Invited Article: Radon and thoron intercomparison experiments for integrated monitors at NIRS, Japan M. Janik, T. Ishikawa, Y. Omori, and N. Kavasi Citation: Review of Scientific Instruments 85, 022001 (2014); doi: 10.1063/1.4865159 View online: http://dx.doi.org/10.1063/1.4865159 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Thoron detection with an active Radon exposure meter—First results Rev. Sci. Instrum. 85, 022106 (2014); 10.1063/1.4865162 Invited Article: In situ comparison of passive radon-thoron discriminative monitors at subsurface workplaces in Hungary Rev. Sci. Instrum. 85, 022002 (2014); 10.1063/1.4865161 Invited Article: Expanded and improved traceability of vibration measurements by laser interferometry Rev. Sci. Instrum. 84, 121601 (2013); 10.1063/1.4845916 Radon and Thoron exhalation rate map in Japan AIP Conf. Proc. 1034, 177 (2008); 10.1063/1.2991202 A simple passive monitor for integrating measurements of indoor thoron concentrations Rev. Sci. Instrum. 73, 2877 (2002); 10.1063/1.1493233
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022001 (2014)
Invited Article: Radon and thoron intercomparison experiments for integrated monitors at NIRS, Japan M. Janik,a) T. Ishikawa, Y. Omori, and N. Kavasi National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, 263-8555 Chiba, Japan
(Received 18 September 2012; accepted 11 November 2013; published online 19 February 2014) Inhalation of radon (222 Rn) and its short-lived decay products and of products of the thoron (220 Rn) series accounts for more than half of the effective dose from natural radiation sources. At this time, many countries have begun large-scale radon and thoron surveys and many different measurement methods and instruments are used in these studies. Consequently, it is necessary to improve and standardize technical methods of measurements and to verify quality assurance by intercomparisons between laboratories. Four international intercomparisons for passive integrating radon and thoron monitors were conducted at the NIRS (National Institute of Radiological Sciences, Japan). Radon exercises were carried out in the 24.4 m3 inner volume walk-in radon chamber that has systems to control radon concentration, temperature, and humidity. Moreover, the NIRS thoron chamber with a 150 dm3 inner volume was utilized to provide three thoron intercomparisons. At present, the NIRS is the only laboratory world-wide that has carried out periodic thoron intercomparison of passive monitors. Fifty laboratories from 26 countries participated in the radon intercomparison, using six types of detectors (charcoal, CR-39, LR 115, polycarbonate film, electret plate, and silicon photodiode). Eighteen laboratories from 12 countries participated in the thoron intercomparisons, using two etch-track types (CR-39 and polycarbonate) detectors. The tests were made under one to three different exposures to radon and thoron. The data presented in this paper indicated that the performance quality of laboratories for radon measurement has been gradually increasing. Results of thoron exercises showed that the quality for thoron measurements still needs further development and additional studies are needed to improve its measuring methods. The present paper provides a summary of all radon and thoron international intercomparisons done at NIRS from 2007 to date and it describes the present status on radon and thoron passive, one-time cycle monitors. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865159] I. INTRODUCTION A. Characteristics of radon and thoron
The natural radioactive gas radon (222 Rn) is a member of the uranium decay series; this means that when uranium decays, after a series of steps, one of its daughter products is radon. One of the radon isotopes is thoron (220 Rn), which is a natural decay product of the most stable thorium isotope (232 Th). Radon was one of the earliest discovered radioactive elements, being identified after uranium, thorium, polonium, and radium. It was detected in 1900 by Dorn, a German chemist, during experiments in which he noticed that radium compounds emanate a radioactive gas.1 In 1908 Ramsay and Gray isolated the gas and named it niton.2 The term thoron (correctly thoron emanation) appeared in the literature for the first time in 1899 by reason of Ernest Rutherford’s experiments that investigated radioactive properties of thorium samples.3 In 1923, the International Committee for Chemical Elements and International Union of Pure and Applied Chemistry (IUPAC) chose the names radon (Rn) and thoron (Tn). The characteristics of parents and daughters of radon (222 Rn) and thoron (220 Rn) are shown in Table I. Radon, its a) Corresponding author email: [email protected]
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parent radium (226 Ra) and its decay products are members of the uranium (238 U) decay chain whereas thoron, its parent radium (224 Ra) and its daughters are members of the thorium (232 Th) decay chain. Significant differences are seen in the half lives of radon and thoron, and many of the behavioural differences in the members of these two chains are visible in indoor air. These differences have had implications for the development of measurement methods for radon, thoron, and their progenies as well as for dosimetric models. B. Radon and thoron problems in radiation protection
Lung cancer occurrence was recognized as a “radon problem” in the late nineteenth century among workers in German mines and was described by Haerting and Hesse.4 In 1924, Ludewig and Lorenser5 suggested that these lung cancer cases could be attributed to radon exposure. In the next 30 years it was recognized that the true cause for high lung dose was not radon itself but the inhalation of its progenies, e.g. Radium A, Radium B, Radium C, and Radium C . It is known that these solid particles are mostly absorbed on the surface of ambient aerosols and during inhalation these radioactive aerosols are accumulated in the lung by filtration. Therefore, the lung is the most exposed organ for radon and thoron in the body.
