- Email: [email protected]
Radon-in-water standard
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 860–862
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radon-in-water standard Miroslav Havelka ´ 1, 102 00 Prague, Czech Republic Czech Metrology Institute, Inspectorate for Ionizing Radiation, Radiova
a r t i c l e in f o
Keywords: Radon-in-water standard 222 Rn standard solution Calibration
a b s t r a c t The radon-in-water standard installed at the Czech Metrology Institute in 1994 is based on a generator producing radium-free radon solution and on a measurement system for generator calibration and stability checking. The generator consists of about 6 L cylindrical vessel with a solid phase 222Rn source with 99.9% air emanation power and an external circuit for solution homogenisation. Standard solutions are prepared by charging water in the vessel with radon for an appropriate period; radon volume activity of the solution may vary from 300 to 2000 Bq/L; its uncertainty is less than 2.5%. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Any radon-in-water standard should provide 222Rn solution free of radium, with a well-defined radon concentration, so that this solution could be used as a standard for any calibration of radon-in-water measuring devices. Such standard was developed at Czech Metrology Institute (CMI) in 1994 after imposing new limits for the exposure of the population to 222Rn, which brought about the need for calibration and routine tests of the instruments measuring radon concentration in drinking water. Similar radon standards are in other institutes, e.g. NIST (Hutchinson et al., 1984), NPL (Dean and Kolkowski, 2004), ENEA (De Felice, 2001) and LPR (Luxembourg). The radon-in-water standard consists of two main components.
NIST and NPL standards use NIST radon emanation standards— sealed polyethylene capsules with standard solution of 226Ra (Volkovitsky, 2006). The CMI radon-in-water standard also differs in the measurement system, which is based on gamma-ray spectrometry, and in the volume of prepared solution. Per one accumulation process it is possible to take up to 4 L of the standard solution, which is sufficient volume for calibrations of all currently used instruments for the measurement of radon concentration in drinking water, including these requiring a large amount of sample (gamma-ray spectrometry systems).
2. Radon-in-water generator The generator consists of a cylindrical vessel, inside which a Rn source is placed, and an external circuit for solution homogenisation (Fig. 1). The accumulation vessel is a glass container of 6.5 L in volume, 75 cm in height and 11 cm in diameter. Its shape was selected with the intention of minimising radon loss due to the contact of the solution with air during sampling and minimising the time necessary for the solution homogenisation. The 222Rn source is a modified source developed at CMI in 1991 for radon gas standard (CMI Catalogue, 2008). The source has a form of 0.3 mm thick foil of dimensions about 40 mm 40 mm. The foil is fixed on a stainless steel holder placed at the upper part of the container and immersed in water during the accumulation phase. The activity of radium in the foil (nominal value 20 kBq) is known with the uncertainty of about 4%. It was measured by means of gamma-ray spectrometry comparing the 226Ra 185 keV peak count rate with that one of a standard 226Ra source of identical size. The source (foil) can be characterised by the emanation power measured in air and water surroundings. The emanation power in air was calcula ted from the activity of 226Ra and of 222Rn remaining in the source. These activities were derived from the results of gamma-ray 222
(a) Radon-in-water generator producing radon solution. (b) Measurement system used for calibration and stability checking of the radon-in-water generator. The radon-in-water generator contains a source with 226Ra that decays to 222Rn. Then radon is separated from radium source and dissolved in water thus forming radium-free radon solution. The separation is usually carried out by diffusion of radon through any plastic layer of the source, while radium remains in the source. Source diffusion parameters have a considerable influence on the source emanation power and subsequently on the radon concentration in the solution, so the source should have as high and stable emanation power as possible. There are two types of sources in all the mentioned standards. CMI, ENEA and LPR standards have solid phase 222Rn sources produced by CMI; Fax: +420 266020466.
E-mail address: [email protected] 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.047
ARTICLE IN PRESS M. Havelka / Applied Radiation and Isotopes 67 (2009) 860–862
Vacuum pump
861
pump with maximum flow rate of 12 L/min. Before the saturation process, radon is removed from the system by three times repeated filling with water and emptying. The standard radon solution is prepared at minimum radon loss from the solution by degasification and by diffusion into plastics, so than the volume activity of radon standard solution (AVRn) can be simply calculated from the time of the charging period, activity of radium in the source, emanation power and volume of the solution inside the apparatus according to theoretical equation (1) without any other correction
Air
Foil with Ra
Container 6.5 l
AVRn ¼
Pump
ARa ð1 elRn t Þ, V
(1)
where ARa, activity of 226Ra in the source (Bq), e, emanation power, V, volume of water in the source (L), lRn, 222Rn decay constant (s1), t, accumulation time (s). The accumulation period is chosen with regard to the desired radon volume activity of the solution. The minimum accumulation period that ensures acceptable uncertainty (2.5%) is about 16 h.
