Radon Concentration Intercomparison in Serbia

RADON CONCENTRATION INTERCOMPARISON IN SERBIA

Gordana Pantelić1, Miloš Živanović1, Maja Eremić Savković2 and Sofija Forkapić3 1Vinča Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia 2Serbian Institute of Occupational Health "Dr Dragomir Karajović", Belgrade, Serbia 3Department of Physics, Faculty of Science, University of Novi Sad, Novi Sad, Serbia [email protected]

INTRODUCTION Radon is a colourless and odourless radioactive gas, produced by the radioactive decay of radium, a naturally occurring radioactive element that is found in trace amounts in all soils as well as building materials. Radon gets into buildings mainly through cracks in ﬂoors or gaps around pipes or cables. Airborne radon can be released during normal household activities, such as showering, dishwashing, etc. Radon is and always has been a natural component of the air we breathe and can pose a huge potential health risk. According to Law on Ionizing Radiation Protection and on Nuclear Safety [1] radioactivity testing in Serbia may be performed only by accredited laboratories. Accredited laboratories ought to participate in interlaboratory comparison or proficiency-testing programme for each accredited method [2]. The call for intercomparison was announced in July 2012 and it was conducted in August 2012. The participants were Serbian Institute of Occupational Health "Dr Dragomir Karajović", Department of Physics at Faculty of Sciences, University of Novi Sad and Radiation Protection Laboratory, Vinča Institute of Nuclear Science. The laboratories are practicing the same method for radon measurement using charcoal canisters US EPA protocol 520/5-87-005 [3]. This method is based on the high affinity of activated charcoal for several gasses and vapors, one of which is 222Rn. Radon is adsorbed onto the charcoal grains and decays to radon short- lived progenies: 218Po, 214Pb, 214Bi, 214Po and 210Pb.

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The results of intercomparison were evaluated by using the u-test. The u-score value was calculated according to the IAEA criteria [4], by using the following formula: | A  A | u  s l (1) 2 2 us  ul where |As – Al| is the difference between the reference value and measured value, ul the total uncertainty of the measured value, and us the total uncertainty of the reference value. It was decided to use the calculated mean value of all the measurements that do not differ more than 10 % from the mean as reference value. Measurements with u-score lower than or equal to 2.58 were considered acceptable.

CHARCOAL EXPOSURE Eight canisters were exposed at two sites in Vinča Institute, four canisters were used in each location simultaneously. Exposure times were between 2 and 3 days (Table 1). Difference in masses before and after exposure was measured in order to perform the correction for humidity. Canisters were positioned at 1 m distance from floor and walls when possible, or as close to 1 m otherwise.

Table 1. Radon canisters exposure data No. of Start Exposure Exposure End Location canister mass (g) start end mass (g) 24.8.2012. 27.8.2012. 1 160.51 164.89 14:54 8:17 Vinča, 24.8.2012. 27.8.2012. 2 160.90 165.33 building 14:54 8:17 24, office 24.8.2012. 27.8.2012. 3 162.22 164.69 14:55 8:18 24.8.2012. 27.8.2012. 4 161.45 163.96 14:55 8:18 24.8.2012. 27.8.2012. 5 161.25 167.79 15:03 9:40 Vinča, 24.8.2012. 27.8.2012. 6 160.77 167.51 building 15:03 9:40 21, ware- 24.8.2012. 27.8.2012. 7 161.42 166.19 house 15:03 9:40 24.8.2012. 27.8.2012. 8 161.76 166.41 15:03 9:40

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The canisters were exposed by removing the metal lid. During the exposure, radon is being adsorbed to and desorbed from the activated charcoal, and it also undergoes radioactive decay. After the exposure, canisters were sealed by replacing the metal lid and vinyl tape.

MEASUREMENT METHOD Measurement was conducted by the gamma spectrometry of radon progeny. Three gamma lines were used, 295 keV and 352 keV lines of 214Pb and 609 keV line of 214Bi. It is necessary to wait at least 3 hours between sealing the canister and measurement, in order to attain radioactive equilibrium. Due to all the processes that take place during and after exposure of canisters, gamma photons count rate is not a linear function of the exposure time of the canister. Canisters are therefore calibrated by using radon calibration chambers (chambers with known radon concentrations and with controlled environmental conditions) for defined exposure periods [3]. Standard and background canisters were used for QA&QC, as well as for the calibration of the measurement equipment. Standard canister is a sealed canister with the same matrix and geometry as the canisters used for measurements, but with the known activity of radon. This is achieved by placing a known amount of 226Ra, which is the parent radionuclide of 222Rn and which has the half-life of 1600 years. This canister was used for determining the efficiency of the detector system. Background canister is a regular radon measurement canister, which has never been exposed. Before every exposure, canisters must be appropriately prepared. This process is done by heating open canisters for a set period of time at 110 °C. During the heating, adsorbed water vapor, radon and its progeny, as well as other adsorbed species, are being desorbed. After the heating, canisters were sealed and were ready for the next exposure.

RESULTS All eight charcoal canisters were measured in all three participating laboratories on gamma spectrometers. The characteristics of HP Ge detectors (manufacturer, resolution and relative efficiency at 1332 keV), thickness of Pb shield, software for peak determination and measurement time are shown in Table 2. Key to the columns in Table 2 is the following: number 1 corresponds to Institute Vinča, number 2 to Institute "Dr Dragomir Karajović" and number 3 to the University of Novi Sad.

