J Radioanal Nucl Chem (2015) 303:1943–1947 DOI 10.1007/s10967-014-3726-5
Measurement of radon exhalation rates from some building materials used in Serbian construction J. M. Stajic • D. Nikezic
Received: 25 June 2014 / Published online: 2 November 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract The rates of radon exhalation from building materials available on the Serbian market were measured using RAD7 device. Fitting the growth curves of radon activity concentration provided the information on radon exhalation rates from the samples. The results were quite low and they ranged from 1.4 to 855 mBq m-2 h-1 for surface exhalation rate and from 1.3 to 11.4 mBq kg-1 h-1 for mass exhalation rate. The effect of moisture on radon exhalation was confirmed. For powdered samples with dimensions smaller than radon diffusion length, the mass exhalation rate is independent of sample mass and its exhalation area. Keywords RAD7
Radon exhalation rate Building materials
Introduction Radon (222Rn: T1/2 = 3.824 days) is a naturally occurring radioactive gas generated by the decay of 226Ra. A certain fraction of radon created in some material escapes the material and migrates into the atmosphere. Radon and its decay products contribute significantly to radiation dose received by general population [1]. When inhaled by human beings, they have high damaging potential to the lung tissue and they have been identified as the second major cause of lung cancer [2]. Two main sources of radon in the indoor air are ground and building materials used in the construction. The J. M. Stajic D. Nikezic (&) Faculty of Science, University of Kragujevac, R. Domanovica 12, 34000 Kragujevac, Serbia e-mail:
[email protected]
amount of radon activity released per unit time from unit surface area (or unit mass) of some sample is termed as radon surface (or mass) exhalation rate [3–5]. It depends on the radium content, the spatial distribution of its atoms within the material and the physical properties of the material itself. Many authors investigated the problem of radon exhalation from different materials [6–11]. A certain amount of a material was usually placed in a closed chamber and measurement of radon activity concentration in the chamber volume was performed during a certain period of time in order to determine radon exhalation rate. Some authors based their experiments on estimating integral radon concentration during the exposure time of solid state nuclear track detectors [12, 13] while others used different active devices for continuous measuring of radon activity concentration [14–16]. The purpose of this work was to measure radon exhalation rates from some building materials often used in Serbian construction. The measuring was performed using closed chamber connected to RAD7 device. Fitting the experimental data provided the information on radon exhalation rates.
Theory If a sample of some material containing 226Ra is placed in a closed chamber, a certain amount of radon gas will constantly be exhaled from the sample and accumulated in the chamber volume. Time evolution of radon concentration in the chamber can be described by the following equation: dNðtÞ En A þ M ¼ ðNðtÞ Next ÞkL NðtÞk dt V
ð1Þ
123
1944
J Radioanal Nucl Chem (2015) 303:1943–1947
where N(t)–number of radon atoms per unit volume of air inside the chamber; Next–the concentration of radon atoms outside the chamber; En–number of radon atoms leaving the unit surface area (or unit mass) of the sample per unit time; A–surface area (or mass) of a sample; V–free volume of the chamber (the total volume reduced by a volume of a sample); M–system background caused by small concentrations of contaminants in the materials of the RAD7’s construction and the chamber itself; k–radioactive decay constant of 222Rn; kL–the leakage rate. In the case of unit surface area (or unit mass), the Eq. 1 does not take into account the sample shape or its volume. Back-diffusion process was not taken into account since it can be neglected in the case when sample volume occupies less than 10 % of the chamber volume or in the case when radon density in the chamber is less than radon density in the pores of a material. The solution of Eq. (1) is given as: En A þ M þ Next kL V NðtÞ ¼ 1 eðkþkL Þt þ N0 eðkþkL Þt ðk þ kL ÞV
Fig. 1 A simplified scheme of the experimental setup
ð2Þ where N0 represents the initial concentration of radon atoms in the chamber. Multiplying the previous equation by k, the following expression was obtained: EA þ M 0 þ Cext kL V CðtÞ ¼ 1 eðkþkL Þt þ C0 eðkþkL Þt ðk þ kL ÞV ð3Þ This equation describes the time evolution of radon activity concentration, C (in Bq m-3), in the chamber volume, where E = kEn represents radon exhalation rate, expressed in Bq m-2 s-1 (or Bq kg-2 s-1). C0 is initial radon activity concentration in the chamber and M0 = kM. Continuous measuring of radon activity concentration in the closed chamber provides the experimental data that can be fitted to the previous function in order to obtain radon exhalation rate as one of the fitting parameters.