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TABLE I. Characteristics of uranium (radium) and thorium decay chains, including radon (222 Rn), thoron (220 Rn), their parents and decay products (daughters). Uranium chain Nuclide (historical 238 U 234Th 234m Pa 234 Pa 234 U 230Th 226 Ra 222 Rn
(Radon) (Radium A) 214 Pb (Radium B) 214 Bi (Radium C) 214 Po (Radium C ) 210 Tl (Radium C ) 210 Pb (Radium D) 210 Bi (Radium E) 210 Po (Radium F) 206 Pb (Radium G) 218 Po
a b
name)a
Half-life
Thorium chain
Radiation emitted (energy
4.49 × 109 y 24.10 d 1.16 min 6.70 h 2.46 × 105 y 7.59 × 104 y 1602 y 3.82 d 3.10 min 26.8 min 19.9 min 0.16 ms 1.30 min 22.3 y 5.01 d 138 d Stable
released)b
α (4.20, 4.15 MeV) ββ-, IT βα (4.77, 4.72 MeV) α (4.69, 4.62 MeV) α (4.78, 4.60 MeV) α (5.49 MeV) α (6.00 MeV), βββα (7.69 MeV) βββα (5.30 MeV) ...
Nuclide (historical name)
232 Th 228 Ra 228 Ac 228 Th 224 Ra 220 Rn
(Thoron) (Thorium A) 212 Pb (Thorium B) 212 Bi (Thorium C) 212 Po (Thorium C ) 208 Tl (Thorium C ) 208 Pb (Thorium D) 216 Po
Half-life
Radiation emitted (energy released)
1.41 × 1010 y 5.75 y 6.25 h 1.91 y 3.63 d 55.6 s 0.15 s 10.6 h 60.5 min 299 ns 3.05 min Stable
α(4.01, 3.95 MeV) β− β− α (5.42, 5.34 MeV) α (5.69, 5.45 MeV) α (6.29 MeV) α (6.78 MeV) β− β−, α (6.050 MeV) α (8.78 MeV) β−
The historical name of the nuclide is added in brackets. The energy released is specific for alpha radiation only.
Because of its short half-life, thoron in contrast to radon can only migrate a short distance from its source before it decays. The mean thoron diffusion length in air under the conditions of stagnant indoor air is about 3–3.5 cm (for radon it is about 2.4 m) and 95% of the thoron can be found within about 7 cm from its source.6–8 From the viewpoint of radiation protection, thoron exposure occurs within a close vicinity to the exhalation surface in the presence of stagnant indoor air conditions. However, measurements in a well-ventilated Japanese house determined the thoron diffusion length was about 20 cm and consequently the thoron concentration at a distance of 1 m from the wall in such a dwelling would be less than 1% of its value at the wall surface.9 This means that the concentration of thoron in a room is inhomogeneous in contrast to that of its daughter 212 Pb which has a homogenous distribution due to its long half-life (10.6 h). It should be noted that 212 Pb is the main contributor to effective dose in the thoron decay chain.10 This is in contrast to dose estimation for indoor radon exposure where the concentration of both the gas and its progeny are much more homogenously distributed. In the 1950s, monitoring programs were started to control worker exposures to radon progeny in uranium mines.11 In 1956 some of the first results about determination of the contents of radon and Thorium B (ThB) were published.12 From the 1970s, increasing numbers of elevated radon level measurements in dwellings were seen in some countries. In 1986 WHO recognized radon as one cause of lung cancer and identified it as a human lung carcinogen.13, 14 At that time, the main source of information on risks of radoninduced lung cancer was epidemiological studies of underground miners.15 Epidemiological studies of radon-exposed miners provided a strong relationship for lung cancer caused by inhaled radon progeny.16 However, coal, potash, and iron miners with low exposure to radon and its progeny have had few or no excess cases of lung cancer.17
For the general public, 13 recent European case-control studies in nine countries showed appreciable hazards from residential radon, particularly for smokers and recent exsmokers and it was further indicated that radon was responsible for about 2% of all deaths from cancer in Europe.18 At present, according to the UNSCEAR 2008 report, the contribution of radon, thoron and their decay products represents the largest fraction of the world mean annual effective dose to the general population from natural radioactivity.19 C. Actions related with the radon and thoron problems
The WHO has collected information on guidelines, programmes and activities about surveys on indoor radon among WHO member states. Its final report provides information on the mean radon levels in different countries as well as on radon action and reference levels. The radon concentration action level values cover a wide range, but the most frequently used are between 100 and 400 Bq m−3 . Many countries have opted for lower levels for new buildings as compared to slightly higher levels for existing buildings.20 In addition, WHO has published a handbook on indoor radon – a public health perspective – to provide a current overview of the major aspects of radon and health with useful information about comprehensive planning, implementation, and evaluation of national radon programmes.21 In many countries, radon levels have not been sufficiently investigated nor are they regulated by any government agency (that is to say, no action level for radon exists). In the past, radon studies were more common than those of thoron, but recent studies in Serbian,22–25 Chinese,26–32 Indian,33–35 Canadian,36–41 Japanese,42–44 Korean,45, 46 Hungarian,47–49 Slovenian,50, 51 Macedonian,52 Romanian,53 Kosovian,54 Pakistanian,55 Irish,56 and USA57–59 dwellings
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have shown that thoron can be a significant contributor to radiation exposure. Therefore, corresponding to this, intercomparisons of radon and thoron measurement methods play an important role not only for domestic radon and thoron surveys but also for international or interregional discussions about radon and thoron mapping in dwellings as well as in the soil. For these purposes, it is necessary to improve and standardize technical methods of measurements and to verify quality assurance by intercomparisons between laboratories. These needs provided the motivation for the intercomparison presented in this paper.
