Radiation Protection Dosimetry (2015), Vol. 164, No. 4, pp. 595 –600 Advance Access publication 13 May 2015
doi:10.1093/rpd/ncv318
THE MOST RECENT INTERNATIONAL INTERCOMPARISONS OF RADON AND THORON MONITORS WITH THE NIRS RADON AND THORON CHAMBERS M. Janik* and H. Yonehara Research Center for Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan *Corresponding author: [email protected]
INTRODUCTION 222
Public exposure to Rn (radon) and its progeny has been a worldwide concern in recent decades. A new European Union Basic Safety Standard was issued as a Council Directive(1) that includes among other things the exposure of members of the public to indoor radon. Therefore, all European countries will be obliged to carry out radon surveys. Consequently, the current and developed techniques and methods should be tested and controlled. In this context, intercomparison studies play a key role in the quality control and quality assurance of radon measurement. Besides the concern with radon, the importance of 220 Rn (thoron) has recently been recognised. Thoron is present everywhere together with radon, and its quantity is sometimes much larger than that of radon at a certain position in dwellings, especially close to walls. On the other hand, developing an accurate measurement procedure for 220Rn would also be beneficial for the accurate measurement of 222Rn, because such measurements can be overestimated due to interference from 220Rn as shown in recent surveys, for example in China(2). The data presented in this article indicated that radon as well as thoron measurement techniques still need improvements in order to solve technical problems in their metrology. MATERIALS AND METHODS Participants Twenty four laboratories from 19 countries participated in the radon intercomparison, using four types
of monitors (passive type: etched track, electret plate and silicon photodiode as well as one active type: ionisation chamber). Twelve laboratories from 11 countries participated in the thoron intercomparison, using two types of detectors (CR-39 and photodiode). A list of participants in the radon and the thoron intercomparisons is given in Table 1. Radon facility The 22.4-m3 inner-volume radon chamber was utilised for radon exposures(3). The radon chamber was run under open-loop mode and stable radon concentration, i.e. radon was injected at a stable (0.5 l min21) flow rate and removed to the atmosphere through active carbon filter. During exposures, the radon level was monitored by an AlphaGUARD, which has been periodically calibrated in the primary atmosphere at the PhysikalischTechnische Bundesanstalt (PTB), Germany(4). Thoron facility A 150-l stainless steel chamber system at National Institute of Radiological Sciences (NIRS) was used to expose monitors to thoron(5). The thoron gas generated in the source, consisting of a fabric wick for lantern mantle with 232Th, was supplied to the chamber in the closed-loop mode with a flow rate of 2 l min21. The thoron system was run in the open-loop mode because externally mixed dry and humidified air obtained by bubbling air through de-ionised water was injected. In the normal mode (one reference thoron monitor), the flow rate of the mixed air was set to 2 l min21. The 220 Rn concentrations in the chamber were measured
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
The fifth international intercomparison for radon and fourth for thoron monitors were conducted at National Institute of Radiological Sciences (Japan) with the radon and thoron chambers. The tests were made under two different exposures to radon and two exposures (in two rounds due to limited space in the thoron chamber) to thoron. In these most recent intercomparisons, two new graphical methods recommended by the ISO standard, Mandel’s h statistic and the Youden plot, were implemented to evaluate the consistency between laboratories and within laboratories. The presented data indicated that the performance quality of laboratories for radon measurement as expressed by the percentage difference parameter has been stable since the first international intercomparison for passive monitors carried out in 2007, and it amounted to around 50 for 10 % of the difference from the reference value. The thoron exercise showed that further development and additional studies to improve its measuring methods and reliability are needed.