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Table 2. HP Ge detectors characteristics Institute 1 2 3 Producer Canberra ORTEC Canberra Resolution (1332 keV) 1.9 keV 1.85 keV 1.79 keV Relative efficiency 50 % 40 % 36 % Thickness of Pb shield 10 cm 10 cm 12 cm Canberra Gamma Canberra Software Genie 2000 Vision®-32 Genie 2000 Measurement time 1800 s 1800 s 3000 s

Radon concentration as well as measurement uncertainty was calculated according to US EPA protocol 520/5-87-005 [3]. For each series of canisters bought from the F&J Speciality Products, INC, laboratories obtained two calibration curves. The first one relates calibration factor to weight gain (water) for the canisters for a 2-day exposure (48 h). The second curve relates exposure time to adjustment factors for 20, 50 and 80 percent humidity. The adjustment factor is used to modify the calibration factor for exposure times different from the desired 2-day exposure time. If a canister is exposed for exactly 2 days, the adjustment factor is 1. Each laboratory corrects the results with calibration factor and with adjustment factor. The activities of radon concentrations were calculated independently, and are given in Table 3. The measurements that meet the criteria to be included in the mean value calculation are shaded. Key to the columns in Table 3 is the same as in Table 2.

Table 3. Results of radon concentration measurements Radon concentration (Bq/m3) No. 1 2 3 mean 1 91 ± 7 104 ± 7 119 ± 9 2 92 ± 7 86 ± 7 91 ± 6 91 ± 6 3 80 ± 6 96 ± 6 89 ± 6 4 74 ± 6 91 ± 6 90 ± 4 5 134 ± 10 103 ± 6 182 ± 10 6 134 ± 10 155 ± 8 160 ± 8 148 ± 22 7 126 ± 9 146 ± 8 106 ± 6 8 121 ± 9 156 ± 8 181 ± 8

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The mean value with 2 standard deviation is shown in the last column. The u-scores were calculated for all the measurements by using formula (1), and it shows that only one measurement does not meet the pre-set criteria. The obtained results are presented in Table 4.

Table 4. The final results of intercomparison Institute "Dr University of Lab. Institute Vinča Dragomir Karajović" Novi Sad

No. u Al/As Final u Al/As Final u Al/As Final 1 0.00 1.00 A 1.41 1.14 A 2.59 1.31 N 2 0.11 1.01 A 0.54 0.95 A 0.00 1.00 A 3 1.30 0.88 A 0.59 1.05 A 0.24 0.98 A 4 2.00 0.81 A 0.00 1.00 A 0.14 0.99 A 5 0.58 0.91 A 1.97 0.70 A 1.41 1.23 A 6 0.58 0.91 A 0.30 1.05 A 0.51 1.08 A 7 0.93 0.85 A 0.09 0.99 A 1.84 0.72 A 8 1.14 0.82 A 0.34 1.05 A 1.41 1.22 A

CONCLUSION The performance evaluation results showed that 95.8 % of all reported results obtained a score of "Acceptable" (A). From the comparison of the performance of these 3 laboratories, it can be seen that all of them had an excellent performance in this intercomparison, which indicates the stability of the performance of the analytical system in these laboratories.

REFERENCES [1] Zakon o zaštiti od jonizujućih zračenja i o nuklearnoj sigurnosti, Sl. Glasnik Republike Srbije, 39, 2009. [2] General requirements for the competence of testing and calibration laboratories, ISO/IEC 17025:2006. [3] Grey DJ, Windham ST, EERF Standard Operating Procedures for Radon- 222 Measurement Using Charcoal Canisters, EPA 520/5-87-005, 1987. [4] ALMERA Proficiency Test on the Determination of Gamma Emitting Radionuclides in Spinach, Soil, and Water, IAEA-CU-2007-04, IAEA Analytical Quality in Nuclear Applications No. IAEA/AQ/3 International Atomic Energy Agency, 2009. ISSN 2074–7659.

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RADON CONCENTRATION INTERCOMPARISON IN SERBIA

According to Law on Ionizing Radiation Protection and on Nuclear Safety radioactivity testing in Serbia may be performed only by accredited laboratories. Accredited laboratories ought to participate in interlaboratory comparison or proficiency-testing programme for each accredited method. The participants in radon concentration intercomparison in 2012 in Serbia were three laboratories. The laboratories are practicing the same method for radon measurement using charcoal canisters US EPA protocol 520/5-87- 005, 1987. The results of intercomparison were evaluated by using the u-test which was calculated according to the IAEA criteria. Measurements with u- score lower than or equal to 2.58 are considered acceptable. Eight canisters were exposed at two sites in Vinča Institute, four canisters were used in each location simultaneously. Exposure times were between 2 and 3 days. Difference in masses before and after exposure was measured in order to perform the correction for humidity. Standard and background canisters are used for the calibration of the measurement equipment. Standard canister is a sealed canister with the same matrix and geometry as the canisters used for measurements, but with the known activity of radon. All eight charcoal canisters were measured in all three participating laboratories on HP Ge detectors. Each laboratory corrects the results with calibration factor and with adjustment factor obtained from canisters manufacturer. The activities of radon concentrations were calculated independently. From the comparison of the performance of these 3 laboratories, it can be seen that all of them had an excellent performance in this intercomparison, which indicates the stability of the performance of the analytical systems in these laboratories.

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