Experimental Case of chamber without sample The rates of radon exhalation from different samples were estimated by placing samples in a chamber with a closedloop arrangement: the chamber was connected through two vents to the inlet and outlet of RAD7 device (Durridge Company). RAD7 can measure radon concentration in a real time using internal hemispheric cell with a volume of 0.7 dm3, coated on the inside with an electrical conductor. It has a low intrinsic background, corresponding to about
123
Fig. 2 Fitting the experimental data to the Eq. (3). Fitting parameters: M0 = 0.00133 Bq h-1, kL = 0.00225 h-1, C0 = 35.85 Bq m-3
0.02 pCi/L. The device was factory calibrated and the overall calibration accuracy was estimated to be better than 5 %. A diagram of the experimental setup is given in Fig. 1. The chamber was made of Plexiglas and it was provided with a door that could be hermetically sealed after placing a sample inside. It has a volume of 30 dm3. In order to find a correct approach to radon exhalation rate measurements from different samples, the characteristics of the measuring system itself were investigated first. It was done by considering the case of the chamber without a sample, but with a certain initial radon activity concentration in the chamber air. Radon in the chamber was left to decay for a month. In the meantime, RAD7 was continuously measuring the activity concentration. The measuring was done in the cycles of 1 h and the concentration obtained at the end of each cycle was assigned to the moment in the middle of the corresponding cycle. Figure 2 shows fitting the experimental data with Eq. (3), using E = 0. Radon activity concentration outside
J Radioanal Nucl Chem (2015) 303:1943–1947
1945
Table 1 Radon surface (EA) and mass (Em) exhalation rates of building materials, obtained by fitting the experimental data No.
Material
Sample mass [kg]
Sample surface [m2]
Em [mBq kg-1 h-1]
EA [mBq m-2 h-1]
1
Alabaster
0.802
–
5.2 ± 0.4
–
2
Paving stone
–
0.104
–
855 ± 7
3
Cement 1
0.574
–
8.0 ± 0.7
–
4
White brick
–
0.232
–
15 ± 1
5
Red brick 1
–
0.244
–
12 ± 1
6
Roof tile
–
0.106
–
46 ± 2
7
Lime
0.439
–
1.3 ± 0.3
–
8
Cement 2
0.864
–
11.4 ± 0.3
–
9
Plaster
1.057
–
1.3 ± 0.1
–
10
Red brick 2
–
0.084
–
53 ± 3
11.
Insulation board
–
0.263
–
29 ± 1
12.
Ceramic tiles 1
–
0.168
–
41 ± 2
Results and discussion
13.
Ceramic tiles 2
–
0.184
–
4.3 ± 0.8
Radon exhalation rates
14.
Marble tiles
–
0.232
–
1.4 ± 0.9
15.
Sand
0.808
–
6.8 ± 0.3
–
16.
Sand ? H2O
1
–
17.0 ± 0.3
–
Table 1 presents radon exhalation rates obtained as fitting parameters for different samples of building materials. Radon exhalation rates were expressed per unit mass (in mBq kg-1 h-1) for samples found in powder form and per unit surface area (in mBq m-2 h-1) for solid samples. Surface exhalation rate ranged from 1.4 to 855 mBq m-2 h-1, while mass exhalation rate had values from 1.3 to 11.4 mBq kg-1 h-1. The last two rows of Table 1 show the values obtained for dry sand and the same sand after adding 20 % of water. The presence of water increased radon exhalation rate for more than 2.5 times. The strong effect of moisture on radon emanation and radon exhalation has already been reported by many authors [17, 18]. Having water instead of air in the pore space among the grains of solid material significantly increases the emanation fraction (the fraction of all radon atoms created in a grain of a material that escapes the grains and becomes available for diffusion through the pore space). The reason for that is the fact that radon range in water is rather small in comparison to the range in air, so there is large probability that most of the recoiled radon atoms that arrive to the pore space will be stopped before reaching some of the neighbouring grains [17, 19]. Figure 4 illustrates the effect of moisture, presenting the growth of radon activity concentrations in the chamber volume for the previously mentioned samples No. 15 and 16 from Table 1.
the chamber, Cext, was measured to be 15 Bq m-3. The following values M0 = 0.00133 Bq h-1 and kL = 0.00225 h-1 were obtained as fitting parameters and they were used for further estimation of rates of radon exhalation from various samples. Radon exhalation rate of building materials Samples of materials often used in building construction were collected from Serbian shops and placed in the chamber described above. They were previously dried at the temperature of 25–30 °C for more than 10 days. Powdered samples were placed in a cylindrical beaker with a diameter of 19 cm. Solid samples were used in their original forms. Sample masses or their free surface areas were presented in Table 1. The chamber was sealed and the changes of radon activity concentration in the chamber volume were detected during 7–10 days for each sample. All tests were conducted at room temperature and normal pressure while the relative humidity in the RAD7’s internal sample cell was kept below 10 %. Experimental data obtained by RAD7 were fitted using Eq. (3). The previously obtained values for M0 and kL were used and new fitting parameters were E and C0. Figure 3 shows the growth of radon concentration in the chamber for the case of paving stone sample.