M. JANIK AND H. YONEHARA Table 1. List of participants. Institute/company
Det. type
IT RO BR PL TH PL NO BE
X X X X X X X X
CR-39 X CR-39 CR-39 CR-39 X CR-39 CR-39 X S.P.a CR-39
CA GE
X X
X CR-39, ELb X CR-39
IR VI PL JP
X X X X
X CR-39 X CR-39 CR-39 I.C.c
SW IT PL
X X X
X CR-39 CR-39 CR-39
CH JP CZ TH CR SP HU JP IT
X X X X X X X X X
X CR-39 X CR-39 LR 115 X CR-39 LR 115 CR-39 X CR-39 I.C.c CR-39
RESULTS
a
Silicon photodiode. Electret. c Ionisation chamber. b
continuously every 30 min using a RAD7 device. For the quality assurance of the continuously measured 220 Rn concentrations, intermittent measurements were also acquired during the exposure test by means of a grab sampling technique using a scintillation cell(6, 7). The RAD7 device was calibrated in the reference thoron atmosphere at the PTB(8) and in the NIRS radon chamber. DATA EVALUATION To standardise the results obtained for all the exposure conditions, the parameter percentage difference (PD) recommended by the Conformity Assessment—General Requirements for Proficiency Testing Standard(9) and the National Association for Proficiency Testing was used. In order to allow comparisons between Public Health England (PHE, UK) and NIRS, the PHE classification parameter was applied as given by the measurement error (MES)(10). Details of these parameters have been described elsewhere(11).
The integrated monitors were all shipped from the participating laboratories to NIRS (and also back) in radon-proof plastic bags or special cases. In addition, the monitors before and after exposures were separately stored in special radon-proof plastic bags outside the NIRS radon chamber. From 4 to 10 monitors per participating laboratory were exposed to radon or thoron and were placed in the chambers. One or two monitors per participating laboratory were exposed outside the radon chamber as a blank (control and transport checking). After exposure, the monitors were sent back to their respective laboratories for analysis. The results of each exposure were then reported to the NIRS for evaluation. After all the reports were received, reference values for radon and thoron and other relevant parameters, presented in Table 2, were sent to the participating laboratories. The uncertainties for radon and thoron concentrations as well as expositions were stated as the expanded uncertainties with the coverage factor k ¼ 2. Results by the parameters PD and MES were categorised into three ranks: Cat-A if jPDj or MES 10 %, Cat-B if 10 % , jPDj or MES , 20 % and Cat-C if jPDj or MES 20 %. The values of 10 and 20 % were based on parameters of the normal distribution of PD from all previous intercomparisons. Referring to results plotted in Figures 1–6 and listed in Table 3, the following conclusions about passive monitors can be drawn:
596
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
ARPA Lazio Babes-Bolyai University CDTN Central Mining Institute Chulalongkorn University CLOR Corentium AS Hainaut Vigilance Sanitaire Health Canada Helmholtz Zentrum Munchen INRA INST IFJ PAN Kobe Pharmaceutical Univ. Landauer Nordic AB MCF Ambiente Srl Medical Univ. of Bialystok NIRP NIRS SUJCHBO Kamenna TINT University in Osijek University of Cantabria University of Pannonia University of Tokyo U-Series
Country Rn Tn
The current intercomparison results were extended in a graphical way to allow evaluation of laboratories. The ISO recommends Mandel’s h statistic and the Youden plot for this purpose. Mandel’s h statistic is a between-laboratory consistency statistic that measures for a selected level the standardised deviation of the mean value obtained by a laboratory from the mean for that level. It is therefore a measure of the laboratory bias. On Mandel’s h plot, the critical level (H0 ¼ 95 %) is indicated. When the critical level is exceeded, the H-null hypothesis ‘no difference between the mean values’ is rejected. In a split-level experiment, useful information can also be obtained from the Youden plot, which consists of plotting the results of two samples against each other. The plot visualises within-laboratory variability as well as between-laboratory variability. The interpretation of the Youden plot is as follows: (1) points that lie near the 45-degree reference line, far from the centre but inside the circle, indicate large systematic error; (2) points that lie far from the 45-degree line but inside the circle indicate large random error and (3) points outside the circle indicate large total (systematic þ random) error.
