Applied Radiation and Isotopes 87 (2014) 435–438
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A review of nationwide radioactivity comparisons on gamma-ray spectrometry organized by the NIRP, China Fei Tuo, Cuihua Xu, Qing Zhang, Jing Zhang, Qiang Zhou, Wenhong Li, Jianfeng Zhang, Xu Su n Key Laboratory of Radiological Protection and Nuclear Emergency, National Institute for Radiological Protection, Chinese Center for Disease Control and Prevention, Beijing 100088, China
H I G H L I G H T S
188 samples were prepared and distributed to 39 participating laboratories. 528 radionuclide assays have been performed. Types of nuclides varied from natural to man-made radionuclides. A perceptible laboratory performance improvement is observed.
art ic l e i nf o
a b s t r a c t
Available online 3 December 2013
Six comparison exercises on radioactivity measurement by γ-spectrometry have been organized by NIRP in China since 2007. The type of measured nuclides changed from natural to man-made over this period. A total of 188 samples were prepared and distributed to 39 different participating laboratories and 528 radionuclide assays have been performed. A perceptible laboratory performance improvement was observed with the average percentage of acceptable scores being 87% in 2008, increasing to 92% in 2012. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Radioactivity Comparison Radionuclide γ-Spectrometry
1. Introduction and objectives Because radioactivity measurements play a vital role in radiological protection and human health, it is essential to obtain accurate and reliable radioactivity values. Comparison exercises are a method for regularly assessing the quality of the analytical data produced by different laboratories (Coquery et al., 1999; Tauhata et al., 2006; Wätjen et al., 2008; Woods et al., 1995). For that reason, comparison exercises have been organized in China for many years by the National Institute for Radiological Protection (NIRP), in cooperation with the National Institute of Metrology (NIM), on radionuclide measurement using γ-spectrometry. The present study summarizes the data we obtained in the past 6 years and the observed improvements are discussed.
2. Material and methods 2.1. Types of samples From 2007 to 2009, bulk samples of environmental matrices (soil and building materials) were prepared and distributed by NIRP. After n
Corresponding author. Tel./fax: þ 86 10 62389720. E-mail address: [email protected] (X. Su).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.072
these bulk materials were dried, ground to a particle size of approximately 2 mm in diameter and homogenized, each kind of material was packed into polythene sacks (containing approximately 500 g of material each), sealed, and then dispatched to the participants for analysis. Each participant received two bags of the material (samples A and B). The compositions of the two samples were essentially identical. The only difference was the activity level. Participants dispensed appropriate weight for analysis according to their lab's own containers type. Participants were instructed to close their containers and bind adhesive tape to the seal to ensure that 222Rn did not escape. They were also instructed to wait for 4 weeks before carrying out the measurement. Since 2010, analysts were presented with a solid matrix, kaolin, which was spiked with three gamma-emitting radionuclides, 241Am, 137 Cs and 60Co by using standard solutions provided by Eckert & Ziegler Isotopes products, U.S.A. The values of the activity concentrations of these radionuclides are traceable to National Institute of Standards and Technology (NIST). Samples were weighed and placed into unified cylindrical polyethylene containers (75 mm diameter and 70 mm height). Approximately 290–300 g of samples was sealed in the container, labeled with weight, and then dispatched to the participants for analysis. For these kaolin samples, the participants make measurements on the samples that they received without transferring the sample to another container.
