Radiation Protection Dosimetry (2014), Vol. 162, No. 1–2, pp. 152 –156 Advance Access publication 1 August 2014
doi:10.1093/rpd/ncu249
RESULTS FROM TIME INTEGRATED MEASUREMENTS OF INDOOR RADON, THORON AND THEIR DECAY PRODUCT CONCENTRATIONS IN SCHOOLS IN THE REPUBLIC OF MACEDONIA
*Corresponding author: [email protected] As part of a survey on concentrations of radon, thoron and their decay products in different indoor environments of the Balkan region involving international collaboration, measurements were performed in 43 schools from 5 municipalities of the Republic of Macedonia. The time-integrated radon and thoron gas concentrations (CRn and CTn) were measured by CR-39 ( placed in chambers with different diffusion barriers), whereas the equilibrium equivalent radon and thoron concentrations (EERC and EETC) were measured using direct radon–thoron progeny sensors consisting of LR-115 nuclear track detectors. The detectors were deployed at a distance of at least 0.5 m from the walls as well as far away from the windows and doors in order to obtain more representative samples of air from the breathing zone; detectors were exposed over a 3-month period (March–May 2012). The geometric mean (GM) values [and geometric standard deviations (GSDs)] of CRn, CTn, EERC and EETC were 76 (1.7), 12 (2.3), 27 (1.4) and 0.75 Bq m23 (2.5), respectively. The equilibrium factors between radon and its decay products (FRn) and >0.5 m >0.5 m ) were evaluated: FRn ranged between 0.10 and 0.84 and FTn ranged between 0.003 and thoron and its decay products (FTn 0.998 with GMs (and GSDs) equal to 0.36 (1.7) and 0.07 (3.4), respectively.
INTRODUCTION Radon, thoron and their decay products are the main sources of public exposure to ionising radiation in indoor environment in most countries worldwide. Differences in half-lives of radon and thoron lead to different indoor concentration profiles. As regards their decay products, they are mainly affected by the dynamical processes including the attachment to aerosol particles, deposition on surfaces present in a room as well as by air movement (1). In most cases, direct measurements of radon and thoron decay product concentrations are limited and they are estimated from radon and thoron gas concentrations using the respective equilibrium factors. The equilibrium factor (F) is defined as a ratio of equilibrium equivalent concentration (EEC) to parent gas concentration (C ). EECs for radon and thoron are defined by the following equations: EERC ¼ 0:105Cð218 Po) þ 0:516Cð214 Pb) þ 0:380Cð214 Bi), 212
EETC ¼ 0:913Cð
ð1Þ 212
Pb) þ 0:087Cð
Bi),
ð2Þ
where EERC and EETC are the equilibrium equivalent concentrations for radon and thoron, respectively and C the activity concentration of a decay product, expressed in Bq m23. This work is part of a wider survey of radon, thoron and their decay product concentrations in indoor environment throughout the Balkan region. Following the Macedonian National survey(2 – 5) where radon and thoron gases were measured in 437 and 300 dwellings, respectively, the main objective of the present work is to investigate radon, thoron and their decay products in schools, determine the indoor equilibrium factors, their variability and to compare the overall results with others reported in similar surveys. MATERIALS AND METHODS Radon and thoron gas concentrations and EERC and EETC in indoor air of 43 schools located in 5 municipalities have been measured using nuclear track detectors (Figure 1). The municipalities belong to a geotectonic unit named the Vardar Zone mainly composed of Neogene– Quaternary sediments, represented mainly by clay and sandstone.
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Zdenka Stojanovska1,*, Zora S. Zunic2, Peter Bossew3, Francesco Bochicchio4, Carmela Carpentieri4, Gennaro Venoso4, Rosaline Mishra5, R.P. Rout5, B.K. Sapra5, Bety D. Burghele6, A. Cucos¸-Dinu6, Blazo Boev7 and C. Cosma6 1 Faculty of Medical Sciences, Goce Delcev University, Stip, Republic of Macedonia 2 Institute of Nuclear Science, Vinca, University of Belgrade, Belgrade, Serbia 3 Bundesamt fu¨r Strahlenschutz (German Federal Office for Radiation Protection), Berlin, Germany 4 Italian National Institute of Health, Rome, Italy 5 Radiological Physics & Advisory Division (RPAD), Bhabha Atomic Research Centre, Mumbai, India 6 Environmental Radioactivity and Nuclear Dating Center, Babes¸-Bolyai University, Cluj-Napoca, Romania 7 Faculty of Mining, Geology and Polytechnic, Goce Delcev University, Stip, Republic of Macedonia
INDOOR RADON, THORON AND THEIR DECAY PRODUCTS IN MACEDONIA Table 1. Descriptive statistics of CRn, CTn, EERC and EETC measured in the schools. CTn
EERC
EETC
43 27 67 242 88 50 0.57 76 1.7
30 1 14 45 16 11 0.71 12 2.3
43 8 29 42 28 7 0.26 27 1.4
43 0.08 0.91 6.10 1.09 1.06 0.98 0.75 2.5
RESULTS AND DISCUSSION
Figure 1. Location of the five municipalities in the Republic of Macedonia where measurements were performed.