Table 2. Reference values and environmental conditions for radon and thoron exposures. Stop
Exposure time, h
Rn conc., kBq m23
Exposure, kBq m23 h
Temperature, 8C
Relative hum., %
Absolute hum., g m23
Rn I II
20/09/2013 15:00 26/09/2013 15:00
24/09/2013 15:00 30/09/2013 15:00
96.00 96.00
1.13+ 0.34 7.08+0.41
109+65 680+79
21.8+0.1 21.8+0.0
57.0+0.2 56.9+0.2
10.9+0.1 10.9+0.1
Tn IA IB IIA IIB
02/10/2013 15:00 03/10/2013 17:30 09/10/2013 14:00 10/10/2013 16:30
03/10/2013 15:00 04/10/2013 17:30 10/10/2013 14:00 11/10/2013 16:30
24.00 24.00 24.00 24.00
4.18+0.74 4.32+0.76 14.62+1.48 15.02+1.49
100+36 104+36 351+71 361+72
30.8+0.5 31.1+0.5 29.4+0.3 30.2+0.4
16.0+1.1 12.9+0.6 31.9+2.0 31.6+0.6
5.1+0.3 4.2+0.2 9.3+0.8 9.7+0.2
Figure 1. Proportions of PD results by exposure normalised to 1.
Figure 2. Proportions of MES results by exposure normalised to 1.
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
- Rn-I exposure: the ratio of the radon concentration given by the participants to that of NIRS expressed by the REF parameter ranged from 0.62 to 1.40 with 1.00 as the average and 0.97 as the median. As shown in Figure 1, 50 % of the results for passive monitors were in Cat-A, e.g. located in the 10 % range around the NIRS reference value. And, 10 % of the results (Figure 2) were included in the MES Cat-A (10 % range). None of them exceeded the critical value of Mandel’s h.
597
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M. JANIK AND H. YONEHARA
Figure 4. Mandel’s h statistic for Tn exposures.
598
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Figure 3. Mandel’s h statistic for Rn exposures.
599
Arithmetic mean [median] (range). a
22.8 [19.2] (7.2 –42) 19.9 [12.5] (3.8 –93) 50.4 [41.8] (9.1 –180.0) 58.4 [41.4] (6.7 –230.0) 20.04 [20.14] (22.4 to 1.8) 20.87 [20.56] (216.0 to 5.5) 20.89 [20.50] (212.0 to 4.5) 20.90 [20.59] (221.0 to 15.0) 20.27 [22.90] (238 to 38) 25.77 [25.29] (293 to 56) 2.03 [29.46] (2100 to 180) 15.70 [26.82] (299 to 220) 1.00 [0.97] (0.62–1.40)a 0.94 [0.95] (0.07–1.60) 1.02 [0.91] (,0.01– 2.80) 1.16 [0.93] (,0.01– 3.20)
MES En
Rn-I Rn-II Tn-I Tn-II
Further evaluation of results expressed by the Youden plot and Mandel’s h statistic showed that some institutions had problems with evaluation of radon data; e.g. IDs 2C, 14, 21, 23, 25A, 27, 27A, 56 and 60 were outside the 2SD limit of the random components (circle), and this indicated a large total (systematic þ random) error. On the other hand, if Mandel’s h was .1.0 for one of the exposures or the difference between values
PD
Rn-II exposure: 46 % of the PDs were lying in Cat-A, but 36 % were in the MES Cat-A. In this exposure, the range of the REF values was wider than that of Rn-I, being from 0.07 to 1.60, and the average value was lower at 0.94, and the median was 0.95. Results of two laboratories exceeded the critical Mandel’s h value.
REF
-
Exposure
Figure 6. The Youden graph for Tn exposures.
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
Figure 5. The Youden graph for Rn exposures.
Table 3. Calculated arithmetic mean, median (in brackets) and range (in parentheses) of REF, PD, En and MES values.