3. Gamma-ray spectrometry measurements
Standard solution
Fig. 1. Scheme of the CMI radon-in-water generator.
spectrometry measurement from 226Ra 185 keV and 214Pb 351 keV peak count rates taking into account differences in the detection efficiencies of both radionuclides. The measurement was carried out after 4 h of air blowing of the source when the equilibrium was reached between 222Rn remaining in the source and the daughter product 214Pb and both activities were equal. The air emanation power of the source before the first use in the generator was 0.9990 (70.0001) and decreased to 0.923 (70.005) after 14 years of usage. This decrease was mainly caused by inconsiderate cleaning procedure that had to be done when plague formed on the source surface, in the early phase of radon-in-water generator testing. No damage effect was observed (less than 0.01% degradation of the source emanation power), when the radon source was treated with distilled water for 80 days. The measurement of emanation power of the source in water (water emanation power) is more complicated then air emanation power measurement, because it is necessary to exclude the source contact with air. Therefore the water emanation power was calculated from the 226Ra activity in the foil, mounted in the generator, and activity of 222Rn in the resulting solution; this procedure led to the uncertainty of about 4%. No difference between the source emanation powers measured in water and air was observed. The activity of radium in the foil and the source air emanation power were measured only as parameters useful for long-term source checking and their values were not used for any calculation of the radon volume activity. For the preparation of the solution, the system is completely filled with water by means of a vacuum pump. The standard solutions are prepared by charging water filled in the vessel with radon for an appropriate period of time. The solution in the container is separated from the water in the mixing circuit (the pump) by valves for almost the whole accumulation period, except for 30 min at the end of this period when it is homogenised by the
4. Results and comments The theoretical equation (1) can be rewritten as AVRn/ (1elt) ¼ ARa e/V, where the right side is constant, if source emanation power is constant. The left side should also be constant for all the measured values of radon volume activity in solution (AVRn) and time (t). Hence the value of ratio AVRn/(1elt) was used for system stability checking of radon-in-water generator. The values of [AVRn/(1elt)] for 30 single solution preparations carried out in a four-year period (1998–2002) are presented in a graph (Fig. 2). The data were normalised to the value calculated from the results in 1994 (installation of the CMI radonin-water standard). All the data are characterised by a standard deviation resulting from the counting statistics of gamma-ray spectrometry measurements. The data were obtained under
1.010 Normalized Activity of Radon Solution
Water
Radon volume activity in the solution (AVRn) from the generator was measured in a 500 mL glass bulb after the establishment of equilibrium of 214Pb and 214Bi using gamma-ray spectrometry with the HPGe semiconductor detector. To exclude sorption of radon daughter products, a complex forming agent (3 g of 10% EDTA, disodium salt dihydrate) was added to the solution in the bulb. The calibration of the system was carried out by a 226Ra solution (with volume activity about 10 kBq/L) introduced in the glass bulb. Thus the radon-in-water standard is traceable to 226Ra.
1.005 1.000 0.995 0.990 0.985 0.980 0.975
0
300
600 Time (days)
900
1200
Fig. 2. Stability and reproducibility of radon-in-water generator. The standard deviation characterising the set of all the measured values is equal to 0.95%.
ARTICLE IN PRESS 862
M. Havelka / Applied Radiation and Isotopes 67 (2009) 860–862
different experimental conditions. Accumulation period varied from 1 to 10 d. Homogenisation time varied from 30 min to 4 h, because most of these results were obtained from the measurements of samples also provided to other laboratories for calibration purposes and the time schedule of this procedure did not permit to keep this period constant. The solutions were prepared from drinking water. The standard deviation characterising the set of all the measured values of normalised radon solution activity is equal to 0.95%, which confirmed good reliability and stability of the radon-in-water standard. The main component of deviations is related to the counting statistics, furthermore it was observed that the results were influenced by the temperature of the solution sample for gamma-ray spectrometry measurement. The temperature of the solution increased due to the heat generated in the pump and was dependent on homogenisation time. After cooling the warmed-up sample to the room temperature, an air bubble grew inside the measuring vessel and influenced detection efficiency. Spread of the results is also caused by the fact that radon diffused into the rubber parts (seals) of the apparatus and back to the solution. This mechanism was verified by a special test in which the source containing radium was removed from the generator and activity of the resulting solution was measured. Since both radon in water generator and the measuring system are stable, it is possible to derive the average value of expression ARa e/V from the measured results of radon volume activity and accumulation time using Eq. (1) and then to utilise this average value for the calculation of radon volume activity from the accumulation time according to Eq. (1). Therefore exact knowledge of the activity of the radium in source, source emanation power and volume of the generator is not necessary. The uncertainty of radon volume activity is then estimated from the uncertainty of the average value of the expression ARa e/V, (uncertainty of calibration of radon solution generator, 1.8%) where the predominant component of the uncertainty originates from the activity of 226Ra standard solution used for system calibration (1.5%) and the uncertainty of accumulation time (o0.5%); furthermore long-term instability effects as diffusion into the rubber parts, etc. also contribute to the final value; these
contributions was estimated from the graph (Fig. 2) such as o0.95%. The total uncertainty about 2.5% (k ¼ 1) relates to activity of the solution in the generator or solution transferred into the measurement bulb; any manipulation with the solution can cause radon loss from the solution, what usually manifests in the increase of the uncertainty.