Sets of three detectors were deployed in classrooms situated mainly on the ground floor (41 of 43) of each school. Radon concentration has been measured using two different detectors. The first one, provided and analysed by the Italian National Institute of Health, Rome, Italy(6), consists of a CR-39 detection material placed on the bottom of a dome-type diffusion chamber and measures radon concentration only. The second one, analysed by the laboratory of Environmental Radioactivity and Nuclear Dating Centre, ClujNapoca, Romania, was a ‘Raduet’ detector that measures simultaneously radon and thoron gas concentrations. On the other hand, a third type of detector was used to measure radon and thoron EECs. It is DRPS– DTPS (direct radon–thoron progeny sensors)(7) based on LR-115 detection, developed, provided and analysed by the Bhabha Atomic Research Centre, Mumbai, India. In each measuring point, the detectors were deployed at 0.5–1 m distance from any wall surface. This distance was maintained to avoid the near-the-wall zone where the spatial gradient of thoron activity concentration is expected. Detectors were exposed for 3 months during March–May 2012. After exposure, detectors were collected and mailed to laboratories for analysis.
Descriptive statistics of all measured quantities are presented in Table 1. All measured concentrations are in a wide interval, with a higher variability for CTn concentration [coefficient of variation (CV) ¼ 71 %] and EETC (CV ¼ 98 %). The measured values of the four quantities fit a log – normal distribution (all of them passed both the Kolmogorov– Smirnov and x 2 tests at a 95 % level of significance). None of the measured indoor radon concentrations in the schools exceeded the action level of 400 Bq m23 for existing buildings adopted in the national regulations(8). The GM value of CRn in schools (76 Bq m23) is similar to that measured in the national survey of dwellings (72 Bq m23) during spring season(2). Only 30 out of 43 Raduet detectors were able to detect thoron. The CTn ranged between 1 and 45 Bq m23. It should be mentioned that measured low CTn values could be attributed to the detectors position (far from the walls) and some of them could be also affected by the presence of high radon concentrations(4) and, in turn, the resulting extreme values of .0.5 m . FTn The related GM [geometric standard deviations (GSD)] of 12 Bq m23 (2.3) for CTn in schools is lower than the 32 Bq m23 (2.8) obtained for Macedonian dwellings in the spring season(4). Moreover, the ranges and GM of CRn and CTn in this survey were generally lower than those published for schools by other countries. In surveys carried out in some European countries the following results were obtained: in schools from Slovenia(9), CRn ranged between 70 and 770 Bq m23, whereas CTn ranged between 4 and 91 Bq m23; in Romania(10), CRn ranged between 31 and 414 Bq m23 and CTn was up to 235 Bq m23. In schools and kindergartens in the Kremikovtsi district, Bulgaria(11), CRn were found to vary between 32 and 1305 Bq m23 with a GM of 220 Bq m23 (GSD ¼ 2.6), whereas in schools from Sokobanja, Serbia(12), the CRn ranged between 17 and 428 Bq m23 with a GM of 97 Bq m23 (GSD ¼ 1.9).