THE MOST RECENT INTERNATIONAL INTERCOMPARISONS
M. JANIK AND H. YONEHARA
exceeded 1.0, then the point was lying outside the circle of the Youden plot:
In order to identify the laboratory performance quality, a similar evaluation as for radon using the Youden plot was applied to thoron exposures. According to this calculation if Mandel’s h statistic was higher than 1 or the difference between calculated values for exposures Tn-I and Tn-II was higher than 1, then the laboratory was found outside the circle in the Youden plot, e.g. ID21—difference was calculated as 1.07, ID24—both Mandel’s h values exceeded 1 and ID28—the difference was 1.27. In addition, the MES values of an appropriate exposure level (comparable Rn-I and Rn-II levels by NIRS) provided by the latest report of the PHE were calculated as 20.1 and 13.4 for the average, 15.0 and 8.8 for the median and 4.2 –92.8 and 3.3–93.3 for the range, for Exposures 2 and 5, respectively(10). The NIRS average and median MES values presented in Table 3 were higher than PHE values; however, the ranges were similar. The difference could be explained by the better reference radon exposure uncertainty reported by PHE (3 %) compared with 5 % reported by NIRS and hence, the lower final calculation results of the PHE MES values. Two active monitors (IDs: 59 and 63) were exposed in the radon atmosphere only. Their PD results indicated radon reference values. Mandel’s h statistic was calculated as a very low value and together with points on the Youden plot was located close to the centre, and it illustrated good reproducibility and low measurement uncertainty. CONCLUSION Results of intercomparisons can reveal monitor calibration problems for laboratories of the participants. Periodical intercomparisons should be carried out as
ACKNOWLEDGEMENT We gratefully acknowledge all of the participants. REFERENCES 1. European Commission. Council Directive laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/ Euratom, 97/43/Euratom and 2003/122/Euratom. 2013/59/EURATOM (2014). 2. Yamada, Y. et al. Radon-thoron discriminative measurements in Gansu Province, China, and their implication for dose estimates. J. Toxicol. Environ. Health. A 69, 723– 734 (2006). 3. Tokonami, S., Ishikawa, T., Sorimachi, A., Takahashi, H. and Miyahara, N. The Japanese radon and thoron reference chambers. AIP Conf. Proc. 1034, 202–205 (2008). 4. Ro¨ttger, A., Honig, A. and Linzmaier, D. Calibration of commercial radon and thoron monitors at stable activity concentrations. Appl. Radiat. Isot. 87, 44– 47 (2014). 5. Sorimachi, A., Ishikawa, T., Janik, M. and Tokonami, S. Quality assurance and quality control for thoron measurement at NIRS. Radiat. Prot. Dosim. 141, 367– 370 (2010). 6. Eappen, K. P., Sapra, B. K. and Mayya, Y. S. A novel methodology for online measurement of thoron using Lucas scintillation cell. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 572, 922– 925 (2007). 7. Tokonami, S., Yang, M., Yonehara, H. and Yamada, Y. Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell. Rev. Sci. Instrum. 73, 69 (2002). 8. Ro¨ttger, A., Honig, A., Dersch, R., Ott, O. and Arnold, D. A primary standard for activity concentration of 220Rn (thoron) in air. Appl. Radiat. Isot. 68, 1292–1296 (2010). 9. International Organization for Standardization and International Electrotechnical Commission. Conformity Assessment—General Requirements for Proficiency Testing. ISO/IEC 17043:2010(E) (2010). 10. Public Health England. Results of the 2013 PHE Intercomparison of Passive Radon Detectors. PHECRCE-011 Report (2013). 11. Janik, M., Ishikawa, T., Omori, Y. and Kavasi, N. Radon and thoron intercomparison experiments for integrated monitors at NIRS. Japan. Rev. Sci. Instrum. 85, 022001 (2014).