436
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
For the 2007–2009 exercises, the individual participants calibrated their detectors by using certified reference materials (CRMs) of traced from NIM or other suppliers. The calibrated samples were of claimed by dispensing a known weight to the container used routinely by that laboratory. For the comparison exercise, participants dispensed the same mass of material to their standard container. For the kaolin samples, almost all participants received a CRM from NIM. The CRM was in the same type of container as used for the comparison. 2.2. Establishment of reference values Samples of each batch were certificated by the NIM, confirmed and guided strongly by the workshop discussions of the scientific committee of NIRP. The NIMs used relative calibration methods to establish the reference values for the activity concentrations of each radionuclide, as well as their uncertainties. 2.3. Scheme and timetable of comparisons Organization of these exercises followed the same scheme each year. Planning took place in February/March with samples being prepared in May to July and then dispatched in August. The deadline for reporting results was 25 October with evaluation taking place in November. After the issue of the report by 15 December, a meeting and technical presentation was held in December or January. Laboratory participation is voluntary, free of charge and all Chinese laboratories have been encouraged to participate in these exercises. Standard report forms are issued with each sample. Participants are requested to analyze the samples by gamma spectrometry and to report the individual radionuclides identified together with their assayed activity levels, uncertainty estimates, the sources of nuclear data used and type of gamma spectrometers. NIRP prepares the performance evaluation reports and sends them to the participants. The results are coded to preserve anonymity: only the laboratory concerned is aware of its own code number. It is recommended that participants use the Monographie 5 of the Bureau International des Poids et Mesures (Bé et al., 2004–2013) as their source of nuclear data. 2.4. Performance evaluation and scoring We perform our evaluation using the 2007 IAEA criteria (IAEA, 2007; Shakhashiro and Mabit, 2009), relative bias and Z-score are calculated just as complementary information for the participating laboratories. 2.4.1. Relative bias To evaluate the bias of the reported results, the relative bias between analyst's value (ValueAnalyst) and the reference value (ValueRef.) is calculated and expressed as a percentage: Relative bias ¼
ValueAnalyst ValueRef : 100% ValueRef :
ð1Þ
2.4.2. The Z-score value The Z-score is calculated from the laboratory results, the assigned value and a standard deviation in accordance with the following equation: Z Score ¼
ValueAnalyst ValueRef : s
ð2Þ
On the basis of the “fitness for purpose” principle, the reference value for the standard deviation (s) is: 0.15 ValueRef. The value of
0.15 is an artitrary value that was agreed by consensus before the comparison. 2.4.3. The U-score value The value of the Utest score is calculated according to the following equation: ValueRef : ValueAnalyst U test ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Þ Unc2Ref : þ Unc2Analyst The choice of the significance level is subjective. For this PT we have set the limiting value for the U-test parameter to 2.58 for a level of probability at 99% to determine whether a result passes the test (Uo 2.58). 2.5. Evaluation criteria of the proficiency test The PT results were evaluated against the acceptance criteria for accuracy and precision (as described below) and assigned the status “acceptable”, “warning” or “not acceptable” accordingly. 2.5.1. Accuracy Participant's result is assigned “acceptable” status if Utest r 2.58. 2.5.2. Precision Participant's result is assigned “acceptable” status if sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi UncAnalyst 2 UncRef : 2 þ 100% P¼ ValueRef : ValueAnalyst
ð4Þ
P directly depends on the measurement uncertainty claimed by the participant. The Limit of Acceptable Precision (LAP) for each analyte is defined for the respective proficiency test in advance. Participants' results are scored as “acceptable” for precision when Po LAP. The LAP value used in the evaluation of radionuclides was 0.15 for 2007–2009 and 0.20 for 2010–2012. As with the Z-score, these values were discussed and agreed by consensus before the comparison. The higher value of 0.2 was chosen because the radionuclides were anthropogenic and it was expected that this would be more difficult to measure. In the final evaluation, both scores for trueness and precision are combined. A result must obtain an “acceptable” score in both criteria to be assigned the final score “acceptable”. 