2013年7月30日火曜日

　体内にセシウム　心臓疾患まねく　 「汚染食品食べない努力を」



バンダジェフスキー博士取材記事


ユーリ・バンダジェフスキー博士　病理解剖学者


博士は「日本の医師は原発事故との関係を否定するのではなく、誠実に対応すべきだ」と述べ、「チェルノブイリよりペースが非常に早く、深刻な事態だ」との認識を示した。


「東京新聞」掲載記事




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2013年7月29日月曜日

バンダジェフスキー博士　時事通信の取材で東京の被曝問題に触れる

２０１３年７月講演のため来日したバンダジェフスキー博士は、時事通信の取材に答え、東京の被爆問題に触れた。

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・
被ばく研究、日本も参加を＝チェルノブイリ調査の博士
ユーリ・バンダジェフスキー博士　病理解剖学者
　１９８６年に起きたチェルノブイリ原発事故で、住民の内部被ばくを調査したベラルーシの病理解剖学者ユーリ・バンダジェフスキー博士（５６）が７月に来日し、時事通信社の取材に応じた。博士は同原発のあるウクライナを拠点に、放射性物質に汚染された土地で健康を維持しながら生活するにはどうすればいいか、新たな研究を進めている。博士は「東京電力福島第１原発事故で苦しむ日本の研究者らは、ぜひこの研究に参加して成果を役立ててほしい」と呼び掛けた。
　博士はチェルノブイリ事故で深刻な影響を受けたベラルーシ・ゴメリで、死亡した住民を病理解剖し、放射性セシウム１３７が心臓疾患に及ぼす影響などを突き止めたことで知られる。新たな研究は欧州連合（ＥＵ）から約３００万ユーロの寄付を受け、フランスやドイツなどの医師や研究者らが参加。チェルノブイリから南に約５０キロのウクライナ・イワンコフ地区で７０００人の子どもを含む住民の健康調査と、食品の放射性物質濃度の測定などを実施する。博士は「内部被ばくしないための施策を進め、住民の健康を守りたい」と意気込む。
　福島原発事故後の日本の現状について、博士は「（政府や東電から）重要な情報が公表されていない」と批判。福島県をはじめ、東京を含む東北・関東地方を中心に広範に放射性物質が飛散したと指摘し、「福島以外でも住民の健康調査を徹底し、内部被ばくを避けるため食品のモニタリング検査をさらに強化すべきだ」と強調した。
　福島県の県民健康管理調査では、事故当時１８歳以下の子ども１２人が甲状腺がんと診断されたが、県の検討委員会は事故との関連を否定している。博士は「日本の医師は原発事故との関係を否定するのではなく、誠実に対応すべきだ」と述べ、「チェルノブイリよりペースが非常に早く、深刻な事態だ」との認識を示した。

引用元：時事通信（2013/07/28-14:32）
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2013年7月26日金曜日

英紙に原発事故当初の実証データ発表　




英紙に原発事故当初の実証データ発表（2013/07/26 22:04）

　東京電力福島第１原発事故が発生した直後に福島、宮城両県内で大気や土壌を測定した弘前大学被ばく医療総合研究所の床次眞司教授（４８）＝放射線防護学＝らの研究グループの論文が２６日、英科学電子雑誌「サイエンティフィック・リポーツ」に掲載された。

　論文では、原発の南側と北西側の放射性プルーム（雲のような塊）で、ヨウ素とセシウムの比率が異なることを指摘。併せて「ヨウ素吸入による被ばくはゼロではないが、人体に影響が出るレベルではない」としている。

　床次教授は取材に「原発事故当初の実証データを示すことで、初期被ばくの実態解明や、シミュレーション研究の精度向上につながるはずだ」と話している。

　研究グループは２０１１年３月１７～１９日、福島県いわき市や川俣町など同県内４市町の４地点で、大気中に浮遊している放射性物質を採取。宮城県を含む９市町の１１地点では、土壌や植物、河川水を調べた。

　解析の結果、セシウム１３７を１とした場合、いわき市で検出されたヨウ素１３１は５０～６０程度。平均６～９程度だった他の地点に比べ、ヨウ素の割合が高かった。

　研究グループは「原発の北西側に流れた放射性プルームと、南側に広がったものとは異なる組成だ」と結論付ける一方、「甲状腺被ばくへの影響は極めて少ない」とした。

　　　　（松倉宏樹）





Activity concentrations of environmental samples collected in Fukushima Prefecture immediately after the Fukushima nuclear accident