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No. of values Minimum (Bq m23) Median (Bq m23) Maximum (Bq m23) AM (Bq m23) SD (Bq m23) CV GM (Bq m23) GSD
CRn
Z. STOJANOVSKA ET AL. Table 2. Descriptive statistics of CTn/CRn, EETC/EERC, >0.5 m . FRn and FTn FRn
FTn
30 0.02 0.17 1.40 0.24 0.27 1.13 0.16 2.4
43 0.004 0.031 0.360 0.042 0.055 1.315 0.028 2.4
On the other hand, the values of the GM for EERC (27 Bq m23) and EETC (0.75 Bq m23) obtained in this study are similar to those measured in 60 schools of Thessaloniki, Northern Greece(13) (22 and 0.9 Bq m23, respectively). The correlations between ln CTn and ln CRn, ln EETC and ln EERC, ln EERC and ln CRn, EETC and ln CTn were also investigated. A significant (at the 90 % confidence level) but weak positive correlation was found only between ln EERC and ln CRn (R 2 ¼ 0.09, P ¼ 0.058). This low correlation could be due to the existence of influencing factors such as ventilation rate and concentration of aerosols in the room that affect the concentration of the gas and EEC differently. The absence of a significant correlation between thoron and its decay products can be explained not only by the influence of environmental factors but also by the short half-life of the thoron gas compared with those of its decay products. Due to the very short half-life of thoron (T1/2 ¼ 55.6 s), its concentration varies considerably with the distance from the walls, while decay products have a half-life of some hours and are essentially homogeneously distributed in room air. Furthermore, the decay product concentrations build up for decay products produced by thoron that was present hours earlier(1). The ratios between CTn and CRn and between EETC and EERC together with equilibrium factors .0.5 m ) were calculated. for radon (FRn) and thoron (FTn The statistics are presented in Table 2. This survey showed that the thoron concentrations were generally lower than the radon concentrations (29 out of 30 measurements), and the CTn/CRn ranged between 0.02 and 1.40, with a GM of 0.16 (GSD ¼ 2.4). The EETC to EERC ratio ranged from 0.004 to 0.36 with a GM of 0.03, which is in good agreement with the world’s range of 0.01– 0.5(1). For FRn, the values were in the interval of 0.10– 0.84 with the CV of 50 %. The GM of 0.36 is in compliance with the generally accepted value of 0.4 by United Nations Scientific Committee on the Effects of Atomic .0.5 m Radiation (UNSCEAR)(1). On the other hand, FTn
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No. of values 43 30 Minimum (Bq m23) 0.10 0.003 0.35 0.088 Median (Bq m23) Maximum (Bq m23) 0.84 0.998 23 0.41 0.127 AM (Bq m ) 0.20 0.183 SD (Bq m23) CV 0.50 1.44 0.36 0.067 GM (Bq m23) GSD 1.7 3.4
CTn/CRn EETC/EERC
varied in a much wider interval of values with a CV of 144 %. The GM value of 0.067 (and AM ¼ 0.13) was higher than the typical value of 0.02, which has also been provided by UNSCEAR(1) and higher than the values of 0.018 and 0.013 reported from Ireland(14, 15) and China(15, 16), respectively. This value is somewhat higher but it is still within the range for the indoor environment of neighbouring countries reported in the literature. For instance, in Kosovo it was observed from the measurements in 48 dwellings where detectors were positioned far from the walls that the FTh values ranged between 0.00019 and 0.25 with a GM of 0.024(17); in Sokobanja, Serbia from measurements performed in 43 dwellings where detectors were positioned on the wall, the results of FTh were in the range between 0.001 and 0.077 with a GM of 0.006 (2.2)(18). The differences between the measured quantities in different municipalities were further tested by the ANOVA and KS tests. The effect of municipality was not significant at the 95 % level of significance. However, the classrooms in Gazi Baba, Kavadarci and Stip seemed to have a higher CRn than the classrooms in the other municipalities. The highest GM of the CTn was obtained in the schools of Petrovec Municipality. Interestingly, during the national survey, in some dwellings of those municipalities, the thoron concentrations were .200 Bq m23. The authors tried to explain the measurement results taking into account the construction of the buildings and their ventilation, although they are from different geographic locations. Ordinarily, all buildings are built on foundations made of concrete and/or brick and their approximate age is from 20 to 40 y. Another common characteristic for schools is ventilation, which is of great influence in particular on concentrations of decay products. Ordinarily, after each shift, classrooms are cleaned and ventilated by opening the windows, twice a day for 30 min, in 5 d a week which implies that in most cases a low level of CRn, EERC and EETC is maintained. Besides, there is high variation in the equilibrium factors FTn.0.5 m especially in some municipalities. For example, in the Aerodrom Municipality the FTn.0.5 m has a GSD of 3.7 with a GM of 0.10, .0.5 m whereas, in the Petrovec Municipality, the FTn has a GSD of 5.1 with a GM of 0.05, which most likely results from the schools being constructed with building materials rich in 232Th, where low levels of thoron (because of the relatively large distance from the detector and the wall) and high levels of EETC, which does not depend on the distance to the source (wall), were measured. This poses an additional issue on the use of the equilibrium factors, in these municipalities, to evaluate the thoron progeny concentration on the basis of the thoron gas measurements because such use can result in significant under- or overestimation.