3. Results and discussion The comparison participants came from a broad spectrum of interests in China, including the local Centers for Disease Control and Prevention, nuclear industries, universities, nuclear power companies, Chinese Academy of Sciences and environmental monitoring laboratories. A summary of the compositions and radionuclides content and reference values of the samples for each exercise is given in Table 1. A total of 188 samples have been prepared and distributed to the participants. The first comparison in 2007 comprised two types of samples named A and B, containing three natural gammaemitting radionuclides. In 2009, we increased the complexity with one of the sample named sample B (soil) containing 137Cs. From 2010, analysts have been presented with a solid matrix, kaolin, spiked with gamma-emitting radionuclides, 137Cs and 60Co. In 2012, 241Am was also included. During the period from 2007 to 2012, 39 different laboratories participated in the comparisons, with some laboratories quitting the program and new ones joining in each year. Now, 28 laboratories are participating in the comparisons. The assessment of
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
437
Table 1 Reference values of radionuclides considered in the comparison samples. All values given in units of Bq kg 1. Radionuclide
226
Ra Th 40 K 137 Cs 60 Co 241 Am 232
Number of assays
108 108 108 88 66 50
2007
2008
2009
Building material
Soil
Building material
Soil
Building material
Sample A
Sample B
Sample A
Sample B
Sample B
Sample A
26.0 70.8 45.17 1.6 463.9 78.2 – – –
109.2 7 1.4 175.2 7 2.8 347.17 7.0 – – –
21.2 7 1.0 26.0 71.5 441.5 7 20.4 – – –
28.17 1.3 46.2 7 2.3 450.17 20.8 – – –
27.9 7 1.0 34.5 71.8 551.3 7 10.3 68.8 7 1.1 – –
100.7 7 3.2 173.5 7 6.3 398.7 7 15.4 – – –
2010
2011
2012
Kaolin
Kaolin
Kaolin
– – – 71.5 7 3.6 96.6 7 4.8 9
– – – 42.0 7 3.0 72.8 7 5.0 180.0 7 13.2
– – – 26.4 71.5 28.0 71.8 90.9 77.4
Note: for all samples the reference date is 1 June of each year, the combined uncertainty is expressed at 1s level.
3.0
2.5 Am-241 Cs-137
Z-scores (|Z|)
2.0
Co-60
1.5
1.0
0.5
0.0
01 02 03 25 16 17 18 05 06 26 04 19 20 21 13 14 15 08 09 07 28 22 23 24 27 10 Laboratory code numbers
Fig. 1. Z-scores of participant laboratories for the determination of
trends in analytical performance is difficult because of the different activity levels of radionuclides in the samples. Therefore, any apparent trends must be interpreted with caution, as the precision of analytical measurements usually decreases as the concentration of the radionuclides decreases. As expected, each exercise has resulted in the accumulation of a vast amount of data. The full results of each comparison were given in the individual reports, together with a detailed discussion. It is impossible to summarize all the results here. Therefore a selective group was examined. Examples of the Z-score results for the determination of 137Cs, 60Co and 241Am in a solid matrix sample in 2012 are shown in Fig. 1. A general satisfactory distribution of Z-values can be seen for most of participants, but further improvement is still possible. An attempt was made to gain some insight into the evolution of laboratory performance with repetitive participation in comparison. Altogether 39 different laboratories have participated in these comparisons. There have been seven of these laboratories who have participated in all of the comparisons. An improvement of the quality of results judged by the pass rate for all participants for the comparison exercises was observed in the past 6 years. At the beginning, some laboratories did not provide complete information of efficiency calibration and applied corrections. A few laboratories reported uncertainty values produced only by their software without knowing details what was included. They now calculated their own uncertainties. The comparisons have technical
137
Cs,
60
Co,
241
Am in a solid matrix sample comparison exercise in 2012.
meetings every year, so we discuss with participants on how they could improve their performance and provide advice, such as to use traceable calibration source correctly, to calibrate γ-spectrometers regularly, to improve evaluation of the measurement uncertainty quantitatively, and train staff to operate systems regularly for quality assurance. After discussions with participants during these technical meeting, participating laboratories have followed the international guides for uncertainty evaluations and combined “all sources” of uncertainty. Also, laboratories now give details of their calibration procedures, including the source of their reference material. The overall performance is continuously improving since the beginning of comparison exercises (Table 2). In the year of 2008 we had 87% of “acceptable results” performance and last years the performance came up to 92%.
4. Conclusions The total number of participant laboratories and complexity of the comparison samples increased over the years of the exercises. It indicates that comparisons have a better acceptance now among the participants, the number of participants has increased from 16 to 28 and their participation in the programmes give confidence to their measurements. Participation in these comparisons has required the laboratories to use traceable reference materials for
438
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
supported financially by the Science Foundation of Ministry of Health (No. 201002009).