Fig. 3 The growth of radon concentration in the chamber (paving stone sample)
Dependence of radon exhalation rate on free surface area of a powdered sample The justification of expressing radon exhalation rate per unit mass of powdered (granular) materials was investigated
123
1946
J Radioanal Nucl Chem (2015) 303:1943–1947
Fig. 4 The growth of radon concentration in the chamber for the samples of dry and moistened sand
Table 2 Radon surface and mass exhalation rates of the soil sample placed in two different sized beakers Free surface area of the sample (cm2)
EA [mBq m-2 h-1]
Em [mBq kg-1 h-1]
44.1
16,575.2
219.1
268.8
2,895.3
233.0
using a certain amount of pulverized soil as a sample. Radon exhalation rate of the same mass of soil was measured twice, placing the sample in two cylindrical beakers with quite different free surface areas. Table 2 presents the results of these measurements. The surface exhalation rate was inversely proportional to the size of the free surface area of the sample. However, similar values of mass exhalation rate were obtained in both cases although the free surface area of the second sample was six times larger than the first one. It is in agreement with the assumption that almost all radon atoms that emanate from grains of a granular material will eventually be exhaled from the material (if the dimensions of the sample are relatively small in comparison to radon diffusion length). That means that radon exhalation is completely determined by the radium activity concentration in a sample and by radon emanation fraction. Figure 5 shows radon emanation fractions of a grain in the surface of a sample and a grain surrounded by infinitely large number of grains, as functions of grain radius. The results were obtained by Monte Carlo method for grains of SiO2 assuming homogenous distribution of 226Ra within the depth of radon range in quartz, calculated by SRIM2008 [20]. Grains were considered to be perfect and identical spheres, packed in a face centred cubic structure. Points of creation of 106 radon atoms and directions of their recoils (following alpha particle emissions) were
123
Fig. 5 The results obtained by Monte Carlo calculation for a grain of SiO2 surrounded by infinitely large number of grains (filled circle) and for a grain at the surface of a sample (open circle)
chosen randomly. The number of radon atoms that escape the grain of their creation and stop in the pore space among the grains was divided by 106 in order to obtain radon emanation fraction. Detailed description of this calculation was given in our previous work [21]. Radon emanation fraction is significantly higher for the grains located at the surface, especially for small grain radius, due to the less probability of radon being embedded in some of the surrounding grains. According to that, increasing free surface of sample should increase radon emanation and radon exhalation. However, since the thickness of the surface layer of grains is rather small in comparison to the thickness of the whole sample, its contribution to total radon emanation fraction is not that significant.
Conclusion The rates of radon exhalation from building materials available on the Serbian market were measured using RAD7 device. The highest surface exhalation rate was detected for paving stone sample (855 mBq m-2 h-1) and the highest mass exhalation rate was estimated for one of two samples of cement (11.4 mBq kg-1 h-1). The results of radon exhalation rate measurements presented in this paper are relatively low in comparison to some values reported from other countries [5, 10, 12, 22– 25]. However, they are in a good agreement with the results previously measured in Serbia (using different methods) by [13], with the exception of a few samples with the highest values of radon exhalation rates that were chosen because of their extremely high content of 226Ra. Using mass exhalation rates for powdered samples with relatively small dimensions in comparison to radon
J Radioanal Nucl Chem (2015) 303:1943–1947
diffusion length was justified due to the fact that almost all radon atoms that arrive to the pores among the grains of such materials eventually diffuse out to the atmosphere. In that case, mass exhalation rate is almost independent on free surface area of a sample. Acknowledgments The present work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under the Project No. 171021.