Scientific Reports
 
3,
 
Article number:
 
2283
 
doi:10.1038/srep02283
Received
 
Accepted
 
Published
 
Radionuclide concentrations in environmental samples such as surface soils, plants and water were evaluated by high purity germanium detector measurements. The contribution rate of short half-life radionuclides such as 132I to the exposure dose to residents was discussed from the measured values. The highest values of the 131I/137Cs activity ratio ranged from 49 to 70 in the environmental samples collected at Iwaki City which is located to the south of the F1-NPS. On the other hand, the 132I/131I activity ratio in the same environmental samples had the lowest values, ranging from 0.01 to 0.02. By assuming that the 132I/131I activity ratio in the atmosphere was equal to the ratio in the environmental samples, the percent contribution to the thyroid equivalent dose by 132I was estimated to be less than 2%. Moreover, the contribution to the thyroid exposure by132I might be negligible if 132I contamination was restricted to Iwaki City.

Subject terms:

Introduction




On March 11, 2011, the power supplies for the cooling systems in the Fukushima Dai-ichi Nuclear Power Station (F1-NPS) were lost due to tsunami damage following the magnitude 9.0 Great East Japan earthquake1, 2. Loss of cooling functions led to hydrogen explosions in three reactor units in the F1-NPS and artificial radionuclides such as radioiodine and radiocesium were released from the reactor buildings. These radionuclides have been detected around the world3, 4, 5, 6, 7. The most contaminated area in Fukushima Prefecture has been observed to the northwest from the F1-NPS8.
On March 12, the Japanese Government had ordered the evacuation of residents within a 20-km radius area from the F1-NPS. A screening survey of radionuclide contamination of evacuees was carried out based on the evacuation decision. A first team of radiological professionals from Hirosaki University carried out the screening survey from March 15 to 19, 2011 at evacuation shelters and public facilities in Fukushima Prefecture9. The team had two purposes: to carry out the screening survey for evacuees and to evaluate ambient dose rate and activity concentrations. In such a nuclear power station accident with large releases of radioactive contaminates from the reactor containment, special attention must be paid to internal exposure to the thyroid by inhalation of released 131I and 132I. 131I and 132I have half-lives of 8 days and 2.3 hours, respectively, and therefore it is necessary to make air-borne activity measurements of these radionuclides quickly. Local health authorities measured the dose rate in the thyroid of 1,149 children under the age of 15 by 1-inch × 1-inch NaI(Tl) scintillation survey meter from March 24 to 30, 201110. Tokonami et al.11measured the thyroid doses for 62 evacuees (including infants) using a 3-inch × 3-inch NaI(Tl) scintillation spectrometer and estimated the thyroid equivalent doses of all of them were below 50 mSv. However, contributions to the thyroid equivalent dose of 132I and 132Te as short half-life radionuclides were not considered. Balanov et al.12 determined the average percent contribution of short half-life radionuclides to thyroid dose of residents in Chernobyl was about 30%.
The authors collected environmental samples such as surface soils, leaves and water immediately after the accident. In this study, radionuclide concentrations in these environmental samples were evaluated, and the percent contribution of short half-life radionuclides to the exposure dose to residents was discussed. On the other hand, air-borne radionuclide concentrations provide important information on the estimation of internal dose due to inhalation in nuclear disasters. In such an emergency, a simple technique to measure air-borne radionuclide concentrations without an AC power supply is needed. Immediately after the accident, radioactive aerosol sampling was also carried out using a glass fiber filter and a battery-powered pump at several locations in Fukushima Prefecture. Furthermore, the inhalation exposure from radioactive aerosols for residents in a measurement period was also discussed.

Results




Ambient dose rate at each measurement site

Ambient dose rate at each measurement site is shown in Table 1 . Ambient dose rates ranged from 0.414 to 9.70 μGy h−1. The highest ambient dose rate of 9.70 μGy h−1 was observed at Koriyama City (KO-2), located to the west of F1-NPS, on March 17, 2011. Ambient dose rate at Iwaki City (IW-2), located to the south of F1-NPS, had the lowest value of 0.414 μGy h−1 on March 18, 2011. As previously reported8, ambient dose rates in the northwest and west directions such as Kawamata Town (KA), Fukushima City (FU) and Koriyama City (KO) were observed to have higher values ranging from 4.50 to 9.70 μGy h−1. However, the ambient dose rate measured for a fourth floor balcony in Fukushima City had the lowest value of 0.252 μGy h−1.