II. METHODOLOGY A. Traceability of radon and thoron measurements
At present, many types of monitors (devices) are currently used for radon and thoron measurements; some of them which were used in the intercomparisons are described in a later section. Radon concentrations estimated with different monitors are not consistent with each other when they are placed in the same environment. Thus, readings by different monitors should be adjusted to a “standard” by changing their calibration factors. “Traceability” of a detector is assured by a chain of calibrations connected with the standard. The traceability can be defined as “measurement results which are traceable to a domestic or international standard by a chain of calibrations.” Radon is a noble gas which emits alpha particles and that makes absolute activity measurements difficult. On the other hand, the traceability of activity for the parent nuclide of radon, 226 Ra, is usually established. Then, radon emanated from traceable radium has been used to assure the traceability of radon activity.60 Calibration of radon monitors using radium with traceable activity needs elaborate experimental devices, so that the number of facilities which can conduct such calibration is limited. In practice, calibration of radon monitors is conducted at a calibration facility by comparing the reading of the monitors to be calibrated with that of a standard monitor (device) which was calibrated using radium with traceable activity. After the comparison, the calibrated monitors can be used as a secondary standard. The traceability of another monitor can be assured by comparing it with the secondary standard monitor (chains of calibration). PTB (Physikalisch-Technische Bundesanstalt) in Germany has been recognized as an international standard facility for radon because it is able to do “absolute activity measurement” of radon by a special technique.61, 62 PTB provides two types of calibration services (primary and secondary). For the primary calibration, a monitor to be calibrated is put into a 21 m3 radon chamber. Then, the radon chamber is filled with radon which was certified with the “absolute measurement.” One of the passive-continuous-longterm radon monitors (AlphaGUARD) of NIRS was certified by the primary calibration at PTB (for three different radon concentrations) and it has since been used as a standard device at NIRS.63 By comparing it with a new monitor to be calibrated at the NIRS radon chamber, the traceability of the new monitor is established by the calibration chain with PTB.