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- Tn-I exposure: the average REF was calculated to be 1.02, with 0.91 as the median, and it ranged from ,0.01 to 2.80. Twenty-five per cent of the PD results but only 8 % of the MES Cat-A were in Cat-A. Only one result exceeded the critical value of Mandel’s h. - Tn-II exposure: REF average for this exposure was found to be 1.16, with the median of 0.93 and ,0.01 –3.20 as a range. In addition, 17 % of the PD and MES results were in Cat-A, and one result exceeded the critical value of Mandel’s h.
there is a need to further investigate various radon and thoron monitors being used today and to improve quality of data.
doi:10.1093/rpd/ncv318
THE MOST RECENT INTERNATIONAL INTERCOMPARISONS OF RADON AND THORON MONITORS WITH THE NIRS RADON AND THORON CHAMBERS M. Janik* and H. Yonehara Research Center for Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan *Corresponding author: [email protected]
INTRODUCTION 222
Public exposure to Rn (radon) and its progeny has been a worldwide concern in recent decades. A new European Union Basic Safety Standard was issued as a Council Directive(1) that includes among other things the exposure of members of the public to indoor radon. Therefore, all European countries will be obliged to carry out radon surveys. Consequently, the current and developed techniques and methods should be tested and controlled. In this context, intercomparison studies play a key role in the quality control and quality assurance of radon measurement. Besides the concern with radon, the importance of 220 Rn (thoron) has recently been recognised. Thoron is present everywhere together with radon, and its quantity is sometimes much larger than that of radon at a certain position in dwellings, especially close to walls. On the other hand, developing an accurate measurement procedure for 220Rn would also be beneficial for the accurate measurement of 222Rn, because such measurements can be overestimated due to interference from 220Rn as shown in recent surveys, for example in China(2). The data presented in this article indicated that radon as well as thoron measurement techniques still need improvements in order to solve technical problems in their metrology. MATERIALS AND METHODS Participants Twenty four laboratories from 19 countries participated in the radon intercomparison, using four types
of monitors (passive type: etched track, electret plate and silicon photodiode as well as one active type: ionisation chamber). Twelve laboratories from 11 countries participated in the thoron intercomparison, using two types of detectors (CR-39 and photodiode). A list of participants in the radon and the thoron intercomparisons is given in Table 1. Radon facility The 22.4-m3 inner-volume radon chamber was utilised for radon exposures(3). The radon chamber was run under open-loop mode and stable radon concentration, i.e. radon was injected at a stable (0.5 l min21) flow rate and removed to the atmosphere through active carbon filter. During exposures, the radon level was monitored by an AlphaGUARD, which has been periodically calibrated in the primary atmosphere at the PhysikalischTechnische Bundesanstalt (PTB), Germany(4). Thoron facility A 150-l stainless steel chamber system at National Institute of Radiological Sciences (NIRS) was used to expose monitors to thoron(5). The thoron gas generated in the source, consisting of a fabric wick for lantern mantle with 232Th, was supplied to the chamber in the closed-loop mode with a flow rate of 2 l min21. The thoron system was run in the open-loop mode because externally mixed dry and humidified air obtained by bubbling air through de-ionised water was injected. In the normal mode (one reference thoron monitor), the flow rate of the mixed air was set to 2 l min21. The 220 Rn concentrations in the chamber were measured
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
The fifth international intercomparison for radon and fourth for thoron monitors were conducted at National Institute of Radiological Sciences (Japan) with the radon and thoron chambers. The tests were made under two different exposures to radon and two exposures (in two rounds due to limited space in the thoron chamber) to thoron. In these most recent intercomparisons, two new graphical methods recommended by the ISO standard, Mandel’s h statistic and the Youden plot, were implemented to evaluate the consistency between laboratories and within laboratories. The presented data indicated that the performance quality of laboratories for radon measurement as expressed by the percentage difference parameter has been stable since the first international intercomparison for passive monitors carried out in 2007, and it amounted to around 50 for 10 % of the difference from the reference value. The thoron exercise showed that further development and additional studies to improve its measuring methods and reliability are needed.