INDOOR RADON, THORON AND THEIR DECAY PRODUCTS IN MACEDONIA
REFERENCES
The present work deals with the simultaneous longterm measurement of indoor radon, thoron and their decay product concentrations by means of nuclear track detectors. Radon and thoron equilibrium factors were also estimated at a distance of 0.5–1 m from the wall. Thoron concentration results showed higher variation compared with the radon ones, as well as thoron decay product concentrations showed higher variation compared with those of radon. It was found that the mean values of CRn, CTn, EERC, .0.5 m obtained for the five municiEETC, FRn and FTn palities included in this study were not statistically different. The radon equilibrium factors FRn obtained in this work (AM ¼ 0.41; range ¼ 0.10 –0.84) is comparable to the typical value (0.4) reported by UNSCEAR(1). .0.5 m should Instead, the thoron equilibrium factor FTn be carefully considered, as it strongly depends on the position where the measurements are performed. In this work FTn was evaluated at a distance of 0.5 m .0.5 m ) where the from the walls (and it was named FTn thoron concentration is lower than close to the wall and leads to a generally higher value of its equilibrium factor. This could explain the higher value of the thoron equilibrium factor found in this work (AM ¼ 0.13) compared with the typical value reported by UNSCEAR(1) (0.02). Moreover, the variability of the thoron equilibrium factor FTn.0.5 m (CV ¼ 144 %) is much higher than the radon equilibrium factor FRn one (CV ¼ 50 %). Considering the large contribution of radon and thoron decay product concentrations in the overall exposure of the population, and in addition their dependence on many different factors related to indoor aerosol dynamics and their resulting variability, more research in that direction is planned.
1. United Nations Scientific Committee on the Effects of Atomic Radiation. Effects of ionizing radiation. Report to the General Assembly with Scientific Annexes (Annex B) (2000). 2. Stojanovska, Z., Januseski, J., Bossew, P., Zunic, Z. S., Tollefsen, T. and Ristova, M. Seasonal indoor radon concentration in FYR of Macedonia. Radiat. Meas. 46(5– 6), 602–610 (2011). 3. Stojanovska, Z., Januseski, J., Boev, B. and Ristova, M. Indoor exposure of population to radon in the FYR of Macedonia. Radiat. Prot. Dosim. 148(2), 162–167 (2012). 4. Stojanovska, Z., Bossew, P., Tokonami, S., Zunic, S. Z., Bochicchio, F., Boev, B., Ristova, M. and Januseski, J. National survey of indoor thoron concentration in FYR of Macedonia (continental Europe—Balkan region). Radiat. Meas. 49, 57–66 (2013). 5. Bossew, P., Stojanovska, Z., Zunic, S. Z. and Ristova, M. Prediction of indoor radon risk from radium concentration in soil: Republic of Macedonia Case Study. Rom. J. Phys. 58, S329–S343 (2013). 6. Carpentieri, C. et al. Assessment of long-term radon concentration measurement precision in field conditions (Serbian schools) for a survey carried out by an international collaboration. Radiat. Prot. Dosim. 145(2– 3), 305–311 (2011). 7. Mishra, R., Prajith, R., Sapra, B. K. and Mayya, Y. S. An integrated approach for the assessment of the thoron progeny exposures using direct thoron progeny sensors. Radiat. Prot. Dosim. 141(4), 363– 366 (2010). 8. Qracjmojl ia oayjopt j nfrf,ftp oa jimphfopsta oa qrpvfsjpoamop jimphfoj mjxa, cpef,f oa fcjefoxj‘a j qpeofsuca,f oa jicfztaj (Smuhbfo cfsojl oa RN br. 29 pe 01.03.2010 dpe) Regulation for occupational exposure on ionizing radiation-measuring, recordkeeping and reporting (Official Gazette of Republic of Macedonia 29, 01.03.2010) [in Macedonian]. 9. Vaupotic, J., Bezek, M., Ka´va´si, N., Ishikawa, T., Yonehara, H. and Tokonami, S. Radon and thoron doses in kindergartens and elementary schools. Radiat. Prot. Dosim. 152(1–3), 247– 252 (2012). 10. Burghele, B. D. and Cosma, C. Thoron and radon measurements in Romanian schools. Radiat. Prot. Dosim. 152(1– 3), 38–41 (2012). 11. Vuchkov, D., Ivanova, K., Stojanovska, Z., Kunovska, B. and Badulin, V. Radon measurement in schools and kindergartens (Kremikovtsi Municipality, Bulgaria), Romanian. J. Phys. 58, S328–S335 (2013). 12. Zˇunic´, Z. S. et al. Some results of a radon survey in 207 Serbian schools. Rom. J. Phys. 58, S320– S327 (2013). 13. Clouvas, A., Xanthos, S. and Antonopoulos-Domis, M. Radon and thoron progeny measurements in dwellings of northern Greece. Sci. Total Environ. 272(1), 249– 250 (2001). 14. McLaughlin, J., Murray, M., Currivan, L., Pollard, D., Smith, V., Tokonami, S., Sorimachi, A. and Janik, M. Long-term measurements of thoron, its airborne progeny and radon in 205 dwellings in Ireland. Radiat. Prot. Dosim. 145(2–3), 189– 193 (2011). 15. Janik, M., Tokonami, S., Kranrod, C., Sorimachi, A., Ishikawa, T., Hosoda, M., McLaughlin, J., Chang, B.-U. and Kim, Y. J. Comparative analysis of radon, thoron and
ACKNOWLEDGEMENTS The authors would like to express their gratitude to the municipality’s mayors and schools directors for their enthusiasm and collaboration during this survey.
