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A primary standard source of radon-222 based on the HPGe detector
Applied Radiation and Isotopes 120 (2017) 101–105
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A primary standard source of radon-222 based on the HPGe detector a,⁎
M.Y.A. Mostafa , M. Vasyanovich a b
a,b
, M. Zhukovsky
b
MARK
Ural Federal University, Ekaterinburg, Russia Institute of Industrial Ecology UB RAS, Ekaterinburg, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Radon measurement Radon standard Solid radium source Emanation
The present paper describes the prototype of a calibration standard system for radon concentrations to be used in establishing the traceability of radon concentration measurements in dwellings. Radon gas was generated with a radium-226 solid source in a certified volume as a closed system. The activity of the radon that was released in the closed system was determined from the difference between the absolute activity of the standard radium solid source and the residual radon decay products (214Bi or 214Pb). A high-purity germanium (HPGe) detector, which was calibrated using gamma reference standard sources, was used to measure the activity of a radium solid source and radon decay products (214Bi or 214Pb). The emanation factor of the 226Ra source was controlled online with the HPGe detector. Radon activity was achieved at ~1500 ± 45 Bq from the radium source at 3.95 ± 0.2 kBq under equilibrium conditions. After this activity, the radon gas was transferred into the closed system producing radon activity concentrations of 31.1 ± 0.3 kBq/m3. Systematic errors were found of less than 4% with a random error around 0.5%. The random error is generally associated with the estimation of the count rate of the measured radon progenies (214Po and 214Po for alpha measurements or 214Pb and 214Bi for gamma measurements), but systematic errors are associated with the errors introduced by the instrumentation and measurement technique. The system that was developed has a high degree of accuracy and can be recommended as a national or regional prototype standard of radon activity concentration to calibrate different working radon measurement devices.
1. Introduction
2012; Lee et al., 2013; Röttger et al., 2014) or by using radon itself in an absolute calibration method (Busch et al., 2002; Spring et al., 2006; Cassette et al., 2006; Nedjadi et al., 2007; Sahagia et al., 2010). Critical analysis of all of these methods was done by De Felice in 2007. All these methods were accomplished through the realization of a complex system for the generation, circulation, and recovery of the radon standard source or system. Because the existence of a radon standard system for radon sources is very important, over the last year, a series of experiments on radon equilibrium equivalent concentrations (EECs) (Mostafa et al., 2015) were conducted and a radon activity concentration standard system was launched (Mostafa et al., 2016). The radon concentration standard system in open flow mode was realised by the pumping of air through an emanation chamber containing the radium source. In this situation, a 226Ra source with high activity (~33 kBq) was used and the controlling of flow rate was necessary (Mostafa et al., 2016). In this present work, we have upgraded the system with a closed system and used a relatively small activity source of ~4 kBq in the calibration. Because the system is closed, the value of the flow rate does not influence the radon concentration because the ratio of (flow rate)/
The quality assurance and state of a device must be calibrated in order to quantitatively measure radiation. Radiation monitoring devices are often calibrated using standard sources. However, in the case of radon monitoring devices, calibration is difficult because of the short half-life of radon and its status as a noble gas. Therefore, the use of the radon (222Rn) primary standards system is important because it provides an opportunity to obtain reference sources or systems to assure the traceability of radon measurements under various conditions. In 1996, Picolo reported the first absolute measurement method for radon based on the detection of alpha particles directly emitted by 222 Rn (Picolo, 1996). This method was also successfully adopted by Dersch (1998, 2004) and Spring et al. (2006). Other methods of standardization systems for 222Rn measurements in the atmosphere have been developed, either by measuring the gamma-emitting decay products of 222Rn gas traceable to a 226Ra standard reference material (SRM) (Hutchinson et al., 1992; Sakamoto et al., 2005; López-Coto et al., 2007; Röttger and Honig, 2011; Heidary et al., 2011; Kim et al.,
⁎
Corresponding author. E-mail address: [email protected] (M.Y.A. Mostafa).
http://dx.doi.org/10.1016/j.apradiso.2016.12.012 Received 24 February 2016; Received in revised form 13 September 2016; Accepted 8 December 2016 Available online 09 December 2016 0969-8043/ © 2016 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 120 (2017) 101–105
M.Y.A. Mostafa et al.