M. JANIK AND H. YONEHARA Table 1. List of participants. Institute/company
Det. type
IT RO BR PL TH PL NO BE
X X X X X X X X
CR-39 X CR-39 CR-39 CR-39 X CR-39 CR-39 X S.P.a CR-39
CA GE
X X
X CR-39, ELb X CR-39
IR VI PL JP
X X X X
X CR-39 X CR-39 CR-39 I.C.c
SW IT PL
X X X
X CR-39 CR-39 CR-39
CH JP CZ TH CR SP HU JP IT
X X X X X X X X X
X CR-39 X CR-39 LR 115 X CR-39 LR 115 CR-39 X CR-39 I.C.c CR-39
RESULTS
a
Silicon photodiode. Electret. c Ionisation chamber. b
continuously every 30 min using a RAD7 device. For the quality assurance of the continuously measured 220 Rn concentrations, intermittent measurements were also acquired during the exposure test by means of a grab sampling technique using a scintillation cell(6, 7). The RAD7 device was calibrated in the reference thoron atmosphere at the PTB(8) and in the NIRS radon chamber. DATA EVALUATION To standardise the results obtained for all the exposure conditions, the parameter percentage difference (PD) recommended by the Conformity Assessment—General Requirements for Proficiency Testing Standard(9) and the National Association for Proficiency Testing was used. In order to allow comparisons between Public Health England (PHE, UK) and NIRS, the PHE classification parameter was applied as given by the measurement error (MES)(10). Details of these parameters have been described elsewhere(11).
The integrated monitors were all shipped from the participating laboratories to NIRS (and also back) in radon-proof plastic bags or special cases. In addition, the monitors before and after exposures were separately stored in special radon-proof plastic bags outside the NIRS radon chamber. From 4 to 10 monitors per participating laboratory were exposed to radon or thoron and were placed in the chambers. One or two monitors per participating laboratory were exposed outside the radon chamber as a blank (control and transport checking). After exposure, the monitors were sent back to their respective laboratories for analysis. The results of each exposure were then reported to the NIRS for evaluation. After all the reports were received, reference values for radon and thoron and other relevant parameters, presented in Table 2, were sent to the participating laboratories. The uncertainties for radon and thoron concentrations as well as expositions were stated as the expanded uncertainties with the coverage factor k ¼ 2. Results by the parameters PD and MES were categorised into three ranks: Cat-A if jPDj or MES 10 %, Cat-B if 10 % , jPDj or MES , 20 % and Cat-C if jPDj or MES 20 %. The values of 10 and 20 % were based on parameters of the normal distribution of PD from all previous intercomparisons. Referring to results plotted in Figures 1–6 and listed in Table 3, the following conclusions about passive monitors can be drawn:
596
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
ARPA Lazio Babes-Bolyai University CDTN Central Mining Institute Chulalongkorn University CLOR Corentium AS Hainaut Vigilance Sanitaire Health Canada Helmholtz Zentrum Munchen INRA INST IFJ PAN Kobe Pharmaceutical Univ. Landauer Nordic AB MCF Ambiente Srl Medical Univ. of Bialystok NIRP NIRS SUJCHBO Kamenna TINT University in Osijek University of Cantabria University of Pannonia University of Tokyo U-Series
Country Rn Tn
The current intercomparison results were extended in a graphical way to allow evaluation of laboratories. The ISO recommends Mandel’s h statistic and the Youden plot for this purpose. Mandel’s h statistic is a between-laboratory consistency statistic that measures for a selected level the standardised deviation of the mean value obtained by a laboratory from the mean for that level. It is therefore a measure of the laboratory bias. On Mandel’s h plot, the critical level (H0 ¼ 95 %) is indicated. When the critical level is exceeded, the H-null hypothesis ‘no difference between the mean values’ is rejected. In a split-level experiment, useful information can also be obtained from the Youden plot, which consists of plotting the results of two samples against each other. The plot visualises within-laboratory variability as well as between-laboratory variability. The interpretation of the Youden plot is as follows: (1) points that lie near the 45-degree reference line, far from the centre but inside the circle, indicate large systematic error; (2) points that lie far from the 45-degree line but inside the circle indicate large random error and (3) points outside the circle indicate large total (systematic þ random) error.