Table 1: Summary of the sampling site locations, date and type of samples
Full table

Air-borne radionuclide concentrations at four sampling sites

131I was detected at three sites (KO-1, FU and IW-2) as shown in Table 2 . Air-borne 131I aerosol concentration at Iwaki City was the highest, 10 ± 3 mBq m−3. On the other hand, air-borne concentrations of 134Cs and 137Cs at the fourth site, Kawamata Town, were the highest with values of 89 ± 23 and 66 ± 18 mBq m−3, respectively. The highest ambient dose rates of 131I, 134Cs and137Cs at Koriyama City were 2 ± 1, 6 ± 4 mBq m−3 and under the detection limit (ND), respectively.


Table 2: Radionuclide concentrations for samples collected on a glass fiber filter at four sampling sites
Full table

Radionuclide concentrations of environmental samples at each sampling site

132Te, 131I, 134Cs and 137Cs were detected in soil samples which were collected at all sampling sites as shown in Table 3 . The maximum values of these radionuclides in the soil samples were observed at Fukushima City (FU), and their respective values were 2.2 × 105, 1.5 × 105, 2.8 × 104and 2.9 × 104 Bq kg−1 wet. 129mTe, 136Cs and 132I were detected in soil samples which were collected at most of the sampling sites in Fukushima Prefecture. Maximum values of these radionuclides (and site ID) were 4.1 × 104 (FU), 9.2 × 103 (KO-2) and 3.3 × 104 Bq kg−1 wet (FU). Furthermore, 140La was also detected at several sampling sites and the maximum value (and site ID) was 1.8 × 104 Bq kg−1 wet (FU). 132Te,131I,134Cs, 136Cs and 137Cs were detected in the plant samples which were collected at all sampling sites as shown in Table 4 . Maximum values of these radionuclides (and site ID) were 4.1 × 105 (KO-1), 3.7 × 105 (IW-1), 1.5 × 105 (KO-1), 2.7 × 104(KO-1) and 1.6 × 105 Bq kg−1 wet (KO-1). 131I activity concentration in Iwaki City (IW) had the highest value. 129mTe and 132I were detected in the plant samples which were collected at most sampling sites in Fukushima Prefecture. Maximum values of these radionuclides (and site ID) were 6.6 × 104 (KO-1) and 5.8 × 104 Bq kg−1 wet (KO-2). Furthermore, 140La was also detected at several sampling sites and the maximum value (and site ID) was 4.8 × 104 Bq kg−1 wet (KO-2).132Te, 131I, 134Cs, 137Cs, 136Cs and 132I were detected from some water samples as summarized in Table 5 . Maximum values of these radionuclides (and site ID) were 1.8 × 103 (AI-SN), 1.3 × 105(FU-RI), 6.8 × 102 (AI-SN), 8.5 × 102 (AI-SN), 1.4 × 102 (AI-SN) and 2.4 × 102 Bq L−1 (AI-SN). A maximum value was observed in a snow sample collected at Aizuwakamatsu City, which is located approximately 96 km from the F1-NPS. On the other hand, the activity concentrations of 129mTe and 140La were below detection limits.


