Radiation Protection Dosimetry (2007), Vol. 123, No. 2, pp. 250–253 Advance Access publication 4 August 2006
doi:10.1093/rpd/ncl102
SCIENTIFIC NOTES
RADON AND THORON PARALLEL MEASUREMENTS IN HUNGARY N. Ka´va´si1, Cs. Ne´meth2, T. Kova´cs1,, S. Tokonami3, V. Jobba´gy1, A. Va´rhegyi4, Z. Gorja´na´cz4, T. Vı´gh5 and J. Somlai1 1 Department of Radiochemistry, Pannon University, H-8201 Veszpre´m, P.O. Box 158, Hungary 2 Department of Physics, Pannon University, H-8201 Veszpre´m, P.O. Box 158, Hungary 3 Radon Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage -ku, Chiba 263-8555, Japan 4 Mecsek Environmental Protection Co. P.O. Box 121, 7614 Pe´cs, Hungary 5 ´ rku´t, Hungary Manganese Mining and Processing Ltd, U Received April 25 2006, amended July 6 2006, accepted July 9 2006 Hungarian detectors modified and developed at the National Institute of Radiological Sciences (NIRS), Japan were placed at different sites, including homes and underground workplaces in Hungary, in order to gain information on the average radon (222Rn) and thoron (220Rn) concentration levels. Measurements were carried out in dwellings in a village and a manganese mine in Hungary. The radon and thoron concentrations in the dwellings of the village in the summer period were found to be 154 (17–1083) and 98 (1–714) Bq m 3, respectively. Considering the results of other radon measurements during the winter (814 Bq m 3) and summer (182 Bq m 3) periods, the thoron concentrations were also expected to be higher in winter. In the manganese mine, radon and thoron were measured at 20 points for 6 months, changing the detectors each month. The averages were 924 (308–1639) and 221 (61–510) Bq m 3 for radon and thoron, respectively. These results showed significant variance with the date and place of the measurement.
INTRODUCTION 222
220
Radon ( Rn), thoron ( Rn) and their progenies are considered to be the major contributors to human exposure from natural radiation sources(1). Recently, more surveys seem to have concluded that radon in the living environment implies an increased health risk(2–5). Although radon presents the main concern as an inhalation dose contributor for the general public, in recent years thoron has gained increasing attention among health physicists. For example, previous studies have shown that thoron (220Rn) was present to a considerable extent in traditional Japanese houses(6–8), it was found to have a higher than average concentration in Mexico City(9), and it has gained an important part in surveys in India(10) and China(11,12). The presence of thoron has two consequences. First, it made confounding effects on the accurate radon measurement(13); second, thoron itself should be considered from a radiological viewpoint because it might result in radiation exposures comparable with those due to radon(14). Traditionally both effects were considered to be relatively small and were, therefore, usually neglected. The measurement of thoron involves more difficulties in comparison with radon. Thoron
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concentrations, mainly because of thoron’s short half-life (55.6 s), is highly inhomogeneous with a strong dependence on the distance from the source(15). In the present paper, radon and thoron parallel integrated measurements at various locations are described.
SITES, METHODS AND MEASUREMENTS Modified Hungarian detectors were placed at different sites, including homes and a mine in Hungary in order to gain information on radon (222Rn) and thoron (220Rn) concentrations over the investigated period. These detectors were developed and evaluated at the National Institute of Radiological Sciences (NIRS), Japan. Namely, a RADOPOT passive 222Rn monitor was remodelled to measure both 222Rn and 220Rn by enhancing its air change. The remodelled monitor was attached to the original 222Rn monitor to become a unified 222Rn and 220Rn discriminative monitor(16). For the detectors to be placed in dwellings, va´go´szo lo s, a village in Hungary, was chosen. Ko This is a village with 1200 inhabitants; the investigated houses were one-storey buildings made of bricks. It is located in the Mecsek Mountains in the southern part of Hungary. This village was chosen because there was uranium mining activity for almost 40 years in the neighbouring areas.
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RADON AND THORON PARALLEL MEASUREMENTS IN HUNGARY
RESULTS AND DISCUSSION Dwellings The average (arithmetical) radon and thoron concentrations in the living areas of 72 dwellings of the va´go´szo lo s were found to be 154 village of Ko (17–1083) and 98 (1–714) Bq m 3, respectively. In the 31 checked cellars the average concentration for radon was 353 (17–1950) Bq m 3 and for thoron was 208 (16–1308) Bq m 3. Considering that the survey was undertaken during the summer period (June–August 2004), these data indicate relatively high levels. The radon concentrations were checked in 46 dwellings from the total 72 in winter periods as well. The results of these measurements are summarised in Table 1. The distribution of the radon and thoron concentrations in the 72 dwellings is plotted in Figure 1. The Tn/Rn ratio in the 72 dwellings is plotted in Figure 2.