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On the other hand, in 1983 the Nuclear Energy Agency of the Organization for Economic Cooperation and Development (OECD) and the Radiation Protection Research Programme of the Commission of European Communities (CED) commenced the joint project of the International Intercalibration and Intercomparison Programme for Radon, Thoron, and Daughters Measuring Equipment (IIIP). In Part I, four laboratories participated to represent different regional groups of OECD and CEC countries, i.e.: (1) ARL – Australian Radiation Laboratory, Australia; (2) EML – Environmental Measurement Laboratory, USA; (3) NRPB – National Radiological Protection Board (from 2005 it became known as the Radiation Protection Division of the Health Protection Agency), UK; and (4) USBM – US Bureau of Mines Denver Research Laboratory, USA. Under this project three experiments were held in 1982, 1984, and 1987 and the main goal was to check the performance of their equipment and to identity differences among the laboratories in the results of radon measurement.64, 65 One of the most important and well-known intercomparisons of integrated passive monitors has been provided by the HPA (Radiation Protection Division of the HPA), UK (formerly the NRPB). The NRPB completed limited exercises non-periodically from 1989 to 1995 but between 1997 and 2007 intercomparisons were carried out every year. In 2011 HPA restarted radon exercises on an annual basis. HPA maintains a 43 m3 walk-in radon chamber and the radon concentration can be varied from around 200 Bq m−3 to 8000 Bq m−3 with the stable condition. ATMOS 12 and AlphaGUARD ionization chambers are utilized as standard devices for monitoring radon concentration in the chamber. These devices have been periodically calibrated at PTB.66 EML is the reference calibration laboratory in North America that provides support to participants from the USA, Canada, etc. Its 30 m3 radon chamber is constructed of stainless steel and provides for a well-controlled, clean, airtight, and uniform test environment. Research with inert aerosols and other pollutants can also be carried out. Radon is generated from a Pylon (Model RN-1025, Ottawa, Ontario, Canada) 226 Ra source with 3810 kBq of radium. The flowthrough scintillation cell monitor that was calibrated against EML’s pulse ionization chamber is used as the reference instrument. The total uncertainty in the ELM radon value is less than 5%. The testing of devices for radon progeny has also been carried out in which the aerosols were generated from Carnauba wax. Results of this exercise showed that monitors for passive or active radon measurements performed very well, indicating proper calibration and continuous maintenance by both the manufacturer and the user.67 Although the traceability of radon has been established, that of thoron is not well established and this is due to the difficulty of its activity certification. Then, for thoron, “intercomparison” is a possible tool for quality assurance. At present, many intercomparison exercises have been conducted for radon and examples were described earlier; those for thoron, however, are scarce. For instance, four methods (the two-filter tube, the ionization chamber, the radon, and thoron discriminative monitor (silicon semiconductor detector), and the etch-track detector) for measuring thoron
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gas were compared using the EML environmental chamber. Thoron, containing 228 Th, was generated from the Pylon source located inside the chamber. It was determined that the nuclear track monitor results could be compared with those of the other three devices.68 In 2004 the first Spanish intercomparison exercise for continuous (active) radon and thoron monitors (ionization chamber, scintillation, and a semiconductor coupled with an electrostatic collection system) was carried out for nine monitoring systems from eight laboratories. The exercise utilized the 20 m3 radon (Spanish reference) chamber as well as the 200 dm3 thoron chamber located at the Institute of Energy Techniques (INTE) of the Technical University of Catalonia (UPC). The main goal of this exercise was checking the influence of a change in environmental conditions on the measurement systems. The responses of monitors showed, in some cases, influences from climatic conditions and the presence of thoron on the results.69 The same institution provided intercomparisons for integrated radon and thoron detectors (SSNTDs, electret plates, and activated carbon) against different climatic conditions as well as radon and thoron concentrations. Analysis of results showed some cases had differences and significant deviations from the true (standard) values.70
B. Intercomparisons for radon and thoron at NIRS
The first intercomparison exercise of measurement techniques for radon, radon decay products and their particle size distributions was carried out with the NIRS radon chamber in 2002. In this exercise, devices were categorized into three types: continuous, grab sampling and integrated. In general, the results for radon concentration by continuous monitors were comparable but those by integrated monitors were distributed in a wide range.71 Since 2007 the NIRS has conducted four international intercomparisons and the main aim of these exercises has been to compare the results of passive type monitors.72, 73 The numbers of laboratories and countries that participated in all intercomparisons are plotted in Figure 1. In the radon case numbers of labs and countries systematically increased from I to IV exercises. In the thoron part, the number of labs increased although the number of countries was almost stable (5–6). In addition, the previous results (I-III Intercomparison) were compared only by percentage difference between results given by each laboratory and average concentration in the chamber registered by the reference instrument. From the IV intercomparison onwards, the new categorization and the new parameter for evaluation of results where uncertainty is taken into consideration were implemented in accordance with the ISO Standard: “Conformity assessment—General requirements for proficiency testing.”74