5. Conclusion The radon-in-water standard showed high stability and the preparation of the standard radon solution was highly reproducible. The standard radon solution has the following parameters:
222Rn volume activity in the solution: 300–2000 Bq/L. The relative standard uncertainty in the whole range of the radon volume activity:o2.5%.
Content of 226Ra:o0.1 Bq/L. Net volume of the standard solution per one accumulation process: 4 L. The standard is used approximately 15 times a year for calibration of devices measuring radon concentration in drinking water. References Czech Metrology Institute, Inspectorate for Ionizing Radiation, Catalogue 2008. CMI-IIR radioactive standards, 222Rn flow through source /http://www.eurostandard.czS. Dean, J.C.J., Kolkowski, P., 2004. The development of a 222Rn standard solution dispenser at NPL. Appl. Radiat. Isot. 61, 95–100. De Felice, P., 2001. Taratura degli strumenti di misura del radon. Proceedings of the Workshop on Esposizioni da attivita lavorative con sorgenti naturali di radiazioni ionizzanti in stabilimenti termali, Montecatini Terme, 28–29 June, 2001. Hutchinson, J.M.R., Mullen, P.A., Colle, R., 1984. Development of regenerative radon-in-water standard. Nucl. Instrum. Methods Phys. Res. 222, 451–457. Volkovitsky, P., 2006. NIST Rn emission standards. Appl. Radiat. Isot. 64, 1249–1252.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radon-in-water standard Miroslav Havelka ´ 1, 102 00 Prague, Czech Republic Czech Metrology Institute, Inspectorate for Ionizing Radiation, Radiova
a r t i c l e in f o
Keywords: Radon-in-water standard 222 Rn standard solution Calibration
a b s t r a c t The radon-in-water standard installed at the Czech Metrology Institute in 1994 is based on a generator producing radium-free radon solution and on a measurement system for generator calibration and stability checking. The generator consists of about 6 L cylindrical vessel with a solid phase 222Rn source with 99.9% air emanation power and an external circuit for solution homogenisation. Standard solutions are prepared by charging water in the vessel with radon for an appropriate period; radon volume activity of the solution may vary from 300 to 2000 Bq/L; its uncertainty is less than 2.5%. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Any radon-in-water standard should provide 222Rn solution free of radium, with a well-defined radon concentration, so that this solution could be used as a standard for any calibration of radon-in-water measuring devices. Such standard was developed at Czech Metrology Institute (CMI) in 1994 after imposing new limits for the exposure of the population to 222Rn, which brought about the need for calibration and routine tests of the instruments measuring radon concentration in drinking water. Similar radon standards are in other institutes, e.g. NIST (Hutchinson et al., 1984), NPL (Dean and Kolkowski, 2004), ENEA (De Felice, 2001) and LPR (Luxembourg). The radon-in-water standard consists of two main components.
NIST and NPL standards use NIST radon emanation standards— sealed polyethylene capsules with standard solution of 226Ra (Volkovitsky, 2006). The CMI radon-in-water standard also differs in the measurement system, which is based on gamma-ray spectrometry, and in the volume of prepared solution. Per one accumulation process it is possible to take up to 4 L of the standard solution, which is sufficient volume for calibrations of all currently used instruments for the measurement of radon concentration in drinking water, including these requiring a large amount of sample (gamma-ray spectrometry systems).