Table 2. Reference values and environmental conditions for radon and thoron exposures. Stop
Exposure time, h
Rn conc., kBq m23
Exposure, kBq m23 h
Temperature, 8C
Relative hum., %
Absolute hum., g m23
Rn I II
20/09/2013 15:00 26/09/2013 15:00
24/09/2013 15:00 30/09/2013 15:00
96.00 96.00
1.13+ 0.34 7.08+0.41
109+65 680+79
21.8+0.1 21.8+0.0
57.0+0.2 56.9+0.2
10.9+0.1 10.9+0.1
Tn IA IB IIA IIB
02/10/2013 15:00 03/10/2013 17:30 09/10/2013 14:00 10/10/2013 16:30
03/10/2013 15:00 04/10/2013 17:30 10/10/2013 14:00 11/10/2013 16:30
24.00 24.00 24.00 24.00
4.18+0.74 4.32+0.76 14.62+1.48 15.02+1.49
100+36 104+36 351+71 361+72
30.8+0.5 31.1+0.5 29.4+0.3 30.2+0.4
16.0+1.1 12.9+0.6 31.9+2.0 31.6+0.6
5.1+0.3 4.2+0.2 9.3+0.8 9.7+0.2
Figure 1. Proportions of PD results by exposure normalised to 1.
Figure 2. Proportions of MES results by exposure normalised to 1.
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
- Rn-I exposure: the ratio of the radon concentration given by the participants to that of NIRS expressed by the REF parameter ranged from 0.62 to 1.40 with 1.00 as the average and 0.97 as the median. As shown in Figure 1, 50 % of the results for passive monitors were in Cat-A, e.g. located in the 10 % range around the NIRS reference value. And, 10 % of the results (Figure 2) were included in the MES Cat-A (10 % range). None of them exceeded the critical value of Mandel’s h.
597
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M. JANIK AND H. YONEHARA
Figure 4. Mandel’s h statistic for Tn exposures.
598
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
Figure 3. Mandel’s h statistic for Rn exposures.
599
Arithmetic mean [median] (range). a
22.8 [19.2] (7.2 –42) 19.9 [12.5] (3.8 –93) 50.4 [41.8] (9.1 –180.0) 58.4 [41.4] (6.7 –230.0) 20.04 [20.14] (22.4 to 1.8) 20.87 [20.56] (216.0 to 5.5) 20.89 [20.50] (212.0 to 4.5) 20.90 [20.59] (221.0 to 15.0) 20.27 [22.90] (238 to 38) 25.77 [25.29] (293 to 56) 2.03 [29.46] (2100 to 180) 15.70 [26.82] (299 to 220) 1.00 [0.97] (0.62–1.40)a 0.94 [0.95] (0.07–1.60) 1.02 [0.91] (,0.01– 2.80) 1.16 [0.93] (,0.01– 3.20)
MES En
Rn-I Rn-II Tn-I Tn-II
Further evaluation of results expressed by the Youden plot and Mandel’s h statistic showed that some institutions had problems with evaluation of radon data; e.g. IDs 2C, 14, 21, 23, 25A, 27, 27A, 56 and 60 were outside the 2SD limit of the random components (circle), and this indicated a large total (systematic þ random) error. On the other hand, if Mandel’s h was .1.0 for one of the exposures or the difference between values
PD
Rn-II exposure: 46 % of the PDs were lying in Cat-A, but 36 % were in the MES Cat-A. In this exposure, the range of the REF values was wider than that of Rn-I, being from 0.07 to 1.60, and the average value was lower at 0.94, and the median was 0.95. Results of two laboratories exceeded the critical Mandel’s h value.
REF
-
Exposure
Figure 6. The Youden graph for Tn exposures.
Downloaded from http://rpd.oxfordjournals.org/ at Stockholms Universitet on November 15, 2015
Figure 5. The Youden graph for Rn exposures.
Table 3. Calculated arithmetic mean, median (in brackets) and range (in parentheses) of REF, PD, En and MES values.
