamber V specimen is volume of the free space in the accumulation chamber, S is specimen area and λRn is radon decay constant. The exhalation rate ðExhÞ of a given amount of material may thus be determined from the equation: Exh ¼
2.3. Software
ð4Þ
3. Results The effect of the humidity in the environmental atmosphere on exhalation rates for the two different materials – brick (upper graphs) and granite (lower graphs) can be seen in Figs. 4 and 5 for radon and thoron, respectively. The RnExh was higher for the brick specimen than for the granite one but the opposite was seen for TnExh. In the examined range of relative (RH: 7–88%) and absolute (AH: 1–24 g m 3) humidities the radon and thoron exhalation rate increased and then above a certain level decreased or reached a plateau. The plotted fits of the polynomial model to RnExh and TnExh with the R2 parameter as 0.80 (RnExh brick), 0.71 (RnExh granite) and 0.82 (TnExh brick) and 0.84 (TnExh granite). Obtained results were compared with other radon and thoron exhalation measurement results. Since experimental conditions were different among studies, with the main differences being the volume of the accumulation chamber, measurement method and measured parameters, the features of the variation were analyzed. It was concluded that the present trend of changes was consistent with other results (Cothern and Smith, 1987; Stranden et al., 1984).
Fig. 3. Radon (upper graph) and thoron (lower graph) concentrations as a function of time inside the accumulation chamber due to radon and thoron exhalation from the granite specimen.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
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Fig. 4. Relationship between radon exhalation rate (brick, upper graph and granite, lower graph) and absolute humidity.
Fig. 5. Relationship between thoron exhalation rate (brick, upper graph and granite, lower graph) and absolute humidity.
On the other hand, the exponential increase of radon and thoron exhalation rates with increasing absolute humidity for granite only was observed by Hassan et al. (2011b). The difference in tendency between their work and current work lies in the different range of AH: 1–19 g cm 3 for the former and 1–24 g cm 3 for the latter. As described in the literature, the effect of moisture on exhalation rate can be explained from a microstructural viewpoint (Stranden et al., 1984) and from a behavioral viewpoint, that is the deposition of aerosols on the surfaces (Yu et al., 1999, 2000). In the radon emanation process most of the radon is generated by alpha recoil (transfer from solids into pore spaces) from the inner grain regions and from grain surfaces. The radon recoils into internal pores (nanopores) and then diffuses out to the intergranular pores. Moisture in pore spaces between grains can increase the direct-recoil fraction relative to air-filled pores because of the shorter recoil distance in water. However, if the nanopores within individual grains are filled with water, the rates of radon diffusion from within the grains are reduced. Experiments carried out by several groups (Chau et al., 2005; Cozmuta et al., 2003; Faheem and Matiullah, 2008; Stranden et al., 1984) confirmed the hypothesis of the presence and role of nanopores in radon transfer from solids. In addition, when relative humidity of the ambient atmosphere is high the higher water content in the air will lead to greater deposition of aerosols on the internal building surfaces, which will
lower the 222Rn and 220Rn emanation rates and reduce the gas concentrations. It should be noted that the mass of the granite specimen did not change with increasing humidity for the range from 7 to 88% (R2 ¼ 0:78) in contrast to the mass of the brick specimen which increased linearity with R2 ¼ 0:95 (Fig. 6). This phenomenon could be explained by the porosity (it was low for granite and high for brick) and density (high for granite and low for brick) of the measured materials.
4. Assessment The following scenario was considered to assess the contribution of humidity of tested materials to dose from indoor radon and thoron. Under steady conditions, the radon or thoron level in a room due to exhalation from an indoor material approaches a steady-state radon or thoron concentration, C in Bq m 3 that can be expressed by following equation: C¼
Exh Aw ðλ þ λw Þ V w
ð6Þ
where Exh is radon or thoron exhalation rate of the material (Bq m 2 s 1), A is the area of the material exhaling radon or
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M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
Fig. 6. Changes of specimen weight versus relative humidity in the accumulation chamber.