Table 1. Radon and thoron concentrations in 46 dwellings.
Average Min–max
222
Rn conc. (Bq m 3) (December– March)
222
220
814 67–3344
182 19–1083
108 4–714
Rn conc. (Bq m 3) (June– August)
Rn conc. (Bq m 3) (June– August)
35 30
Number of dwellings
The rock under the village is grey-sandstone. The analysis of the drill cores taken from a depth of 1–2 m in the soil of the village showed that the average uranium and thorium contents were 136 and 77 Bq kg 1, respectively. These values exceed world averages(1) (33 and 45 Bq kg 1). The detectors were mostly placed in the inhabited areas of the houses, such as bedrooms and living-rooms, at a height of 1–1.5 m and 20 cm from the wall. Detectors were also placed in some cellars of the buildings, at least 1 m above ground level and 10 cm from the nearest wall. Radon measurements executed earlier in this village showed higher levels here than the characteristic one for Hungarian villages(17). To obtain information about the parallel radon and thoron average concentrations in underground workplaces, a manganese mine was chosen. The ´ rku´t, in the manganese mine can be found in U middle-west part of Hungary. The manganite ores found here contain relatively little amounts of 226Ra and 232Th. [226Ra: 7.83 (1.7–22.3) Bq kg 1; 232Th: 10.1 (3.76–25.7) Bq kg 1]. Despite this fact, the radon concentration measured here earlier was relatively high (annual average 575–997 Bq m 3 depending on the place where the measurements were conducted). In this study the radon and thoron concentrations were measured at 20 different locations in the mine, such as the blowing road, the back-heading, air channel, pump building, back-stope, active faces, etc., over 6 months: the detectors were changed every month. The detectors were placed 10–20 cm from the nearest mine wall or roof.
25
Rn winter Rn summer
20
Tn summer
15 10 5 0
0-100 100-200 200-400 400-600 600-1000 1000-2000 2000-
Rn/Tn concentrations [Bq m−3]
Figure 1. The distribution of the radon and thoron concentrations in the 72 dwellings.
The radon concentration is considerable lower in the summer than the winter months owing to increased ventilation, but even in summer it exceeds the Hungarian yearly average in villages(17) (134 Bq m 3). The thoron concentration was also expected to be lower when the summer measurements were taken, but the 98 Bq m 3 average for the 72 dwellings does not indicate a low level. The yearly average is expected to be higher according to the experiences of the seasonal variations of thoron levels in dwellings(9). Our future work will investigate this seasonal variation of thoron concentrations. Manganese mine The average radon and thoron concentrations measured over 6 months (May–Oct 2004) in the 20 locations in the mine were 924 (308–1639) Bq m 3and 221 (61–510) Bq m 3, respectively. Consequently, the average radon concentration does not exceed the action level established in Hungary (1000 Bq m 3). The radon and thoron concentrations (6 month averages) measured at 20 locations in the mine are shown in Figure 3. It can be seen that the concentrations are strongly dependent on the location of the measurement. There are differences of five to eight times for radon and thoron, respectively, depending on the location of the detectors in the mine. The monthly
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7
6
Tn/Rn ratio
5
4
3
2
1
0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71
dwellings Figure 2. The ratio of the thoron to radon concentrations in the 72 dwellings.
−3
Radon and Thoron concentration [Bqm ]
1800 1600 Radon 1400
Thoron
1200 1000 800 600 400 200 0 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
Number of measurement point
Figure 3. The radon and thoron concentrations in the manganese mine.
averages at the ninth measuring point in the out-take shaft of the mine (air measurement station) can be seen in Figure 4. The radon concentration data come from the airendway, where the air represents the average air of the mine and show characteristics of radon concentration variations of the caves, namely that they are higher in the warmer months in comparison with colder periods. But the characteristic cannot be found in the case of thoron. The monthly averages show strong
variability. The considerable differences in the radon and thoron concentrations with the place and date of the measurements may be due to the air change differences and the different types of work in the diverse parts of the mine; in the case of thoron, its short half-life also influences the results. CONCLUSIONS The 98 Bq m 3 thoron concentration measured in the summer period in 72 dwellings of the village
252
Avtivity concentration [Bqm−3]
RADON AND THORON PARALLEL MEASUREMENTS IN HUNGARY 1600
Rn
Tn
1400 1200 1000 800 600 400 200 0 May June July
Aug Sept Oct average
Months
Figure 4. The monthly averages in the out-take shaft of the manganese mine.