THE MOST RECENT INTERNATIONAL INTERCOMPARISONS
M. JANIK AND H. YONEHARA
exceeded 1.0, then the point was lying outside the circle of the Youden plot:
In order to identify the laboratory performance quality, a similar evaluation as for radon using the Youden plot was applied to thoron exposures. According to this calculation if Mandel’s h statistic was higher than 1 or the difference between calculated values for exposures Tn-I and Tn-II was higher than 1, then the laboratory was found outside the circle in the Youden plot, e.g. ID21—difference was calculated as 1.07, ID24—both Mandel’s h values exceeded 1 and ID28—the difference was 1.27. In addition, the MES values of an appropriate exposure level (comparable Rn-I and Rn-II levels by NIRS) provided by the latest report of the PHE were calculated as 20.1 and 13.4 for the average, 15.0 and 8.8 for the median and 4.2 –92.8 and 3.3–93.3 for the range, for Exposures 2 and 5, respectively(10). The NIRS average and median MES values presented in Table 3 were higher than PHE values; however, the ranges were similar. The difference could be explained by the better reference radon exposure uncertainty reported by PHE (3 %) compared with 5 % reported by NIRS and hence, the lower final calculation results of the PHE MES values. Two active monitors (IDs: 59 and 63) were exposed in the radon atmosphere only. Their PD results indicated radon reference values. Mandel’s h statistic was calculated as a very low value and together with points on the Youden plot was located close to the centre, and it illustrated good reproducibility and low measurement uncertainty. CONCLUSION Results of intercomparisons can reveal monitor calibration problems for laboratories of the participants. Periodical intercomparisons should be carried out as
ACKNOWLEDGEMENT We gratefully acknowledge all of the participants. REFERENCES 1. European Commission. Council Directive laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/ Euratom, 97/43/Euratom and 2003/122/Euratom. 2013/59/EURATOM (2014). 2. Yamada, Y. et al. Radon-thoron discriminative measurements in Gansu Province, China, and their implication for dose estimates. J. Toxicol. Environ. Health. A 69, 723– 734 (2006). 3. Tokonami, S., Ishikawa, T., Sorimachi, A., Takahashi, H. and Miyahara, N. The Japanese radon and thoron reference chambers. AIP Conf. Proc. 1034, 202–205 (2008). 4. Ro¨ttger, A., Honig, A. and Linzmaier, D. Calibration of commercial radon and thoron monitors at stable activity concentrations. Appl. Radiat. Isot. 87, 44– 47 (2014). 5. Sorimachi, A., Ishikawa, T., Janik, M. and Tokonami, S. Quality assurance and quality control for thoron measurement at NIRS. Radiat. Prot. Dosim. 141, 367– 370 (2010). 6. Eappen, K. P., Sapra, B. K. and Mayya, Y. S. A novel methodology for online measurement of thoron using Lucas scintillation cell. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 572, 922– 925 (2007). 7. Tokonami, S., Yang, M., Yonehara, H. and Yamada, Y. Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell. Rev. Sci. Instrum. 73, 69 (2002). 8. Ro¨ttger, A., Honig, A., Dersch, R., Ott, O. and Arnold, D. A primary standard for activity concentration of 220Rn (thoron) in air. Appl. Radiat. Isot. 68, 1292–1296 (2010). 9. International Organization for Standardization and International Electrotechnical Commission. Conformity Assessment—General Requirements for Proficiency Testing. ISO/IEC 17043:2010(E) (2010). 10. Public Health England. Results of the 2013 PHE Intercomparison of Passive Radon Detectors. PHECRCE-011 Report (2013). 11. Janik, M., Ishikawa, T., Omori, Y. and Kavasi, N. Radon and thoron intercomparison experiments for integrated monitors at NIRS. Japan. Rev. Sci. Instrum. 85, 022001 (2014).
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- Tn-I exposure: the average REF was calculated to be 1.02, with 0.91 as the median, and it ranged from ,0.01 to 2.80. Twenty-five per cent of the PD results but only 8 % of the MES Cat-A were in Cat-A. Only one result exceeded the critical value of Mandel’s h. - Tn-II exposure: REF average for this exposure was found to be 1.16, with the median of 0.93 and ,0.01 –3.20 as a range. In addition, 17 % of the PD and MES results were in Cat-A, and one result exceeded the critical value of Mandel’s h.
there is a need to further investigate various radon and thoron monitors being used today and to improve quality of data.