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rate standard of measurement and quality. Monitors consist of at least two parts: a detector (CR-39, LR 115, polycarbonate, and electret plate) or collector (e.g., active charcoal) and a diffusion chamber or other parts where the detector or collector is mounted. Many difficulties, especially in thoron calibration and measurements, have been found (e.g., influence of humidity, wind speed, type of infiltration barrier) because of its relatively short half-life (55 s).75 In addition, new institutions continually begin to undertake measurements of radon, thoron, and their progenies and new techniques are still being developed. Therefore, it is important to maintain the reliability of radon and thoron measurements among other things through comparisons with other laboratories. At this time, many countries have begun large-scale radon and thoron surveys as well as participating in international intercomparisons of measuring devices. Many different measurement methods and instruments are used in these surveys. The instruments can be divided into two main sampling method categories: active or passive (Figure 2). Most of the measurement techniques are based on the detection and recording of alpha particles emitted in the uranium decay chain from 222 Rn, 218 Po, 214 Po as well as in the thorium chain from 220 Rn, 216 Po, 212 Bi, and 212 Po.76, 77 It should be mentioned that some measurement methods utilizing activated carbon are analysed with HPGe or NaI spectrometers by the 609 keV or 1764 keV γ -rays from 214 Bi.78 Routine radon concentration measurements should be done with a minimum financial investment and their results should be available within a short time. However, the extrapolation of single short-term measurements to long-term mean values is difficult due to variable parameters and diurnal as well as seasonal fluctuations.79 The parameters include such items as pressure gradient, outdoor–indoor temperature differences, wind speed and direction, and other environmental parameters as well as occupant behaviour and room ventilation.80, 81 The suggested minimum time for screening measurement to achieve a discrepancy from the long-term value on the level of
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 022001 (2014)
Invited Article: Radon and thoron intercomparison experiments for integrated monitors at NIRS, Japan M. Janik,a) T. Ishikawa, Y. Omori, and N. Kavasi National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, 263-8555 Chiba, Japan
(Received 18 September 2012; accepted 11 November 2013; published online 19 February 2014) Inhalation of radon (222 Rn) and its short-lived decay products and of products of the thoron (220 Rn) series accounts for more than half of the effective dose from natural radiation sources. At this time, many countries have begun large-scale radon and thoron surveys and many different measurement methods and instruments are used in these studies. Consequently, it is necessary to improve and standardize technical methods of measurements and to verify quality assurance by intercomparisons between laboratories. Four international intercomparisons for passive integrating radon and thoron monitors were conducted at the NIRS (National Institute of Radiological Sciences, Japan). Radon exercises were carried out in the 24.4 m3 inner volume walk-in radon chamber that has systems to control radon concentration, temperature, and humidity. Moreover, the NIRS thoron chamber with a 150 dm3 inner volume was utilized to provide three thoron intercomparisons. At present, the NIRS is the only laboratory world-wide that has carried out periodic thoron intercomparison of passive monitors. Fifty laboratories from 26 countries participated in the radon intercomparison, using six types of detectors (charcoal, CR-39, LR 115, polycarbonate film, electret plate, and silicon photodiode). Eighteen laboratories from 12 countries participated in the thoron intercomparisons, using two etch-track types (CR-39 and polycarbonate) detectors. The tests were made under one to three different exposures to radon and thoron. The data presented in this paper indicated that the performance quality of laboratories for radon measurement has been gradually increasing. Results of thoron exercises showed that the quality for thoron measurements still needs further development and additional studies are needed to improve its measuring methods. The present paper provides a summary of all radon and thoron international intercomparisons done at NIRS from 2007 to date and it describes the present status on radon and thoron passive, one-time cycle monitors. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865159] I. INTRODUCTION A. Characteristics of radon and thoron
The natural radioactive gas radon (222 Rn) is a member of the uranium decay series; this means that when uranium decays, after a series of steps, one of its daughter products is radon. One of the radon isotopes is thoron (220 Rn), which is a natural decay product of the most stable thorium isotope (232 Th). Radon was one of the earliest discovered radioactive elements, being identified after uranium, thorium, polonium, and radium. It was detected in 1900 by Dorn, a German chemist, during experiments in which he noticed that radium compounds emanate a radioactive gas.1 In 1908 Ramsay and Gray isolated the gas and named it niton.2 The term thoron (correctly thoron emanation) appeared in the literature for the first time in 1899 by reason of Ernest Rutherford’s experiments that investigated radioactive properties of thorium samples.3 In 1923, the International Committee for Chemical Elements and International Union of Pure and Applied Chemistry (IUPAC) chose the names radon (Rn) and thoron (Tn). The characteristics of parents and daughters of radon (222 Rn) and thoron (220 Rn) are shown in Table I. Radon, its a) Corresponding author email: [email protected]
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parent radium (226 Ra) and its decay products are members of the uranium (238 U) decay chain whereas thoron, its parent radium (224 Ra) and its daughters are members of the thorium (232 Th) decay chain. Significant differences are seen in the half lives of radon and thoron, and many of the behavioural differences in the members of these two chains are visible in indoor air. These differences have had implications for the development of measurement methods for radon, thoron, and their progenies as well as for dosimetric models. B. Radon and thoron problems in radiation protection
Lung cancer occurrence was recognized as a “radon problem” in the late nineteenth century among workers in German mines and was described by Haerting and Hesse.4 In 1924, Ludewig and Lorenser5 suggested that these lung cancer cases could be attributed to radon exposure. In the next 30 years it was recognized that the true cause for high lung dose was not radon itself but the inhalation of its progenies, e.g. Radium A, Radium B, Radium C, and Radium C . It is known that these solid particles are mostly absorbed on the surface of ambient aerosols and during inhalation these radioactive aerosols are accumulated in the lung by filtration. Therefore, the lung is the most exposed organ for radon and thoron in the body.