Table 2 Radionuclides and percentage of pass rate by year. Year
Gamma nuclide
Number of participating laboratories
Percentage of pass rate (%)
226
16 16 25
50 87 88
21 25 28
76 92 92
Ra, 232Th, 40K Ra, 232Th, 40K 226 Ra, 232Th, 40K, 137 Cs 2010 137Cs, 60Co 2011 137Cs, 60Co, 241Am 2012 137Cs, 60Co, 241Am
2007 2008 2009
226
their calibration which underpins the traceabilities of their results. Comparisons are an important tool to assess the Chinese laboratories performances. It is hoped that comparisons could contribute to the improvement of measurement quality and will contribute to an increased level of public confidence. However, there is also a need to calibrate γ-spectrometers regularly for accurate measurements, and to improve evaluation of the measurement uncertainty quantitatively for some laboratories. The future comparison exercises will focus on food and water characterized low levels of manmade radionuclides, which may be more challengeable for the analytical measurements. Acknowledgments The authors would like to thank the participants and laboratories participating in these comparison exercises. This work was
References Bé, M.-M., et al., 2004–2013. Table of Radionuclides, Vol. 1 to Vol. 7, Monographie BIPM-5. Bureau International des Poids et Mesures, Sèvres, France 〈http://www. bipm.org/en/publications/monographie-ri-5.html〉. Coquery, M., Carvalho, F.P., Azemard, S., Horvat, M., 1999. The IAEA worldwide intercomparison exercises (1990–1997): determination of trace elements in marine sediments and biological samples. Sci. Total Environ. 237–238, 501–508. IAEA, 2007. Report on the IAEA-CU-2006-03 World-Wide Open Proficiency Test on the Determination of Gamma Emitting Radionuclides, Report IAEA/AL/171, Seibersdorf. International Atomic Energy Agency, Vienna, Austria. Shakhashiro, A., Mabit, L., 2009. Results of an IAEA inter-comparison exercise to assess 137Cs and total 210Pb analytical performance in soil. Appl. Radiat. Isot. 67 (1), 139–146. Tauhata, L., Vianna, M.E.C.M., Oliveira, A.E.D., Ferreira, A.C., Braganca, M.J.C., Clain, A.F., Faria, R.Q.D., 2006. The Brazilian national intercomparison program (PNI/IRD/ CNEN): evaluation of 15 years of data. J. Environ. Radioact. 86 (3), 384–390. Wätjen, U., Spasova, Y., Altzitzoglou, T., 2008. Measurement comparisons of radioactivity among European monitoring laboratories for the environment and food stuff. Appl. Radiat. Isot. 66 (6–7), 742–749. Woods, M.J., Jerome, S.M., Dean, J.C.J., Perkin, E.M.E., 1995. A review of data obtained in a series of radioactivity measurement intercomparison exercises in the UK: γ-ray emitters 1989–1994. Sci. Total Environ. 173–174, 247–257.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A review of nationwide radioactivity comparisons on gamma-ray spectrometry organized by the NIRP, China Fei Tuo, Cuihua Xu, Qing Zhang, Jing Zhang, Qiang Zhou, Wenhong Li, Jianfeng Zhang, Xu Su n Key Laboratory of Radiological Protection and Nuclear Emergency, National Institute for Radiological Protection, Chinese Center for Disease Control and Prevention, Beijing 100088, China
H I G H L I G H T S
188 samples were prepared and distributed to 39 participating laboratories. 528 radionuclide assays have been performed. Types of nuclides varied from natural to man-made radionuclides. A perceptible laboratory performance improvement is observed.