2. Radon-in-water generator The generator consists of a cylindrical vessel, inside which a Rn source is placed, and an external circuit for solution homogenisation (Fig. 1). The accumulation vessel is a glass container of 6.5 L in volume, 75 cm in height and 11 cm in diameter. Its shape was selected with the intention of minimising radon loss due to the contact of the solution with air during sampling and minimising the time necessary for the solution homogenisation. The 222Rn source is a modified source developed at CMI in 1991 for radon gas standard (CMI Catalogue, 2008). The source has a form of 0.3 mm thick foil of dimensions about 40 mm 40 mm. The foil is fixed on a stainless steel holder placed at the upper part of the container and immersed in water during the accumulation phase. The activity of radium in the foil (nominal value 20 kBq) is known with the uncertainty of about 4%. It was measured by means of gamma-ray spectrometry comparing the 226Ra 185 keV peak count rate with that one of a standard 226Ra source of identical size. The source (foil) can be characterised by the emanation power measured in air and water surroundings. The emanation power in air was calcula ted from the activity of 226Ra and of 222Rn remaining in the source. These activities were derived from the results of gamma-ray 222
(a) Radon-in-water generator producing radon solution. (b) Measurement system used for calibration and stability checking of the radon-in-water generator. The radon-in-water generator contains a source with 226Ra that decays to 222Rn. Then radon is separated from radium source and dissolved in water thus forming radium-free radon solution. The separation is usually carried out by diffusion of radon through any plastic layer of the source, while radium remains in the source. Source diffusion parameters have a considerable influence on the source emanation power and subsequently on the radon concentration in the solution, so the source should have as high and stable emanation power as possible. There are two types of sources in all the mentioned standards. CMI, ENEA and LPR standards have solid phase 222Rn sources produced by CMI; Fax: +420 266020466.
E-mail address: [email protected] 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.047
ARTICLE IN PRESS M. Havelka / Applied Radiation and Isotopes 67 (2009) 860–862
Vacuum pump
861
pump with maximum flow rate of 12 L/min. Before the saturation process, radon is removed from the system by three times repeated filling with water and emptying. The standard radon solution is prepared at minimum radon loss from the solution by degasification and by diffusion into plastics, so than the volume activity of radon standard solution (AVRn) can be simply calculated from the time of the charging period, activity of radium in the source, emanation power and volume of the solution inside the apparatus according to theoretical equation (1) without any other correction
Air
Foil with Ra
Container 6.5 l
AVRn ¼
Pump
ARa ð1 elRn t Þ, V
(1)
where ARa, activity of 226Ra in the source (Bq), e, emanation power, V, volume of water in the source (L), lRn, 222Rn decay constant (s1), t, accumulation time (s). The accumulation period is chosen with regard to the desired radon volume activity of the solution. The minimum accumulation period that ensures acceptable uncertainty (2.5%) is about 16 h.
3. Gamma-ray spectrometry measurements
Standard solution
Fig. 1. Scheme of the CMI radon-in-water generator.
spectrometry measurement from 226Ra 185 keV and 214Pb 351 keV peak count rates taking into account differences in the detection efficiencies of both radionuclides. The measurement was carried out after 4 h of air blowing of the source when the equilibrium was reached between 222Rn remaining in the source and the daughter product 214Pb and both activities were equal. The air emanation power of the source before the first use in the generator was 0.9990 (70.0001) and decreased to 0.923 (70.005) after 14 years of usage. This decrease was mainly caused by inconsiderate cleaning procedure that had to be done when plague formed on the source surface, in the early phase of radon-in-water generator testing. No damage effect was observed (less than 0.01% degradation of the source emanation power), when the radon source was treated with distilled water for 80 days. The measurement of emanation power of the source in water (water emanation power) is more complicated then air emanation power measurement, because it is necessary to exclude the source contact with air. Therefore the water emanation power was calculated from the 226Ra activity in the foil, mounted in the generator, and activity of 222Rn in the resulting solution; this procedure led to the uncertainty of about 4%. No difference between the source emanation powers measured in water and air was observed. The activity of radium in the foil and the source air emanation power were measured only as parameters useful for long-term source checking and their values were not used for any calculation of the radon volume activity. For the preparation of the solution, the system is completely filled with water by means of a vacuum pump. The standard solutions are prepared by charging water filled in the vessel with radon for an appropriate period of time. The solution in the container is separated from the water in the mixing circuit (the pump) by valves for almost the whole accumulation period, except for 30 min at the end of this period when it is homogenised by the
4. Results and comments The theoretical equation (1) can be rewritten as AVRn/ (1elt) ¼ ARa e/V, where the right side is constant, if source emanation power is constant. The left side should also be constant for all the measured values of radon volume activity in solution (AVRn) and time (t). Hence the value of ratio AVRn/(1elt) was used for system stability checking of radon-in-water generator. The values of [AVRn/(1elt)] for 30 single solution preparations carried out in a four-year period (1998–2002) are presented in a graph (Fig. 2). The data were normalised to the value calculated from the results in 1994 (installation of the CMI radonin-water standard). All the data are characterised by a standard deviation resulting from the counting statistics of gamma-ray spectrometry measurements. The data were obtained under
1.010 Normalized Activity of Radon Solution
Water
Radon volume activity in the solution (AVRn) from the generator was measured in a 500 mL glass bulb after the establishment of equilibrium of 214Pb and 214Bi using gamma-ray spectrometry with the HPGe semiconductor detector. To exclude sorption of radon daughter products, a complex forming agent (3 g of 10% EDTA, disodium salt dihydrate) was added to the solution in the bulb. The calibration of the system was carried out by a 226Ra solution (with volume activity about 10 kBq/L) introduced in the glass bulb. Thus the radon-in-water standard is traceable to 226Ra.