References 1. UNSCEAR (2000) United Nations Scientific Committee on the effects of atomic radiations sources, effects and risks of ionizing radiation. United Nations, New York 2. The World Health Organization (2005) Fact Sheet No. 291: radon and cancer. http://www.who.int/mediacentre/factsheets/fs291/en/ index.html 3. Mustonen R (1984) Natural radioactivity in and radon exhalation from Finish building materials. Health Phys 46:1195–1203 4. Tufail M, Mirza SM, Chughati MK, Ahmad N, Khan HA (1991) Preliminary measurements of exhalation rates and diffusion coefficients for radon in cements. Nucl Tracks Radiat Meas 19:427–428 5. Zhang L, Lei X, Guo Q, Wang S, Ma X, Shi Z (2012) Accurate measurement of the radon exhalation rate of building materials using the closed chamber method. J Radiol Prot 32:315–323 6. Ackers JG (1984) Direct measurement of radon exhalation from surfaces. Radiat Prot Dosim 7:199–201 7. De Martino S, Sabbarese C, Monetti G (1998) Radon emanation and exhalation rates from soils measured with an electrostatic collector. Appl Radiat Isot 49:407–413 8. Khan AJ, Prasad R, Tyagi RK (1992) Measurement of radon exhalation rate from some building materials. Nucl Tracks Radiat Meas 20(4):609–610 9. Poffijn A, Bourgoignie R, Mirijins R, Uyttenhove J, Janssens A, Jacobs R (1984) Laboratory measurements of radon exhalation and diffusion. Radiat Prot Dosim 7:77–79 10. Petropoulos NP, Anagnostakis MJ, Simopoulos SE (2001) Building materials radon exhalation rate: ERRICCA intercomparison exercise results. Sci Total Environ 272:109–118 11. Righi S, Bruzzi L (2006) Natural radioactivity and radon exhalation in building materials used in Italian dwellings. J Environ Radioact 88:158–170
1947 12. Najam LA, Tawfiq NF, Mahmood RH (2013) Radon concentration in some building materials in Iraq using CR-39 track detector. Int J Phys 1(3):73–76 13. Ujic P, Celikovic I, Kandic A, Vukanac I, Djurasevic M, Dragosavac D, Zunic ZS (2010) Internal exposure from building materials exhaling 222Rn and 220Rn as compared to external exposure due to their natural radioactivity content. Appl Radiat Isot 68:201–206 14. Chen J, Rahman NM, Atiya IA (2010) Radon exhalation from building materials for decorative use. J Environ Radioact 101:317–322 15. Hassan NM (2014) Radon emanation coefficient and its exhalation rate of wasted petroleum samples associated with petroleum industry in Egypt. J Radioanal Nucl Chem 299:111–117 16. Tan Y, Xiao D, Liu X, Tang Q (2013) Measuring radon exhalation rate by tracing the radon concentration of ventilation-type accumulation chamber. Radiat Meas 58:33–36 17. Sasaki T, Gunji Y, Okuda T (2004) Mathematical modeling of radon emanation. J Nucl Sci Technol 41(2):142–151 18. Sakoda A, Nishiyama Y, Hanamoto K, Ishimori Y, Yamamoto Y, Kataoka T, Kawabe A, Yamaoka K (2010) Differences of natural radioactivity and radon emanation fraction among constituent minerals of rock or soil. Appl Radiat Isot 68:1180–1184 19. Sakoda A, Hanamoto K, Ishimori Y, Kataoka T, Kawabe A, Yamaoka K (2010) First model of the effect of grain size on radon emanation. Appl Radiat Isot 68:1169–1172 20. Ziegler JF, Ziegler MD, Biersack JP (2008) SRIM: stopping and range of ions in matter http://www.srim.org 21. Stajic JM, Nikezic D (2014) Theoretical calculation of radon emanation fraction. Nucl Instrum Methods Phys Res, Sect B 336:19–25 22. Cosma C, Dancea F, Jurcut T, Ristoiu D (2001) Determination of 222 Rn emanation fraction and diffusion coefficient in concrete using accumulation chambers and the influence of humidity and radium distribution. Appl Radiat Isot 54:467–473 23. Keller G, Hoffmann B, Feigenspan T (2001) Radon permeability and radon exhalation of building materials. Sci Total Environ 272:85–89 24. Stoulos S, Manolopoulou M, Papastefanou C (2003) Assessment of natural radiation exposure and radon exhalation from building materials in Greece. J Environ Radioact 69:225–240 25. Tuccimei P, Moroni M, Norcia D (2006) Simultaneous determination of 222Rn and 220Rn exhalation rates from building materials used in Central Italy with accumulation chambers and a continuous solid state alpha detector: influence of particle size, humidity and precursors concentration. Appl Radiat Isot 64:254–263
123