art ic l e i nf o
a b s t r a c t
Available online 3 December 2013
Six comparison exercises on radioactivity measurement by γ-spectrometry have been organized by NIRP in China since 2007. The type of measured nuclides changed from natural to man-made over this period. A total of 188 samples were prepared and distributed to 39 different participating laboratories and 528 radionuclide assays have been performed. A perceptible laboratory performance improvement was observed with the average percentage of acceptable scores being 87% in 2008, increasing to 92% in 2012. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Radioactivity Comparison Radionuclide γ-Spectrometry
1. Introduction and objectives Because radioactivity measurements play a vital role in radiological protection and human health, it is essential to obtain accurate and reliable radioactivity values. Comparison exercises are a method for regularly assessing the quality of the analytical data produced by different laboratories (Coquery et al., 1999; Tauhata et al., 2006; Wätjen et al., 2008; Woods et al., 1995). For that reason, comparison exercises have been organized in China for many years by the National Institute for Radiological Protection (NIRP), in cooperation with the National Institute of Metrology (NIM), on radionuclide measurement using γ-spectrometry. The present study summarizes the data we obtained in the past 6 years and the observed improvements are discussed.
2. Material and methods 2.1. Types of samples From 2007 to 2009, bulk samples of environmental matrices (soil and building materials) were prepared and distributed by NIRP. After n
Corresponding author. Tel./fax: þ 86 10 62389720. E-mail address: [email protected] (X. Su).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.072
these bulk materials were dried, ground to a particle size of approximately 2 mm in diameter and homogenized, each kind of material was packed into polythene sacks (containing approximately 500 g of material each), sealed, and then dispatched to the participants for analysis. Each participant received two bags of the material (samples A and B). The compositions of the two samples were essentially identical. The only difference was the activity level. Participants dispensed appropriate weight for analysis according to their lab's own containers type. Participants were instructed to close their containers and bind adhesive tape to the seal to ensure that 222Rn did not escape. They were also instructed to wait for 4 weeks before carrying out the measurement. Since 2010, analysts were presented with a solid matrix, kaolin, which was spiked with three gamma-emitting radionuclides, 241Am, 137 Cs and 60Co by using standard solutions provided by Eckert & Ziegler Isotopes products, U.S.A. The values of the activity concentrations of these radionuclides are traceable to National Institute of Standards and Technology (NIST). Samples were weighed and placed into unified cylindrical polyethylene containers (75 mm diameter and 70 mm height). Approximately 290–300 g of samples was sealed in the container, labeled with weight, and then dispatched to the participants for analysis. For these kaolin samples, the participants make measurements on the samples that they received without transferring the sample to another container.
436
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
For the 2007–2009 exercises, the individual participants calibrated their detectors by using certified reference materials (CRMs) of traced from NIM or other suppliers. The calibrated samples were of claimed by dispensing a known weight to the container used routinely by that laboratory. For the comparison exercise, participants dispensed the same mass of material to their standard container. For the kaolin samples, almost all participants received a CRM from NIM. The CRM was in the same type of container as used for the comparison. 2.2. Establishment of reference values Samples of each batch were certificated by the NIM, confirmed and guided strongly by the workshop discussions of the scientific committee of NIRP. The NIMs used relative calibration methods to establish the reference values for the activity concentrations of each radionuclide, as well as their uncertainties. 2.3. Scheme and timetable of comparisons Organization of these exercises followed the same scheme each year. Planning took place in February/March with samples being prepared in May to July and then dispatched in August. The deadline for reporting results was 25 October with evaluation taking place in November. After the issue of the report by 15 December, a meeting and technical presentation was held in December or January. Laboratory participation is voluntary, free of charge and all Chinese laboratories have been encouraged to participate in these exercises. Standard report forms are issued with each sample. Participants are requested to analyze the samples by gamma spectrometry and to report the individual radionuclides identified together with their assayed activity levels, uncertainty estimates, the sources of nuclear data used and type of gamma spectrometers. NIRP prepares the performance evaluation reports and sends them to the participants. The results are coded to preserve anonymity: only the laboratory concerned is aware of its own code number. It is recommended that participants use the Monographie 5 of the Bureau International des Poids et Mesures (Bé et al., 2004–2013) as their source of nuclear data. 2.4. Performance evaluation and scoring We perform our evaluation using the 2007 IAEA criteria (IAEA, 2007; Shakhashiro and Mabit, 2009), relative bias and Z-score are calculated just as complementary information for the participating laboratories. 