FUNDING This survey was funded by the research fund of the Goce Delcev University, Stip, Republic of Macedonia. The financial support of the Ministry of Science and Technological Development of the Republic of Serbia, University of Belgrade (Project: P41028), is appreciated.
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CONCLUSIONS
Z. STOJANOVSKA ET AL. thoron progeny concentration measurements. J. Radiat. Res. 54(4), 597–610 (2013). 16. Yamada, Y. et al. Radon-thoron discriminative measurements in Gansu Province, China, and their implication for dose estimates. J. Toxicol. Environ. Health A 69, 723– 739 (2006). 17. Gulan, L., Milic, G., Bossew, P., Omori, Y., Ishikawa, T., Mishra, R., Mayya, Y. S., Stojanovska, Z., Vuckovic, B.
and Zunic, Z. S. Field experience on indoor radon, thoron and their progenies with solid state detectors in a survey of Kosovo and Metohija. Radiat. Prot. Dosim. 152(1– 3), 189– 197 (2012). 18. Mishra, R. et al. An evaluation of thoron (and radon) equilibrium factor close to wall based on long-term measurements in dwellings. Radiat. Prot. Dosim. 160(1-3), 164– 168 (2014).
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156
doi:10.1093/rpd/ncu249
RESULTS FROM TIME INTEGRATED MEASUREMENTS OF INDOOR RADON, THORON AND THEIR DECAY PRODUCT CONCENTRATIONS IN SCHOOLS IN THE REPUBLIC OF MACEDONIA
*Corresponding author: [email protected] As part of a survey on concentrations of radon, thoron and their decay products in different indoor environments of the Balkan region involving international collaboration, measurements were performed in 43 schools from 5 municipalities of the Republic of Macedonia. The time-integrated radon and thoron gas concentrations (CRn and CTn) were measured by CR-39 ( placed in chambers with different diffusion barriers), whereas the equilibrium equivalent radon and thoron concentrations (EERC and EETC) were measured using direct radon–thoron progeny sensors consisting of LR-115 nuclear track detectors. The detectors were deployed at a distance of at least 0.5 m from the walls as well as far away from the windows and doors in order to obtain more representative samples of air from the breathing zone; detectors were exposed over a 3-month period (March–May 2012). The geometric mean (GM) values [and geometric standard deviations (GSDs)] of CRn, CTn, EERC and EETC were 76 (1.7), 12 (2.3), 27 (1.4) and 0.75 Bq m23 (2.5), respectively. The equilibrium factors between radon and its decay products (FRn) and >0.5 m >0.5 m ) were evaluated: FRn ranged between 0.10 and 0.84 and FTn ranged between 0.003 and thoron and its decay products (FTn 0.998 with GMs (and GSDs) equal to 0.36 (1.7) and 0.07 (3.4), respectively.
