preliminary results from an indoor radon thoron survey in hungary

Radiation Protection Dosimetry Advance Access published August 26, 2012 Radiation Protection Dosimetry (2012), pp. 1–4

doi:10.1093/rpd/ncs231

PRELIMINARY RESULTS FROM AN INDOOR RADON THORON SURVEY IN HUNGARY G. Szeiler1, J. Somlai1, T. Ishikawa2, Y. Omori2, R. Mishra3, B. K. Sapra3, Y. S. Mayya3, S. Tokonami4, A. Csorda´s5 and T. Kovaćs1,* 1 Institute of Radiochemistry and Radioecology, University of Pannonia, PO Box 158, H-8201 Veszprem, Hungary 2 National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan 3 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai I-400 085, India 4 Institute of Radiation Emergency Medicine, Hirosaki University, 1-Bunkyocho, Hirosaki, Japan 5 Social Organization for Radioecological Cleanliness, Veszprem, Hungary

More than half of the radiation dose of natural origin comes from radon. However, according to some surveys in certain cases, the radiation dose originating from thoron may be considerable. Among the factors disturbing the measurement of radon, the presence of thoron may also influence the measured radon value, making the estimated radiation exposure imprecise. Thoron has previously been surveyed, mainly in Asia; however, recent surveys for some European locations have found that significant thoron concentrations also need to be considered. In this survey, several types of commercially available SSNTDs (solid-state nuclear track detectors) capable of measuring both radon and thoron were placed at the same time in 73 houses and 7 workplaces in Hungary with 3-month exposition periods. In order to measure thoron, the distance of the detector sets was fixed as 15–20 cm from the walls. The radon concentration was measured with five types of SSNTDs: NRPB, NRPB SSI, Raduet, DTPS and DRPS. The first four types had relatively good accordance (within+ + 10 %), but the results of the DRPS detectors were considerably lower when compared with other detectors for radon concentrations over 100 Bq m23. The thoron averages were provided by two different types of detectors: Raduet and DTPS. The difference between their average results was more than 30 % and was six times the maximum values. Therefore, the thoron measurement results were judged to be erroneous, and their measurement protocol should be clearly established for future work.

INTRODUCTION 222

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Radon ( Rn), thoron ( Rn) and their progenies are considered to be the major contributors to human exposure from natural sources(1). Although radon and its progenies are the main contributors in inhalation dose for the general public, thoron has gained increasing attention among health physicists in recent years. The presence of thoron has two consequences. On the one hand, it confounds accurate radon measurements, and on the other hand, thoron itself should be considered from a radiological aspect as it might result in radiation exposure comparable with that originating from radon. Both effects have been considered to be relatively small and they have usually been neglected. Despite the fact that the parent element of thoron 232 Th is usually present in soil and rocks in a higher concentration than the parent element of radon, the measurable thoron concentration is usually negligible relative to radon in indoor air. The short lifetime of thoron (half-life, 55.6 s) and its inhomogeneous distribution, with a strong dependence on the distance from the source, are responsible for this. However, there are some longer lifetime isotopes among the thoron progenies that are capable of spreading in the air by sticking to aerosol particles, and establishing a

more or less homogeneous concentration distribution; therefore, the radiation dose contribution is not negligible. Despite this, the number of indoor thoron surveys is surprisingly small. This is for several reasons, but is primarily because the measurement of thoron involves more difficulties compared with that of radon. In countries where significant thoron concentrations are expectable, measurements of radon and thoron concentrations for the general population have not been done widely (primarily due to economical issues). Therefore, thoron reports in the literature have appeared more or less as a phenomenon specific in Asia(2, 3). However, nowadays, more and more studies are dealing with the role of 220Rn (thoron) in the radiation exposure of the general population. During the past few years, several studies on indoor thoron measurements have been published for Europe too(4 – 6). Indoor radon measurements are relatively a great tradition in Hungary; however, rather few studies have called attention to thoron measurements. Specific research work related to thoron was carried out by Hunyadi et al.(7). In 2003, the Institute of Radiochemistry and Radioecology, the University of Pannonia, and the Social Organisation for Radioecological Cleanliness

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G. SZEILER ET AL.

MATERIALS AND METHODS Detector set-up used for the measurements There are several detectors generally used for identifying integral radon concentrations and they have several harmonized protocols(10, 11) concerning the placement, optimum measurement period, placement within a building of the detectors etc. However, for the measurement of average thoron concentration, despite the great number of surveys and measurements, there is no unified protocol, different evaluation protocols are used almost everywhere, even within so-called standard SSNTD techniques(10 – 12). Therefore, during this work, the detectors of different kinds were compared under identical measurement conditions. The detectors were placed on a plate positioned 15–20 cm away from a wall. CR-39 and LR-115 track detectors were placed in the different detector holders. Types of detectors and their features (1) NRPB and NRPB SSI (CR-39): Two types of NRPB diffusion chamber (electrically non-conductive plastic and electrically conductive) with the CR-39 detector were used for measuring radon concentration(8). (2) Raduet (CR-39): The radon and thoron measurements were performed using Raduet discriminative passive measuring devices. Each Raduet detector includes two CR-39 detectors in two diffusion chambers of different permeability(5, 8). One of them lets both radon and thoron into the track detector holder, while the other one lets only radon into it, so the radon concentration can be calculated from the difference of the two. (3) DRPS-DTPS (LR-115) (DRPS: direct radon progeny sensor; DTPS: direct thoron progeny sensor): These sensors are based on the LR-