2.4.1. Relative bias To evaluate the bias of the reported results, the relative bias between analyst's value (ValueAnalyst) and the reference value (ValueRef.) is calculated and expressed as a percentage: Relative bias ¼
ValueAnalyst ValueRef : 100% ValueRef :
ð1Þ
2.4.2. The Z-score value The Z-score is calculated from the laboratory results, the assigned value and a standard deviation in accordance with the following equation: Z Score ¼
ValueAnalyst ValueRef : s
ð2Þ
On the basis of the “fitness for purpose” principle, the reference value for the standard deviation (s) is: 0.15 ValueRef. The value of
0.15 is an artitrary value that was agreed by consensus before the comparison. 2.4.3. The U-score value The value of the Utest score is calculated according to the following equation: ValueRef : ValueAnalyst U test ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Þ Unc2Ref : þ Unc2Analyst The choice of the significance level is subjective. For this PT we have set the limiting value for the U-test parameter to 2.58 for a level of probability at 99% to determine whether a result passes the test (Uo 2.58). 2.5. Evaluation criteria of the proficiency test The PT results were evaluated against the acceptance criteria for accuracy and precision (as described below) and assigned the status “acceptable”, “warning” or “not acceptable” accordingly. 2.5.1. Accuracy Participant's result is assigned “acceptable” status if Utest r 2.58. 2.5.2. Precision Participant's result is assigned “acceptable” status if sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi UncAnalyst 2 UncRef : 2 þ 100% P¼ ValueRef : ValueAnalyst
ð4Þ
P directly depends on the measurement uncertainty claimed by the participant. The Limit of Acceptable Precision (LAP) for each analyte is defined for the respective proficiency test in advance. Participants' results are scored as “acceptable” for precision when Po LAP. The LAP value used in the evaluation of radionuclides was 0.15 for 2007–2009 and 0.20 for 2010–2012. As with the Z-score, these values were discussed and agreed by consensus before the comparison. The higher value of 0.2 was chosen because the radionuclides were anthropogenic and it was expected that this would be more difficult to measure. In the final evaluation, both scores for trueness and precision are combined. A result must obtain an “acceptable” score in both criteria to be assigned the final score “acceptable”. 3. Results and discussion The comparison participants came from a broad spectrum of interests in China, including the local Centers for Disease Control and Prevention, nuclear industries, universities, nuclear power companies, Chinese Academy of Sciences and environmental monitoring laboratories. A summary of the compositions and radionuclides content and reference values of the samples for each exercise is given in Table 1. A total of 188 samples have been prepared and distributed to the participants. The first comparison in 2007 comprised two types of samples named A and B, containing three natural gammaemitting radionuclides. In 2009, we increased the complexity with one of the sample named sample B (soil) containing 137Cs. From 2010, analysts have been presented with a solid matrix, kaolin, spiked with gamma-emitting radionuclides, 137Cs and 60Co. In 2012, 241Am was also included. During the period from 2007 to 2012, 39 different laboratories participated in the comparisons, with some laboratories quitting the program and new ones joining in each year. Now, 28 laboratories are participating in the comparisons. The assessment of
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
437
Table 1 Reference values of radionuclides considered in the comparison samples. All values given in units of Bq kg 1. Radionuclide
226
Ra Th 40 K 137 Cs 60 Co 241 Am 232
Number of assays
108 108 108 88 66 50
2007
2008
2009
Building material
Soil
Building material
Soil
Building material
Sample A
Sample B
Sample A
Sample B
Sample B
Sample A
26.0 70.8 45.17 1.6 463.9 78.2 – – –
109.2 7 1.4 175.2 7 2.8 347.17 7.0 – – –
21.2 7 1.0 26.0 71.5 441.5 7 20.4 – – –
28.17 1.3 46.2 7 2.3 450.17 20.8 – – –
27.9 7 1.0 34.5 71.8 551.3 7 10.3 68.8 7 1.1 – –
100.7 7 3.2 173.5 7 6.3 398.7 7 15.4 – – –
2010
2011
2012
Kaolin
Kaolin
Kaolin
– – – 71.5 7 3.6 96.6 7 4.8 9
– – – 42.0 7 3.0 72.8 7 5.0 180.0 7 13.2
– – – 26.4 71.5 28.0 71.8 90.9 77.4
Note: for all samples the reference date is 1 June of each year, the combined uncertainty is expressed at 1s level.