Table 3: Radionuclide concentrations for soil samples
Full table


Table 4: Radionuclide concentrations for plant samples
Full table


Table 5: Radionuclide concentrations for water samples
Full table

Discussion




Radon decay products were collected more than 99% on the 1st stage as the result of performance test of the filter sampling system in the radon chamber of the National Institute of Radiological Sciences, Japan (NIRS). Moreover, radionuclide distributions on the glass fiber filter obtained by the imaging plate measurements seemed to be homogeneous. Therefore, the simple filter sampling system used for this study was an effective technique for the collection of airborne radionuclide in an emergency situation. The airborne 131I activity concentration at Iwaki City was observed as the highest value of 10 mBq m−3 on March 18, 2011. According to the estimation of thyroid equivalent dose for an infant by SPEEDI (System for Prediction of Environmental Emergency Dose Information), high equivalent doses were shown not only in the northwest direction from F1-NPS but also in the south direction such as along the coast in Iwaki City13. According to the simulation results by Katata et al.14, a radioactive plume including 131I was released in the south direction from F1-NPS in the morning on March 15, and it reached Iwaki City. No rainfall was observed around Iwaki City (Yamada monitoring station) on March 15 according to meteorological observation data of the Japan Meteorological Agency15. Rainfall of 0.5–2.0 mm was observed at Iwaki City from 2 PM to 4 PM on March 16, and no rainfall was observed until 7 AM on March 2115. This fact suggested that the contamination in Iwaki City was dry deposition. The maximum values of thyroid equivalent dose for residents in Namie Town were estimated to be 33 mSv according to Tokonamiet al.11. Moreover, they estimated the atmospheric 131I activity concentration on March 15 was 23 kBq m−3. This estimated value was the 131I activity concentration of particulate and gaseous forms. According to Momoshima et al.16, the 131I collected on activated charcoal accounted for 30 to 67% of the total 131I. 131I activity concentration was corrected to the value of March 15, 2011 for the physical half-life, and it was evaluated as 13 mBq m−3. 131I activity concentration as gaseous forms was estimated to be 30 mBq m−3, assuming that 131I gaseous forms were 70% of the total amount. Moreover, according to the simulation results by Morino et al.17, all the species in the radioactive plume from F1-NPS were released toward the Pacific Ocean during the period from March 17 to 19. This fact might indicate the internal exposure by inhalation of 131I at Iwaki City during the period from March 17 to March 19, 2011 was negligible.
Radionuclide concentrations in the environmental samples collected at Fukushima City and Koriyama City were higher than those for samples collected in other sites ( Table 2 ). Radionuclide concentrations of soil samples collected at Fukushima City on March 22 were reported by Taira et al.18. Although 132Te, 132I and 140La were not detected, activity concentrations of other radionuclides in that report were similar values to the present results. According to the simulation results by Katata et al.14, the radioactive plume including 131I was released to the northwest direction from F1-NPS in the evening on March 15, and it reached Fukushima City and Koriyama City. Katata et al. also reported that the radioactive contamination by wet deposition (rainfall) was observed around these areas in the evening on March 15. Activity ratio of each radionuclide based on 137Cs activity concentration is shown in Table 6 and Table 7 . The obtained radionuclide concentrations were corrected to the value of March 15, 2011 for each physical half-life. Since the number of detected radionuclides in water samples was small, only the activity ratio results for soil and plant samples are shown in this table. Tagami et al.19 reported the average value of the134Cs/137Cs activity ratio of soil samples which were collected 20 km south of F1-NPS was 0.9. Moreover, 134Cs/137Cs activity ratios of tea leaves (collected 300 km southwest from F1-NPS) and camellia leaves (collected 220 km south from F1-NPS) were also reported to be 0.98 ± 0.09 and 0.92 ± 0.0520. In this study, the average values (range) of the 134Cs/137Cs activity ratio of all soil and plant samples were 1.0 (0.89–1.1) and 1.0 (0.93–1.0), respectively. These values were similar to the previous study19, 20. Average values (range) of 131I/137Cs activity ratio of all soil and plant samples were 16.4 (2.9–54) and 16.5 (0.85–70), respectively. Average values (range) of 131I/137Cs activity ratio of soil and plant samples at Iwaki City were 51 (49 and 54) and 57 (43 and 70), respectively. On the other hand, average values of 131I/137Cs activity ratio of soil and plant samples excluding Iwaki City were 8.7 and 6.4, respectively. These results suggested that the generation sources of radioactive plume which was released on March 15, 2011 to each area differed.