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TABLE I. Characteristics of uranium (radium) and thorium decay chains, including radon (222 Rn), thoron (220 Rn), their parents and decay products (daughters). Uranium chain Nuclide (historical 238 U 234Th 234m Pa 234 Pa 234 U 230Th 226 Ra 222 Rn
(Radon) (Radium A) 214 Pb (Radium B) 214 Bi (Radium C) 214 Po (Radium C ) 210 Tl (Radium C ) 210 Pb (Radium D) 210 Bi (Radium E) 210 Po (Radium F) 206 Pb (Radium G) 218 Po
a b
name)a
Half-life
Thorium chain
Radiation emitted (energy
4.49 × 109 y 24.10 d 1.16 min 6.70 h 2.46 × 105 y 7.59 × 104 y 1602 y 3.82 d 3.10 min 26.8 min 19.9 min 0.16 ms 1.30 min 22.3 y 5.01 d 138 d Stable
released)b
α (4.20, 4.15 MeV) ββ-, IT βα (4.77, 4.72 MeV) α (4.69, 4.62 MeV) α (4.78, 4.60 MeV) α (5.49 MeV) α (6.00 MeV), βββα (7.69 MeV) βββα (5.30 MeV) ...
Nuclide (historical name)
232 Th 228 Ra 228 Ac 228 Th 224 Ra 220 Rn
(Thoron) (Thorium A) 212 Pb (Thorium B) 212 Bi (Thorium C) 212 Po (Thorium C ) 208 Tl (Thorium C ) 208 Pb (Thorium D) 216 Po
Half-life
Radiation emitted (energy released)
1.41 × 1010 y 5.75 y 6.25 h 1.91 y 3.63 d 55.6 s 0.15 s 10.6 h 60.5 min 299 ns 3.05 min Stable
α(4.01, 3.95 MeV) β− β− α (5.42, 5.34 MeV) α (5.69, 5.45 MeV) α (6.29 MeV) α (6.78 MeV) β− β−, α (6.050 MeV) α (8.78 MeV) β−
The historical name of the nuclide is added in brackets. The energy released is specific for alpha radiation only.
Because of its short half-life, thoron in contrast to radon can only migrate a short distance from its source before it decays. The mean thoron diffusion length in air under the conditions of stagnant indoor air is about 3–3.5 cm (for radon it is about 2.4 m) and 95% of the thoron can be found within about 7 cm from its source.6–8 From the viewpoint of radiation protection, thoron exposure occurs within a close vicinity to the exhalation surface in the presence of stagnant indoor air conditions. However, measurements in a well-ventilated Japanese house determined the thoron diffusion length was about 20 cm and consequently the thoron concentration at a distance of 1 m from the wall in such a dwelling would be less than 1% of its value at the wall surface.9 This means that the concentration of thoron in a room is inhomogeneous in contrast to that of its daughter 212 Pb which has a homogenous distribution due to its long half-life (10.6 h). It should be noted that 212 Pb is the main contributor to effective dose in the thoron decay chain.10 This is in contrast to dose estimation for indoor radon exposure where the concentration of both the gas and its progeny are much more homogenously distributed. In the 1950s, monitoring programs were started to control worker exposures to radon progeny in uranium mines.11 In 1956 some of the first results about determination of the contents of radon and Thorium B (ThB) were published.12 From the 1970s, increasing numbers of elevated radon level measurements in dwellings were seen in some countries. In 1986 WHO recognized radon as one cause of lung cancer and identified it as a human lung carcinogen.13, 14 At that time, the main source of information on risks of radoninduced lung cancer was epidemiological studies of underground miners.15 Epidemiological studies of radon-exposed miners provided a strong relationship for lung cancer caused by inhaled radon progeny.16 However, coal, potash, and iron miners with low exposure to radon and its progeny have had few or no excess cases of lung cancer.17
For the general public, 13 recent European case-control studies in nine countries showed appreciable hazards from residential radon, particularly for smokers and recent exsmokers and it was further indicated that radon was responsible for about 2% of all deaths from cancer in Europe.18 At present, according to the UNSCEAR 2008 report, the contribution of radon, thoron and their decay products represents the largest fraction of the world mean annual effective dose to the general population from natural radioactivity.19 C. Actions related with the radon and thoron problems
The WHO has collected information on guidelines, programmes and activities about surveys on indoor radon among WHO member states. Its final report provides information on the mean radon levels in different countries as well as on radon action and reference levels. The radon concentration action level values cover a wide range, but the most frequently used are between 100 and 400 Bq m−3 . Many countries have opted for lower levels for new buildings as compared to slightly higher levels for existing buildings.20 In addition, WHO has published a handbook on indoor radon – a public health perspective – to provide a current overview of the major aspects of radon and health with useful information about comprehensive planning, implementation, and evaluation of national radon programmes.21 In many countries, radon levels have not been sufficiently investigated nor are they regulated by any government agency (that is to say, no action level for radon exists). In the past, radon studies were more common than those of thoron, but recent studies in Serbian,22–25 Chinese,26–32 Indian,33–35 Canadian,36–41 Japanese,42–44 Korean,45, 46 Hungarian,47–49 Slovenian,50, 51 Macedonian,52 Romanian,53 Kosovian,54 Pakistanian,55 Irish,56 and USA57–59 dwellings
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have shown that thoron can be a significant contributor to radiation exposure. Therefore, corresponding to this, intercomparisons of radon and thoron measurement methods play an important role not only for domestic radon and thoron surveys but also for international or interregional discussions about radon and thoron mapping in dwellings as well as in the soil. For these purposes, it is necessary to improve and standardize technical methods of measurements and to verify quality assurance by intercomparisons between laboratories. These needs provided the motivation for the intercomparison presented in this paper.