INTRODUCTION Radon, thoron and their decay products are the main sources of public exposure to ionising radiation in indoor environment in most countries worldwide. Differences in half-lives of radon and thoron lead to different indoor concentration profiles. As regards their decay products, they are mainly affected by the dynamical processes including the attachment to aerosol particles, deposition on surfaces present in a room as well as by air movement (1). In most cases, direct measurements of radon and thoron decay product concentrations are limited and they are estimated from radon and thoron gas concentrations using the respective equilibrium factors. The equilibrium factor (F) is defined as a ratio of equilibrium equivalent concentration (EEC) to parent gas concentration (C ). EECs for radon and thoron are defined by the following equations: EERC ¼ 0:105Cð218 Po) þ 0:516Cð214 Pb) þ 0:380Cð214 Bi), 212
EETC ¼ 0:913Cð
ð1Þ 212
Pb) þ 0:087Cð
Bi),
ð2Þ
where EERC and EETC are the equilibrium equivalent concentrations for radon and thoron, respectively and C the activity concentration of a decay product, expressed in Bq m23. This work is part of a wider survey of radon, thoron and their decay product concentrations in indoor environment throughout the Balkan region. Following the Macedonian National survey(2 – 5) where radon and thoron gases were measured in 437 and 300 dwellings, respectively, the main objective of the present work is to investigate radon, thoron and their decay products in schools, determine the indoor equilibrium factors, their variability and to compare the overall results with others reported in similar surveys. MATERIALS AND METHODS Radon and thoron gas concentrations and EERC and EETC in indoor air of 43 schools located in 5 municipalities have been measured using nuclear track detectors (Figure 1). The municipalities belong to a geotectonic unit named the Vardar Zone mainly composed of Neogene– Quaternary sediments, represented mainly by clay and sandstone.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Lethbridge on December 12, 2014
Zdenka Stojanovska1,*, Zora S. Zunic2, Peter Bossew3, Francesco Bochicchio4, Carmela Carpentieri4, Gennaro Venoso4, Rosaline Mishra5, R.P. Rout5, B.K. Sapra5, Bety D. Burghele6, A. Cucos¸-Dinu6, Blazo Boev7 and C. Cosma6 1 Faculty of Medical Sciences, Goce Delcev University, Stip, Republic of Macedonia 2 Institute of Nuclear Science, Vinca, University of Belgrade, Belgrade, Serbia 3 Bundesamt fu¨r Strahlenschutz (German Federal Office for Radiation Protection), Berlin, Germany 4 Italian National Institute of Health, Rome, Italy 5 Radiological Physics & Advisory Division (RPAD), Bhabha Atomic Research Centre, Mumbai, India 6 Environmental Radioactivity and Nuclear Dating Center, Babes¸-Bolyai University, Cluj-Napoca, Romania 7 Faculty of Mining, Geology and Polytechnic, Goce Delcev University, Stip, Republic of Macedonia
INDOOR RADON, THORON AND THEIR DECAY PRODUCTS IN MACEDONIA Table 1. Descriptive statistics of CRn, CTn, EERC and EETC measured in the schools. CTn
EERC
EETC
43 27 67 242 88 50 0.57 76 1.7
30 1 14 45 16 11 0.71 12 2.3
43 8 29 42 28 7 0.26 27 1.4
43 0.08 0.91 6.10 1.09 1.06 0.98 0.75 2.5
RESULTS AND DISCUSSION
Figure 1. Location of the five municipalities in the Republic of Macedonia where measurements were performed.
Sets of three detectors were deployed in classrooms situated mainly on the ground floor (41 of 43) of each school. Radon concentration has been measured using two different detectors. The first one, provided and analysed by the Italian National Institute of Health, Rome, Italy(6), consists of a CR-39 detection material placed on the bottom of a dome-type diffusion chamber and measures radon concentration only. The second one, analysed by the laboratory of Environmental Radioactivity and Nuclear Dating Centre, ClujNapoca, Romania, was a ‘Raduet’ detector that measures simultaneously radon and thoron gas concentrations. On the other hand, a third type of detector was used to measure radon and thoron EECs. It is DRPS– DTPS (direct radon–thoron progeny sensors)(7) based on LR-115 detection, developed, provided and analysed by the Bhabha Atomic Research Centre, Mumbai, India. In each measuring point, the detectors were deployed at 0.5–1 m distance from any wall surface. This distance was maintained to avoid the near-the-wall zone where the spatial gradient of thoron activity concentration is expected. Detectors were exposed for 3 months during March–May 2012. After exposure, detectors were collected and mailed to laboratories for analysis.