Table 6: Activity ratio in soil samples of seven radionuclides to 137Cs and the 132I/131I activity ratio
Full table


Table 7: Activity ratio in plant samples of seven radionuclides to 137Cs and the 132I/131I activity ratio
Full table
Average values (range) of 132I/131I activity ratio of all soil and plant samples were 0.09 (0.02–0.32) and 0.19 (0.01–0.59), respectively ( Table 6 and Table 7 ). Especially, 132I/131I activity ratio of soil and plant samples at Iwaki City had the lowest values, and they were 0.02 and 0.01, respectively. The percent contribution to the thyroid equivalent dose of 132I was not considered in the report by Tokonami et al11. According to ICRP Publication 72, dose coefficients of 131I and 132I to an adult are 2.2 × 10−8 and 2.9 × 10−10 (Sv/Bq), respectively21. If it was assumed that the 132I/131I activity ratio in the atmosphere was equal to the ratio in the environmental samples, the percent contribution to the thyroid equivalent dose by 132I was estimated to be less than 2%. Moreover, the contribution by 132I to the thyroid exposure might be negligible (less than 0.03%) if 132I was restricted to Iwaki City. However, if 132Te is taken into the body, 132I will be generated by radioactive decay of 132Te, and the generated 132I will accumulate in the thyroid12. Thus, it will be necessary to examine this process in the human body.
The authors have already reported on the thyroid equivalent dose for residents who lived in the northwest direction from F1-NPS11. Although the local health authorities were reported on the screening survey of the thyroid dose in Iwaki City which was contaminated by 131I at the same level as the northwest region, no detailed examination in this area was carried out by the Japanese government. Furthermore, since the residents were not evacuated from Iwaki City, many children who lived in this city might have been exposed to radioiodine. Therefore, it is important to clarify the thyroid equivalent dose for children who lived in a south direction from F1-NPS (especially coastal areas) immediately after the accident, and it is also important to continue to make ultrasound examinations of the thyroid for residents.

Methods




Environmental sampling

Environmental sampling sites in Fukushima Prefecture are shown in Fig. 1 . This figure was made using the Generic Mapping Tools (GMT) created in 1988 by Wessel and Smith22. The types of environmental samples were summarized in Table 1 . The sampling sites were selected after considering direction and distance from the F1-NPS. Moreover, the environmental sampling sites were located at evacuation shelters and public facilities. The distance between F1-NPS and each sampling site was about 44–96 km. A 1 kg soil sample from 5 cm below the surface was collected at each sampling site. Moreover, a plant sample was also collected at each soil sampling site. Plant species are summarized in Table 1 . Rain water, river water and snow were collected at some sampling sites. Ambient radioactive aerosols were collected by a two-stage sampling technique with glass fiber filters (Whatman GF/F, ϕ = 47 mm) and a battery-powered pump (MP-Σ300, Sibata Scientific Technology Ltd.). The sampling flow rate was set to 2.0 L min−1 at each sampling site. The sampling time and total sampling volume are given in Table 1 . The weather at each site was fair during the period from March 17 to 19.


Figure 1: The location of the environmental sampling points.
The location of the environmental sampling points.
Sampling sites in Fukushima Prefecture were selected after considering their direction and distance from F1-NPS.

Measurement of ambient dose rate

In-situ gamma-ray spectra at several sampling sites for the estimation of ambient dose rate were obtained using a 3-inch × 3-inch NaI(Tl) scintillation spectrometer (JSM-112, Hitachi-Aloka Co.). Measurements at every site were carried out 1 m above the uncovered soil surface. Measurements at Fukushima City (FU) were carried out not only outside but also on a balcony of a fourth floor building (the disaster countermeasures office). This balcony was made of concrete. Counting time was set to 300 s at every site. The obtained gamma-ray pulse height distributions were unfolded by a 60 × 60 response matrix for the evaluation of ambient dose rates23. This calculation software assumed that the fallout formed an infinite plane source on the ground.

Evaluation of radionuclide concentrations

Quick measurement is necessary for evaluation of the short half-life nuclide concentrations. Although gravel and roots were removed from the soil samples, a drying processing was not carried out for the environmental samples. The plant sample was cut about 1 square centimeter from a leaf. The water sample (100 mL) had several grams of NaCl added as a carrier for the evaluation of radionuclide concentration. Two filter samples were enclosed in a container. Every environmental sample was enclosed in a cylindrical polypropylene container of 48 mm × 55 mm. Radionuclide concentrations of each sample were measured with a high-purity germanium (HPGe) detector (GEM-100210, ORTEC). The measurement time was set at 600 s for the evaluations of short half-life radionuclides such as 131I, 132I and 132Te. For evaluation of long half-life radionuclides such as134Cs and 137Cs, measurement time was set at more than 16,000 s. The radionuclide concentrations in environmental samples excluding filter samples were corrected to the value on March 15, 2011 by each physical half-life. On the other hand, radionuclide concentrations of filter samples were corrected to the sampling date.