II. METHODOLOGY A. Traceability of radon and thoron measurements
At present, many types of monitors (devices) are currently used for radon and thoron measurements; some of them which were used in the intercomparisons are described in a later section. Radon concentrations estimated with different monitors are not consistent with each other when they are placed in the same environment. Thus, readings by different monitors should be adjusted to a “standard” by changing their calibration factors. “Traceability” of a detector is assured by a chain of calibrations connected with the standard. The traceability can be defined as “measurement results which are traceable to a domestic or international standard by a chain of calibrations.” Radon is a noble gas which emits alpha particles and that makes absolute activity measurements difficult. On the other hand, the traceability of activity for the parent nuclide of radon, 226 Ra, is usually established. Then, radon emanated from traceable radium has been used to assure the traceability of radon activity.60 Calibration of radon monitors using radium with traceable activity needs elaborate experimental devices, so that the number of facilities which can conduct such calibration is limited. In practice, calibration of radon monitors is conducted at a calibration facility by comparing the reading of the monitors to be calibrated with that of a standard monitor (device) which was calibrated using radium with traceable activity. After the comparison, the calibrated monitors can be used as a secondary standard. The traceability of another monitor can be assured by comparing it with the secondary standard monitor (chains of calibration). PTB (Physikalisch-Technische Bundesanstalt) in Germany has been recognized as an international standard facility for radon because it is able to do “absolute activity measurement” of radon by a special technique.61, 62 PTB provides two types of calibration services (primary and secondary). For the primary calibration, a monitor to be calibrated is put into a 21 m3 radon chamber. Then, the radon chamber is filled with radon which was certified with the “absolute measurement.” One of the passive-continuous-longterm radon monitors (AlphaGUARD) of NIRS was certified by the primary calibration at PTB (for three different radon concentrations) and it has since been used as a standard device at NIRS.63 By comparing it with a new monitor to be calibrated at the NIRS radon chamber, the traceability of the new monitor is established by the calibration chain with PTB.
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On the other hand, in 1983 the Nuclear Energy Agency of the Organization for Economic Cooperation and Development (OECD) and the Radiation Protection Research Programme of the Commission of European Communities (CED) commenced the joint project of the International Intercalibration and Intercomparison Programme for Radon, Thoron, and Daughters Measuring Equipment (IIIP). In Part I, four laboratories participated to represent different regional groups of OECD and CEC countries, i.e.: (1) ARL – Australian Radiation Laboratory, Australia; (2) EML – Environmental Measurement Laboratory, USA; (3) NRPB – National Radiological Protection Board (from 2005 it became known as the Radiation Protection Division of the Health Protection Agency), UK; and (4) USBM – US Bureau of Mines Denver Research Laboratory, USA. Under this project three experiments were held in 1982, 1984, and 1987 and the main goal was to check the performance of their equipment and to identity differences among the laboratories in the results of radon measurement.64, 65 One of the most important and well-known intercomparisons of integrated passive monitors has been provided by the HPA (Radiation Protection Division of the HPA), UK (formerly the NRPB). The NRPB completed limited exercises non-periodically from 1989 to 1995 but between 1997 and 2007 intercomparisons were carried out every year. In 2011 HPA restarted radon exercises on an annual basis. HPA maintains a 43 m3 walk-in radon chamber and the radon concentration can be varied from around 200 Bq m−3 to 8000 Bq m−3 with the stable condition. ATMOS 12 and AlphaGUARD ionization chambers are utilized as standard devices for monitoring radon concentration in the chamber. These devices have been periodically calibrated at PTB.66 EML is the reference calibration laboratory in North America that provides support to participants from the USA, Canada, etc. Its 30 m3 radon chamber is constructed of stainless steel and provides for a well-controlled, clean, airtight, and uniform test environment. Research with inert aerosols and other pollutants can also be carried out. Radon is generated from a Pylon (Model RN-1025, Ottawa, Ontario, Canada) 226 Ra source with 3810 kBq of radium. The flowthrough scintillation cell monitor that was calibrated against EML’s pulse ionization chamber is used as the reference instrument. The total uncertainty in the ELM radon value is less than 5%. The testing of devices for radon progeny has also been carried out in which the aerosols were generated from Carnauba wax. Results of this exercise showed that monitors for passive or active radon measurements performed very well, indicating proper calibration and continuous maintenance by both the manufacturer and the user.67 Although the traceability of radon has been established, that of thoron is not well established and this is due to the difficulty of its activity certification. Then, for thoron, “intercomparison” is a possible tool for quality assurance. At present, many intercomparison exercises have been conducted for radon and examples were described earlier; those for thoron, however, are scarce. For instance, four methods (the two-filter tube, the ionization chamber, the radon, and thoron discriminative monitor (silicon semiconductor detector), and the etch-track detector) for measuring thoron