Descriptive statistics of all measured quantities are presented in Table 1. All measured concentrations are in a wide interval, with a higher variability for CTn concentration [coefficient of variation (CV) ¼ 71 %] and EETC (CV ¼ 98 %). The measured values of the four quantities fit a log – normal distribution (all of them passed both the Kolmogorov– Smirnov and x 2 tests at a 95 % level of significance). None of the measured indoor radon concentrations in the schools exceeded the action level of 400 Bq m23 for existing buildings adopted in the national regulations(8). The GM value of CRn in schools (76 Bq m23) is similar to that measured in the national survey of dwellings (72 Bq m23) during spring season(2). Only 30 out of 43 Raduet detectors were able to detect thoron. The CTn ranged between 1 and 45 Bq m23. It should be mentioned that measured low CTn values could be attributed to the detectors position (far from the walls) and some of them could be also affected by the presence of high radon concentrations(4) and, in turn, the resulting extreme values of .0.5 m . FTn The related GM [geometric standard deviations (GSD)] of 12 Bq m23 (2.3) for CTn in schools is lower than the 32 Bq m23 (2.8) obtained for Macedonian dwellings in the spring season(4). Moreover, the ranges and GM of CRn and CTn in this survey were generally lower than those published for schools by other countries. In surveys carried out in some European countries the following results were obtained: in schools from Slovenia(9), CRn ranged between 70 and 770 Bq m23, whereas CTn ranged between 4 and 91 Bq m23; in Romania(10), CRn ranged between 31 and 414 Bq m23 and CTn was up to 235 Bq m23. In schools and kindergartens in the Kremikovtsi district, Bulgaria(11), CRn were found to vary between 32 and 1305 Bq m23 with a GM of 220 Bq m23 (GSD ¼ 2.6), whereas in schools from Sokobanja, Serbia(12), the CRn ranged between 17 and 428 Bq m23 with a GM of 97 Bq m23 (GSD ¼ 1.9).
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No. of values Minimum (Bq m23) Median (Bq m23) Maximum (Bq m23) AM (Bq m23) SD (Bq m23) CV GM (Bq m23) GSD
CRn
Z. STOJANOVSKA ET AL. Table 2. Descriptive statistics of CTn/CRn, EETC/EERC, >0.5 m . FRn and FTn FRn
FTn
30 0.02 0.17 1.40 0.24 0.27 1.13 0.16 2.4
43 0.004 0.031 0.360 0.042 0.055 1.315 0.028 2.4
On the other hand, the values of the GM for EERC (27 Bq m23) and EETC (0.75 Bq m23) obtained in this study are similar to those measured in 60 schools of Thessaloniki, Northern Greece(13) (22 and 0.9 Bq m23, respectively). The correlations between ln CTn and ln CRn, ln EETC and ln EERC, ln EERC and ln CRn, EETC and ln CTn were also investigated. A significant (at the 90 % confidence level) but weak positive correlation was found only between ln EERC and ln CRn (R 2 ¼ 0.09, P ¼ 0.058). This low correlation could be due to the existence of influencing factors such as ventilation rate and concentration of aerosols in the room that affect the concentration of the gas and EEC differently. The absence of a significant correlation between thoron and its decay products can be explained not only by the influence of environmental factors but also by the short half-life of the thoron gas compared with those of its decay products. Due to the very short half-life of thoron (T1/2 ¼ 55.6 s), its concentration varies considerably with the distance from the walls, while decay products have a half-life of some hours and are essentially homogeneously distributed in room air. Furthermore, the decay product concentrations build up for decay products produced by thoron that was present hours earlier(1). The ratios between CTn and CRn and between EETC and EERC together with equilibrium factors .0.5 m ) were calculated. for radon (FRn) and thoron (FTn The statistics are presented in Table 2. This survey showed that the thoron concentrations were generally lower than the radon concentrations (29 out of 30 measurements), and the CTn/CRn ranged between 0.02 and 1.40, with a GM of 0.16 (GSD ¼ 2.4). The EETC to EERC ratio ranged from 0.004 to 0.36 with a GM of 0.03, which is in good agreement with the world’s range of 0.01– 0.5(1). For FRn, the values were in the interval of 0.10– 0.84 with the CV of 50 %. The GM of 0.36 is in compliance with the generally accepted value of 0.4 by United Nations Scientific Committee on the Effects of Atomic .0.5 m Radiation (UNSCEAR)(1). On the other hand, FTn
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No. of values 43 30 Minimum (Bq m23) 0.10 0.003 0.35 0.088 Median (Bq m23) Maximum (Bq m23) 0.84 0.998 23 0.41 0.127 AM (Bq m ) 0.20 0.183 SD (Bq m23) CV 0.50 1.44 0.36 0.067 GM (Bq m23) GSD 1.7 3.4
CTn/CRn EETC/EERC