Evaluation of surface distribution of radioactive aerosols on the filter

The surface distribution of radioactive aerosols on the filter is also important due to the counting efficiency in the HPGe detector measurement. Therefore the surface distribution with the same system as for in-situ sampling was evaluated using a radioactive aerosol chamber (internal volume: approximately 25-m3) at NIRS. This radioactive aerosol chamber is environmentally controlled for temperature and relative humidity. Radon is used as the radioactive source. The temperature and relative humidity can be controlled in the range of 5 to 30°C with an error of 0.5°C, and 30 to 90% with an error of 3%, respectively24. In this study, radon concentration, temperature and relative humidity were set to 10,000 Bq m−3, 20°C and 60%, respectively. Carnauba wax was used as the aerosol material and the particle size had the distribution which made approximately 100 nm maximum25. Two glass fiber filters with a battery-powered pump which were used for the in-situ sampling were used for the performance test. The sampling flow rate was set to 2 L min−1 and radon decay products were collected during 3.5 h. After aerosol samples were collected, the gross alpha measurements were recorded over consecutive 1 minute intervals during a total recording period of 60 minutes. Moreover, an imaging plate technique (BAS-MS 2025, Fuji Film Co.) was used in order to obtain the distribution images of the radon decay products on the glass fiber filters. All radionuclides other than 3H, which has a low beta energy of 18.6 keV, can be detected by this technique26. Information in the imaging plate was read out after 3 days using a reading system (FLA-5100, Fuji Film Co.). Gradation and resolution for the reading system were set to 16 bits and 25 μm, respectively.

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Acknowledgements




This work was partly supported by a Grant for the Co-medical Education Program in Radiation Emergency Medicine by the Ministry of Education, Culture, Sports, Science and Technology, Japan (2011). The authors thank Drs. Shun'ichi Hisamatsu and Yoshihito Ohtsuka, Institute for Environmental Sciences for their kind assistance in carrying out measurement of radionuclide concentrations with the HPGe detector. Mr. Junya Ishikawa and Mr. Masaru Yamaguchi, Hirosaki University Graduate School of Health Sciences assisted in preparing the environmental samples.

Author information




Affiliations

  1. Department of Radiological Life Sciences, Hirosaki University Graduate School of Health Sciences 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan

    • Masahiro Hosoda,
      •  
    • Satoru Monzen,
      •  
    • Minoru Osanai,
      •  
    • Hironori Yoshino,
      •  
    • Yasushi Mariya &
      •  
    • Ikuo Kashiwakura

  2. Institute of Radiation Emergency Medicine, Hirosaki University 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan

    • Shinji Tokonami,
      •  
    • Hirofumi Tazoe,
      •  
    • Atsuyuki Sorimachi,
      •  
    • Masatoshi Yamada,
      •  
    • Akifumi Nakata,
      •  
    • Mitsuaki Yoshida &
      •  
    • Ikuo Kashiwakura

  3. Department of Radioecology, Institute for Environmental Sciences 1-7 Ienomae, Obuchi, Rokkasho, Aomori 039-3212, Japan

    • Naofumi Akata &
      •  
    • Hideki Kakiuchi

  4. Research Center for Radiation Protection, National Institute of Radiological Sciences 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

    • Yasutaka Omori,
      •  
    • Tetsuo Ishikawa &
      •  
    • Sarata K. Sahoo

  5. Institute of Radiochemistry and Radioecology, University of Pannonia Egyetem St. 10, Veszprem, H-8200, Hungary

    • Tibor Kovács

Contributions

M.H., S.T. and I.K. designed the study; M.H., S.T., S.M., M.O., Masatoshi Yamada, A.N., Mitsuaki Yoshida, H.Y. and I.K. carried out field measurements and sample preparations; H.T. and Masatoshi Yamada measured filter samples; M.H., A.S., S.S., N.A., H.K. and Y.M. analyzed gamma spectrum; M.H., Y.O. and T.I. carried out experiment at NIRS radon chamber; M.H., S.T. and T.K. wrote the manuscript; S.T. supervised the study. All authors contributed extensively to discussions about this work and in reviewing the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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