Table 2 Radon and thoron concentrations in Bq m 3 (annual effective dose in mSv y 1 in parenthesis) calculated for different (minimal and maximum) absolute humidity (AH), exhalation rates (Exh) and air exchange rates (ACH). AH (g cm 3)
Material
Radon 1.35 20 1.35 22
Granite Granite Brick Brick
Thoron 1.35 22 1.35 20
Brick Brick Granite Granite
TnExh (mBq s 1 m 2)
0.70 1.33 3.09 7.07 100 640 2515 5900
thoron (m2), V is the air volume of the room (m 3) i.e. the room volume minus the volume occupied by the room contents, λ is the radon or thoron decay constant (s 1), and λw is the air removal rate due to air exchange rate (s 1). In this scenario granite and brick were used as raw (without painting) decorative materials for walls and flooring. The room size was assumed to be 5 (length) 4 (width) 2.5 (height) m. The area of the measured materials that exhaled the radon or thoron was 17 m2 (about 20% of the total wall, ceiling and floor area). The concentration depends also on air exchange rate (ACH) of the room. In a typical dwelling the ACH ranges from 0.20 to 1.20 h 1 (Brelih and Seppnen, 2011) with 0.5 as average. These boundary conditions were used for the calculation and the results are presented in Table 2. The difference in behavior of radon and thoron with respect to the air exhalation rate is observed. The higher ACH reduce radon concentration whereas there is no effect on thoron. Latest investigations (Kavasi et al., 2012) in the NIRS thoron experimental room showed that unmixed condition clearly causes inhomogeneous thoron concentration but increase the concentration of thoron progeny. On the other hand a mixed condition was reported to cause homogeneity of thoron gas with one order magnitude lower concentration than for unmixed air. This was attributed mainly to relatively long-life thoron decay product, i.e. 212Pb (t 1=2 ¼ 10:6 h) which can migrate from the source and finally decay to the important, from a dosimetric point of view, alpha-emitter 212Bi (t 1=2 ¼ 60:6 min). The contribution of radon at the average ACH ¼ 0.5 and the highest measured value of RnExh ¼0.7 mBq s 1 m 2 is relatively
ACH ¼ 0 (h 1)
ACH ¼ 0.2
113 (2.86) 215 (5.43) 500 (12.62) 1146 (28.88) 3 17 69 161
(0.02) (0.10) (0.38) (0.90)
ACH ¼ 0.5
ACH ¼ 1.2
4 8 18 42
(0.10) (0.20) (0.46) (1.05)
2 3 7 17
(0.04) (0.08) (0.19) (0.43)
1 1 3 7
(0.02) (0.03) (0.08) (0.18)
3 17 68 160
(0.02) (0.10) (0.38) (0.90)
3 17 68 159
(0.02) (0.10) (0.38) (0.89)
3 17 67 157
(0.01) (0.10) (0.37) (0.88)
small, only 17 Bq m 3. For the same ACH and the highest TnExh the thoron concentration on the material surface is ten times higher (159 Bq m 3) than that of radon. The annual effective dose due to indoor exposure was calculated based on formulas given by UNSCEAR (2000) report: E ¼ C F T EEC
ð7Þ
where C is the average radon or thoron concentration (Bq m 3), F is the equilibrium factor, i.e. typically 0.4 for radon indoor and 0.02 for thoron indoor, T is home occupancy time, i.e. 7000 h, and EEC is the recommended value to convert radon or thoron equilibrium-equivalent concentration to population effective dose, i.e. 9 nSv (Bq h m 3) 1 for radon and 40 nSv (Bq h m 3) 1 for thoron. For radon and based on the highest concentration calculation and ACH¼ 0.5, the population dose due to indoor radon exposure is determined to be 0.43 mSv whereas the population dose due to indoor thoron exposure is 0.89 mSv. On the other hand for the lowest radon and thoron concentrations it is 0.04 and 0.02 mSv, respectively. In the worst case when there was no ventilation (ACH ¼0) the indoor radon concentration will rise significantly above 100 Bq m 3 with the annual dose being about 3 mSv if the examined area is covered with a material having a radon exhalation rate of 0.7 mBq s 1 m 2 (minimal RnExh). In the thoron case the dose does not exceed 0.90 mSv for maximum measured TnExh but is constant and does not depends on ACH.
M. Janik et al. / Applied Radiation and Isotopes 95 (2015) 102–107
The total population dose due to indoor radon and thoron exposure, with the assumption of ACH ¼ 0.5, is found to be 0.42 mSv for the lowest humidity (AH¼ 1.35 g cm 3) up to 0.97 mSv for the highest humidity (AH 20 g cm 3 ) for granite while the corresponding values are 0.21 and 0.53 mSv for brick. These results show that effective dose can change by the factor 2 due to humidity. Further investigations will be necessary to find relations between radon and thoron exhalation to other environmental parameters, e.g., temperature and pressure as well as their gradients. 5. Conclusion The main purpose of this study was to find a relationship between the ambient atmosphere, especially its humidity and radon and thoron exhalation rates. Two different materials with high radium (brick) and high thorium (granite) concentrations were chosen for measurements. The findings are summarized below. (1) The obtained results showed that for both materials radon and thoron exhalation rates increased and that above a certain level they decreased or reached a plateau; these trends were consistent with results of previous studies. (2) The results showed the same trends of exhalations for both materials with different structural characteristics (porosity and density) regardless of their weight. (3) It was concluded that effective dose could be changed by the factor of 2 due to humidity. (4) The present results indicated that humidity should be taken into consideration to calculate the equivalent dose from radon and thoron.
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