1.005 1.000 0.995 0.990 0.985 0.980 0.975
0
300
600 Time (days)
900
1200
Fig. 2. Stability and reproducibility of radon-in-water generator. The standard deviation characterising the set of all the measured values is equal to 0.95%.
ARTICLE IN PRESS 862
M. Havelka / Applied Radiation and Isotopes 67 (2009) 860–862
different experimental conditions. Accumulation period varied from 1 to 10 d. Homogenisation time varied from 30 min to 4 h, because most of these results were obtained from the measurements of samples also provided to other laboratories for calibration purposes and the time schedule of this procedure did not permit to keep this period constant. The solutions were prepared from drinking water. The standard deviation characterising the set of all the measured values of normalised radon solution activity is equal to 0.95%, which confirmed good reliability and stability of the radon-in-water standard. The main component of deviations is related to the counting statistics, furthermore it was observed that the results were influenced by the temperature of the solution sample for gamma-ray spectrometry measurement. The temperature of the solution increased due to the heat generated in the pump and was dependent on homogenisation time. After cooling the warmed-up sample to the room temperature, an air bubble grew inside the measuring vessel and influenced detection efficiency. Spread of the results is also caused by the fact that radon diffused into the rubber parts (seals) of the apparatus and back to the solution. This mechanism was verified by a special test in which the source containing radium was removed from the generator and activity of the resulting solution was measured. Since both radon in water generator and the measuring system are stable, it is possible to derive the average value of expression ARa e/V from the measured results of radon volume activity and accumulation time using Eq. (1) and then to utilise this average value for the calculation of radon volume activity from the accumulation time according to Eq. (1). Therefore exact knowledge of the activity of the radium in source, source emanation power and volume of the generator is not necessary. The uncertainty of radon volume activity is then estimated from the uncertainty of the average value of the expression ARa e/V, (uncertainty of calibration of radon solution generator, 1.8%) where the predominant component of the uncertainty originates from the activity of 226Ra standard solution used for system calibration (1.5%) and the uncertainty of accumulation time (o0.5%); furthermore long-term instability effects as diffusion into the rubber parts, etc. also contribute to the final value; these
contributions was estimated from the graph (Fig. 2) such as o0.95%. The total uncertainty about 2.5% (k ¼ 1) relates to activity of the solution in the generator or solution transferred into the measurement bulb; any manipulation with the solution can cause radon loss from the solution, what usually manifests in the increase of the uncertainty.
5. Conclusion The radon-in-water standard showed high stability and the preparation of the standard radon solution was highly reproducible. The standard radon solution has the following parameters:
222Rn volume activity in the solution: 300–2000 Bq/L. The relative standard uncertainty in the whole range of the radon volume activity:o2.5%.
Content of 226Ra:o0.1 Bq/L. Net volume of the standard solution per one accumulation process: 4 L. The standard is used approximately 15 times a year for calibration of devices measuring radon concentration in drinking water. References Czech Metrology Institute, Inspectorate for Ionizing Radiation, Catalogue 2008. CMI-IIR radioactive standards, 222Rn flow through source /http://www.eurostandard.czS. Dean, J.C.J., Kolkowski, P., 2004. The development of a 222Rn standard solution dispenser at NPL. Appl. Radiat. Isot. 61, 95–100. De Felice, P., 2001. Taratura degli strumenti di misura del radon. Proceedings of the Workshop on Esposizioni da attivita lavorative con sorgenti naturali di radiazioni ionizzanti in stabilimenti termali, Montecatini Terme, 28–29 June, 2001. Hutchinson, J.M.R., Mullen, P.A., Colle, R., 1984. Development of regenerative radon-in-water standard. Nucl. Instrum. Methods Phys. Res. 222, 451–457. Volkovitsky, P., 2006. NIST Rn emission standards. Appl. Radiat. Isot. 64, 1249–1252.