varied in a much wider interval of values with a CV of 144 %. The GM value of 0.067 (and AM ¼ 0.13) was higher than the typical value of 0.02, which has also been provided by UNSCEAR(1) and higher than the values of 0.018 and 0.013 reported from Ireland(14, 15) and China(15, 16), respectively. This value is somewhat higher but it is still within the range for the indoor environment of neighbouring countries reported in the literature. For instance, in Kosovo it was observed from the measurements in 48 dwellings where detectors were positioned far from the walls that the FTh values ranged between 0.00019 and 0.25 with a GM of 0.024(17); in Sokobanja, Serbia from measurements performed in 43 dwellings where detectors were positioned on the wall, the results of FTh were in the range between 0.001 and 0.077 with a GM of 0.006 (2.2)(18). The differences between the measured quantities in different municipalities were further tested by the ANOVA and KS tests. The effect of municipality was not significant at the 95 % level of significance. However, the classrooms in Gazi Baba, Kavadarci and Stip seemed to have a higher CRn than the classrooms in the other municipalities. The highest GM of the CTn was obtained in the schools of Petrovec Municipality. Interestingly, during the national survey, in some dwellings of those municipalities, the thoron concentrations were .200 Bq m23. The authors tried to explain the measurement results taking into account the construction of the buildings and their ventilation, although they are from different geographic locations. Ordinarily, all buildings are built on foundations made of concrete and/or brick and their approximate age is from 20 to 40 y. Another common characteristic for schools is ventilation, which is of great influence in particular on concentrations of decay products. Ordinarily, after each shift, classrooms are cleaned and ventilated by opening the windows, twice a day for 30 min, in 5 d a week which implies that in most cases a low level of CRn, EERC and EETC is maintained. Besides, there is high variation in the equilibrium factors FTn.0.5 m especially in some municipalities. For example, in the Aerodrom Municipality the FTn.0.5 m has a GSD of 3.7 with a GM of 0.10, .0.5 m whereas, in the Petrovec Municipality, the FTn has a GSD of 5.1 with a GM of 0.05, which most likely results from the schools being constructed with building materials rich in 232Th, where low levels of thoron (because of the relatively large distance from the detector and the wall) and high levels of EETC, which does not depend on the distance to the source (wall), were measured. This poses an additional issue on the use of the equilibrium factors, in these municipalities, to evaluate the thoron progeny concentration on the basis of the thoron gas measurements because such use can result in significant under- or overestimation.
INDOOR RADON, THORON AND THEIR DECAY PRODUCTS IN MACEDONIA
REFERENCES
The present work deals with the simultaneous longterm measurement of indoor radon, thoron and their decay product concentrations by means of nuclear track detectors. Radon and thoron equilibrium factors were also estimated at a distance of 0.5–1 m from the wall. Thoron concentration results showed higher variation compared with the radon ones, as well as thoron decay product concentrations showed higher variation compared with those of radon. It was found that the mean values of CRn, CTn, EERC, .0.5 m obtained for the five municiEETC, FRn and FTn palities included in this study were not statistically different. The radon equilibrium factors FRn obtained in this work (AM ¼ 0.41; range ¼ 0.10 –0.84) is comparable to the typical value (0.4) reported by UNSCEAR(1). .0.5 m should Instead, the thoron equilibrium factor FTn be carefully considered, as it strongly depends on the position where the measurements are performed. In this work FTn was evaluated at a distance of 0.5 m .0.5 m ) where the from the walls (and it was named FTn thoron concentration is lower than close to the wall and leads to a generally higher value of its equilibrium factor. This could explain the higher value of the thoron equilibrium factor found in this work (AM ¼ 0.13) compared with the typical value reported by UNSCEAR(1) (0.02). Moreover, the variability of the thoron equilibrium factor FTn.0.5 m (CV ¼ 144 %) is much higher than the radon equilibrium factor FRn one (CV ¼ 50 %). Considering the large contribution of radon and thoron decay product concentrations in the overall exposure of the population, and in addition their dependence on many different factors related to indoor aerosol dynamics and their resulting variability, more research in that direction is planned.
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ACKNOWLEDGEMENTS The authors would like to express their gratitude to the municipality’s mayors and schools directors for their enthusiasm and collaboration during this survey.
FUNDING This survey was funded by the research fund of the Goce Delcev University, Stip, Republic of Macedonia. The financial support of the Ministry of Science and Technological Development of the Republic of Serbia, University of Belgrade (Project: P41028), is appreciated.
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CONCLUSIONS
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and Zunic, Z. S. Field experience on indoor radon, thoron and their progenies with solid state detectors in a survey of Kosovo and Metohija. Radiat. Prot. Dosim. 152(1– 3), 189– 197 (2012). 18. Mishra, R. et al. An evaluation of thoron (and radon) equilibrium factor close to wall based on long-term measurements in dwellings. Radiat. Prot. Dosim. 160(1-3), 164– 168 (2014).
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