3.0
2.5 Am-241 Cs-137
Z-scores (|Z|)
2.0
Co-60
1.5
1.0
0.5
0.0
01 02 03 25 16 17 18 05 06 26 04 19 20 21 13 14 15 08 09 07 28 22 23 24 27 10 Laboratory code numbers
Fig. 1. Z-scores of participant laboratories for the determination of
trends in analytical performance is difficult because of the different activity levels of radionuclides in the samples. Therefore, any apparent trends must be interpreted with caution, as the precision of analytical measurements usually decreases as the concentration of the radionuclides decreases. As expected, each exercise has resulted in the accumulation of a vast amount of data. The full results of each comparison were given in the individual reports, together with a detailed discussion. It is impossible to summarize all the results here. Therefore a selective group was examined. Examples of the Z-score results for the determination of 137Cs, 60Co and 241Am in a solid matrix sample in 2012 are shown in Fig. 1. A general satisfactory distribution of Z-values can be seen for most of participants, but further improvement is still possible. An attempt was made to gain some insight into the evolution of laboratory performance with repetitive participation in comparison. Altogether 39 different laboratories have participated in these comparisons. There have been seven of these laboratories who have participated in all of the comparisons. An improvement of the quality of results judged by the pass rate for all participants for the comparison exercises was observed in the past 6 years. At the beginning, some laboratories did not provide complete information of efficiency calibration and applied corrections. A few laboratories reported uncertainty values produced only by their software without knowing details what was included. They now calculated their own uncertainties. The comparisons have technical
137
Cs,
60
Co,
241
Am in a solid matrix sample comparison exercise in 2012.
meetings every year, so we discuss with participants on how they could improve their performance and provide advice, such as to use traceable calibration source correctly, to calibrate γ-spectrometers regularly, to improve evaluation of the measurement uncertainty quantitatively, and train staff to operate systems regularly for quality assurance. After discussions with participants during these technical meeting, participating laboratories have followed the international guides for uncertainty evaluations and combined “all sources” of uncertainty. Also, laboratories now give details of their calibration procedures, including the source of their reference material. The overall performance is continuously improving since the beginning of comparison exercises (Table 2). In the year of 2008 we had 87% of “acceptable results” performance and last years the performance came up to 92%.
4. Conclusions The total number of participant laboratories and complexity of the comparison samples increased over the years of the exercises. It indicates that comparisons have a better acceptance now among the participants, the number of participants has increased from 16 to 28 and their participation in the programmes give confidence to their measurements. Participation in these comparisons has required the laboratories to use traceable reference materials for
438
F. Tuo et al. / Applied Radiation and Isotopes 87 (2014) 435–438
supported financially by the Science Foundation of Ministry of Health (No. 201002009).
Table 2 Radionuclides and percentage of pass rate by year. Year
Gamma nuclide
Number of participating laboratories
Percentage of pass rate (%)
226
16 16 25
50 87 88
21 25 28
76 92 92
Ra, 232Th, 40K Ra, 232Th, 40K 226 Ra, 232Th, 40K, 137 Cs 2010 137Cs, 60Co 2011 137Cs, 60Co, 241Am 2012 137Cs, 60Co, 241Am
2007 2008 2009
226
their calibration which underpins the traceabilities of their results. Comparisons are an important tool to assess the Chinese laboratories performances. It is hoped that comparisons could contribute to the improvement of measurement quality and will contribute to an increased level of public confidence. However, there is also a need to calibrate γ-spectrometers regularly for accurate measurements, and to improve evaluation of the measurement uncertainty quantitatively for some laboratories. The future comparison exercises will focus on food and water characterized low levels of manmade radionuclides, which may be more challengeable for the analytical measurements. Acknowledgments The authors would like to thank the participants and laboratories participating in these comparison exercises. This work was
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