115(13) and the tracks formed are counted with an automatic track-counting technique using a spark counter(13). This sensor measures the tracks left by radon and/or thoron progenies of different energy levels. Significantly, this type is capable of measuring progeny concentration and the radon and thoron concentration ratios calculated from this are only an estimated value as there was no opportunity to measure EEC, from which the actual equilibrium factor can be identified. CR-39 detectors were chemically etched for 3 h in a 6.25 M NaOH solution at 908C. Etching for the LR115 detectors was performed using 2.5 M NaOH, on 608C for 1 h, with continuous stirring(2). The tracks were viewed using a Virginia 99 image analysis system. The CR-39 track detectors were calibrated in an airtight radon chamber (an EV 03209, produced and calibrated by Genitron Instruments GmbH), using a PYLON RN 2000A calibration standard source. For the LR-115 detectors, the calibration factor given by the manufacturer was used. The standard deviation of the measurement results of the track detectors capable of radon measurement was 7 %, while the standard deviation for the DRPSs was 28 %, which is acceptable. Great differences were found between the data given by the widely used Raduet detectors capable also of measuring thoron, and that of the DTPSs made in India. The average deviation was 42 %. MEASURING SITES Workplaces and dwellings Seven detectors were placed in offices where employees worked 8 h d21, and the building was not equipped with air-conditioners. The exposure period was 3 months. Seventy-three dwellings located in five counties were selected. Eighty per cent of the selected houses were one-storey houses, and the rest were houses with an attic. Concerning their building materials, 90 % of the houses had been built using traditional building materials (clay and straw mortar or brick-clay). The rest of the houses used TENORM as building material too. The sets of detectors were placed at the same time for periods of 3 months. RESULTS AND DISCUSSION In evaluation of the results, two aspects were taken into consideration: first, how capable the different track detector types were of performing measurements in accordance with each other, and second, to gain information about the radon and thoron concentrations of the investigated places.

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in a joint work started wide-scale thoron surveys in Hungary, using radon –thoron discriminative passive detectors(8). This study has introduced the results from dwellings in a village located near the closed and remediated uranium mines and from several high radon risk workplaces as mine, cave and therapeutic bath according to Hungarian Regulations(9). Based on the results, it could be stated that in certain regions of Hungary thoron is present in a significant amount in the air of certain homes. The next survey was performed in 2006–2007, where one type of CR39-based Raduet dosimeters were used(5). In the present paper, attention was turned to eliminating the errors made in previous surveys. Measurements were made throughout 3 months at 80 places (73 dwellings and 7 workplaces) using commercially available SSNTDs.

INDOOR RADON THORON SURVEY IN HUNGARY

level 300 Bq m23 given in the 2009 WHO recommendation was not exceeded by the measured average values in any cases (although in one case a higher level was measured using the Raduet detector). Figure 3 shows the average values of the different detectors. The lower values of the DRPS detectors comes from the fact that these type of detectors showed considerable underestimation with respect to other detectors at radon concentrations over 100 Bq m23—as it is shown in Figure 1. The average thoron concentration provided by Raduet and DRPS detectors are 31 (1–285) and 22 (8 –44) Bq m23, respectively..The min– max values had rather great differences during the measurement of thoron. Further surveys are planned to clarify the reasons. The average thoron concentration distribution is shown in Figure 4. Based on the results, it can be stated that the thoron concentration was generally low, but in some cases (10 %) it exceeded 100 Bq m23. In these rooms, the contribution of thoron to the radiation dose may be considerable and therefore thoron measurements seem to be necessary. Elaborating the appropriate measurement protocol and finding the reasons for the great differences are foreseen in further works. SUMMARY

Figure 1. Distribution of radon concentration measured using different types of detectors.

Figure 2. Average radon concentration distribution.

Within this study, 73 dwellings and 7 workplaces in 5 Hungarian counties were surveyed for radon and toron, with 3-month exposure periods. For that purpose, several types of track detectors (NRPB, NRPB SSI, Raduet, DRPS-DTPS) were used, which behaved in different ways depending on the radon and thoron concentrations. Three types of the detectors measured radon and thoron concentrations directly, while the fourth one measured progeny concentration. Actually, this is advantageous, as from

Figure 3. Average radon concentration values measured using different detectors.

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Figure 1 shows the distribution of the radon concentration measured using different radon detectors in 73 dwellings and at 7 workplaces (offices). It is apparent from the figure that in case of three detectors types the distribution was similar, while the DRPS detectors (made in India) considerably underestimated the values in places with high radon concentration. In order to find out the causes, further surveys will be performed. The distribution of the radon concentration average values of the four detectors is shown in Figure 2. In the 80 measurement places where the average radon concentration measured using the four detectors was 79 Bq m23. In 58 cases, the average radon concentration was below 100 Bq m23, and it only exceeded the value 200 Bq m23 in one case (14, 15). The reference

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2. 3. 4. 5. 6.

the aspect of radiation exposure the major part of the dose does not originate radon and thoron, but their progenies. Raduet and NRPB detectors gave similar values in workplaces and dwellings, and at lower and higher activity concentration. On the contrary, the DRPSDTPS (LR115) track detectors generally showed underestimation at higher radon concentration (.100 Bq m23). This was probably caused by the fact that the DTPS-DRPS types measured the progeny concentration. Further radon concentration and progeny concentration measurements are planned. Considering the thoron measurements, it could be stated that thoron was also present in parallel with radon; therefore, it was not always negligible. Unfortunately, the detectors used for measurements so far have showed great differences. Further measurements are planned in order to establish an appropriate measurement protocol. FUNDING

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11. 12.

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This research is supported by the Hungarian Research Fund [OTKA Nos. K 81975, K 81933]. 14.

REFERENCES 1. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and Effects of Ionizing

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7. Figure 4. Distribution of thoron concentration.

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