seems to be high considering that the radon concentration differences measured in the summer and winter periods were significant (814 and 182 Bq m 3). This can influence the accuracy of the radon measurements and has its own impact on the radiological hazard. The radon and thoron concentrations in the manganese mine air seem to be high despite low uranium and thorium concentrations in the neighbouring rocks. The results show strong dependency on the locations and the date of the measurements. This may be due to the different type of mining activity and the differences in the air change at particular places in the mine. Although the thoron problem was considered to be Asia-specific, our data refutes conventional ideas and practice. Therefore, the importance of radon–thoron discriminative measurements should be recognised.
REFERENCES 1. United Nations Scientific Committee on the effects of Atomic Radiation. Sources and effects of ionizing radiation. UNSCEAR 2000. Report to the General Assembly with Scientific Annexes (NY: United Nations) (2000). 2. Krewski, D. et al. Residential radon and risk of lung cancer. A combined analysis of 7 North American casecontrol studies. Epidemiology 16, 137–145 (2005). 3. Darby, S. et al. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case–control studies. Brit. Med. J. 330, 223 (2005).
4. Chen, J. Estimated risks of lung cancer for different exposure profiles based on the new EPA model. Health Phys. 88, 323–333 (2005). 5. Wichmann, H. E., Rosario, A. S., Heid, I. M., Kreuzer, M., Heinrich, J. and Kreienbrock, L. Increased lung cancer risk due to residential radon in a pooled and extended analysis of studies in Germany. Health Phys. 88, 71–75 (2005). 6. Doi, M. and Kobayashi, S. Spatial distribution of radon and thoron concentrations in the indoor air of a traditional Japanese wooden house. Health Phys. 66, 43–49 (1994). 7. Ma, J., Yonehara, H., Aoyama, T., Doi, M., Kobayashi, S. and Sakaunoe, M. Influence of air flow on the behavior of thoron and its progeny in a traditional Japanese house. Health Phys. 72, 86–91 (1997). 8. Tokonami, S., Yonehara, H., Zhuo, W., Sun, Q., Sanada, T. and Yamada, Y. Understanding of high radon concentrations observed in a well- ventilated Japanese wooden house. In: Proceedings of the ninth International Conference on Indoor Air Quality and Climate, Monterey, USA, 30 June–5 July. Vol. 1, pp. 665–669 (2002). 9. Martinez, T., Navarrete, M., Gonzalez, P. and Ramirez, A. Variation in indoor thoron levels in Mexico City dwellings. Radiat. Prot. Dosim. 111, 111–113 (2004). 10. Mishara, R., Tripathy, S. P., Khating, D. T. and Dwivedi, K. K. An extensive indoor 222Rn/220Rn monitoring in Shillong, India. Radiat. Prot. Dosim. 112, 429–433 (2004). 11. Guo, Q., Sun, J., Chemg, J., Shang, B. and Sun, J. The levels of indoor thoron and its progeny in four areas of China. J. Nucl. Sci. Technol. 38, 799–803 (2001). 12. Tokonami, S. et al. Radon and thoron exposures for cave residents in Shanxi and Shaanxi provinces. Radiat. Res. 162(4), 390–396 (2004). 13. Tokonami, S., Yang, M. and Sanada, T. Contribution from thoron on the response of passive radon detectors. Health Phys. 80, 612–615 (2001). 14. Chung, W., Tokonami, S. and Furukawa, M. Preliminary survey on radon and thoron concentrations in Korea. Radiat. Prot. Dosim. 80, 423–426 (1998). 15. Gargioni, E., Honig, A. and Ro¨ttger, A. Development of a calibration facility for measurements of the thoron activity concentration. Nucl. Instrum. Methods Phys. Res. 506, 166–172 (2003). 16. Zhuo, W., Tokonami, S., Yonehara, H. and Yamada, Y. A simple passive monitor for integrating measurements of indoor thoron concentrations. Rev. Sci. Instrum. 73, 2877–2881 (2002). 17. To´th, E. Radon a magyar falvakban (in Hungarian). Fizikai Szemle 2, 44–49 (1999).
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