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gas were compared using the EML environmental chamber. Thoron, containing 228 Th, was generated from the Pylon source located inside the chamber. It was determined that the nuclear track monitor results could be compared with those of the other three devices.68 In 2004 the first Spanish intercomparison exercise for continuous (active) radon and thoron monitors (ionization chamber, scintillation, and a semiconductor coupled with an electrostatic collection system) was carried out for nine monitoring systems from eight laboratories. The exercise utilized the 20 m3 radon (Spanish reference) chamber as well as the 200 dm3 thoron chamber located at the Institute of Energy Techniques (INTE) of the Technical University of Catalonia (UPC). The main goal of this exercise was checking the influence of a change in environmental conditions on the measurement systems. The responses of monitors showed, in some cases, influences from climatic conditions and the presence of thoron on the results.69 The same institution provided intercomparisons for integrated radon and thoron detectors (SSNTDs, electret plates, and activated carbon) against different climatic conditions as well as radon and thoron concentrations. Analysis of results showed some cases had differences and significant deviations from the true (standard) values.70
B. Intercomparisons for radon and thoron at NIRS
The first intercomparison exercise of measurement techniques for radon, radon decay products and their particle size distributions was carried out with the NIRS radon chamber in 2002. In this exercise, devices were categorized into three types: continuous, grab sampling and integrated. In general, the results for radon concentration by continuous monitors were comparable but those by integrated monitors were distributed in a wide range.71 Since 2007 the NIRS has conducted four international intercomparisons and the main aim of these exercises has been to compare the results of passive type monitors.72, 73 The numbers of laboratories and countries that participated in all intercomparisons are plotted in Figure 1. In the radon case numbers of labs and countries systematically increased from I to IV exercises. In the thoron part, the number of labs increased although the number of countries was almost stable (5–6). In addition, the previous results (I-III Intercomparison) were compared only by percentage difference between results given by each laboratory and average concentration in the chamber registered by the reference instrument. From the IV intercomparison onwards, the new categorization and the new parameter for evaluation of results where uncertainty is taken into consideration were implemented in accordance with the ISO Standard: “Conformity assessment—General requirements for proficiency testing.”74
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rate standard of measurement and quality. Monitors consist of at least two parts: a detector (CR-39, LR 115, polycarbonate, and electret plate) or collector (e.g., active charcoal) and a diffusion chamber or other parts where the detector or collector is mounted. Many difficulties, especially in thoron calibration and measurements, have been found (e.g., influence of humidity, wind speed, type of infiltration barrier) because of its relatively short half-life (55 s).75 In addition, new institutions continually begin to undertake measurements of radon, thoron, and their progenies and new techniques are still being developed. Therefore, it is important to maintain the reliability of radon and thoron measurements among other things through comparisons with other laboratories. At this time, many countries have begun large-scale radon and thoron surveys as well as participating in international intercomparisons of measuring devices. Many different measurement methods and instruments are used in these surveys. The instruments can be divided into two main sampling method categories: active or passive (Figure 2). Most of the measurement techniques are based on the detection and recording of alpha particles emitted in the uranium decay chain from 222 Rn, 218 Po, 214 Po as well as in the thorium chain from 220 Rn, 216 Po, 212 Bi, and 212 Po.76, 77 It should be mentioned that some measurement methods utilizing activated carbon are analysed with HPGe or NaI spectrometers by the 609 keV or 1764 keV γ -rays from 214 Bi.78 Routine radon concentration measurements should be done with a minimum financial investment and their results should be available within a short time. However, the extrapolation of single short-term measurements to long-term mean values is difficult due to variable parameters and diurnal as well as seasonal fluctuations.79 The parameters include such items as pressure gradient, outdoor–indoor temperature differences, wind speed and direction, and other environmental parameters as well as occupant behaviour and room ventilation.80, 81 The suggested minimum time for screening measurement to achieve a discrepancy from the long-term value on the level of