AlphaGUARD monitor in a diffusion mode and the results stored in the AlphaGUARD memory. At the same time, the gamma spectrum of the 226 Ra solid source was measured continuously every 24 h during the calibration period. The environmental parameters for pressure, temperature, and humidity were measured with the AlphaGUARD sensors in the closed measurement circuit during the whole experiment. In this experiment, an AlphaGUARD (PQ2000Pro) radon monitor was used as a reference device (a secondary standard to test the radon activity concentration). A 60-min diffusion mode was standard for our version of the AlphaGUARD PQ2000Pro equipment. This AlphaGUARD was calibrated using a NIST calibration source (Mostafa et al., 2016). Using the NIST source, the calibration error (including source activity error, random errors during measurements, etc.) was approximately 8– 9%. As in Fig. 1, the solid 226Ra source was assembled in the emanation box and the box was installed on the HPGe detector for online γ-ray spectrometry. The emanation box included a slot for the volume flow and the transport of the radon gas to the reference volume. The emanation factor of the 222Rn gas from the solid 226Ra source was controlled (measured continually) online with the HPGe detector by measuring the residual activity of 214Bi (Eγ=609.3 keV) activity in the solid radium source and comparing this value with the absolute activity of the 226Ra source. The stability of gamma spectrometry was controlled with standard 137Cs source installed in the emanation box. The calibration process with reference atmosphere was performed over a time period of almost 40 days. During this period, the reference volume had to be completely isolated from any surrounding influences. Thus, the components of the radon reference chamber are made of stainless steel. Therefore, there is no chance for any additional source of radon to enter (Linzmaier and Röttger 2013). For this procedure, if the radon gas emanation is constant, the emanation factor χ is calculated using:
(emanation chamber volume) is considerably greater than the half-life of radon. In this case, we can neglect the error from flow rate fluctuations, which improve the quality of the calibration. In general, the calibration of the 222Rn activity concentration standard for further transfer is possible using a radium standard source with the release of an activity concentration C by means of a reference atmosphere. In this case, the released radon activity concentration (C) equals A/V Bq/m3, where A is the activity (Bq) of released radon from a solid radium standard source, and V is the calibration volume (m3). The radon gas is emanated from a solid 226Ra standard source and the emanation factor was controlled online with a gamma spectrometer. The radon activity, which was emanated and released in the closed system, was calculated absolutely from the residual activity in the solid 226Ra standard source. A high-purity germanium (HPGe) detector was used to measure the absolute activity of the 226Ra with online controlling of the radon emanation (by measuring the 214Bi activity, the progeny of 226Ra) to produce the released absolute activity of 222Rn. An alternative approach to 222Rn metrology, based on the use of this prototype of the 222Rn concentration standard was proposed. In this approach, no reference measurements for the 222Rn are needed in the calibration process and the uncertainty of the reference 222Rn activity concentrations in the standard radon test system was kept to around 3% (P=0.95). Using this prototype of the radon concentration standard source, the calibration of radon concentration measurement devices is greatly simplified and the measurement accuracy is improved. 2. Experimental set-up and procedure A schematic drawing of the calibration system is shown in Fig. 1. The traceable volume for the generation of the reference radon atmosphere consists of a standard chamber with a certified volume of (5.04×10−2 ± 5×10−4 m3) 50.4 l and an emanation box (2.1×10−4 ± 1×10−6 m3). The total volume of the complete system including all experimental components is (48.1 ± 0.5)×10−3 m3 with a systematic volume uncertainty ΔVsys= ± 1%. The emanation box was installed on the HPGe detector for online gamma measurement. A 226Ra standard source (national primary standard source from D.I. Mendeleyev Institute for Metrology) was placed inside this box. The certified activity according to original certification was (3.95 ± 0.2 kBq (K=2)). The activity of the 226Ra measured using the 186-keV line directly with our HPGe detector was (3.98 ± 0.12 kBq). The emanated radon from the solid radium source was cycled and accumulated through the radon chamber in a closed system with a flow rate of (10−3 m3/min). An AlphaGUARD (PQ2000Pro) radon monitor was located inside the radon chamber. Radon concentration in the chamber was measured directly with the
χ222Rn=
A (222Rn) =1 − A (226Ra)
A (214Bi) A (226Ra)
(1)
where A is the activity. In general, for the emanation source, the emanation factor is fulfilled by the equation 0≤χ≤1. The velocity of the generated radon activity in the closed radon system is (ARn= NRn.λRn) and λRn is the radon decay constant. At the constant χ, the radon activity concentration CRn at time (t) is calculated by Eq. (2). The radon activity concentration in the reference volume (total volume of the whole closed system) starts to increase according to this equation until the equilibrium is achieved (99% after ~600 h). If the emanation factor changes with time, in this case, Eq. (2) cannot describe the activity concentration of radon and the source cannot be used in the radon standard system. Eqs. (3) and (4) present the dependence of the radon activity concentration on the variance of the emanation factor
Fig. 1. Schematic drawing of the calibration system.
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M.Y.A. Mostafa et al.
Table 1 Systematic and random error values for the gamma spectrometry (HPGe) and (AlphaGUARD). Type of measurement
AlphaGUARD
HPGe detector
Random errors Systematic errors
Δrα = ± 4. 5% Δεsys= ± 6%
Δrγ = ± 0. 5% Δεsys= ± 1. 8%
ΔVsys= ± 1%
ΔVsys= ± 1%
—
Δεsys (Cs−137) = ± 0. 54%
—
Δεsys (Ra−226) = ± 3%
Total systematic error (HPGe)
∆γsys = ± K√ (Δεsys (214Bi))2+ (ΔVsys )2+ (Δεsys (137Cs))2+ (Δεsys (226Ra))2= ± 3.6%
Total systematic error (AlphaGUARD)
A ∆sys = ± K√ (Δεsys) 2 +(ΔVsys) = ± 6.7%
Total errors
ΔTα = (∆αsys )2+(Δrα )2 = ± 8.8%
2
ΔTγ = (∆γsys )2+(Δrγ )2 = ± 3.65%
*K=1.1.
0.1
t
CRn (t)=
∫
Relative Efficiency
0
dCRn dt dt
(4)
Because the radon gas was removed very quickly from the emanation box by airflow, there were no effects observed of wall attachment of radon decay products in the volume of the emanation box. Wall attachment of radon decay products can be checked at the end of the experiment when the emanation source is quickly removed from the emanation box and the empty emanation box is measured with γspectrometry. Within the limits of the measurement error and the detection limit, there were no additional background counts in the emanation box.
0.01
1E-3
3. Results and discussion 100
3.1. Systematic and random errors
1000
Energy (KeV)
In general, experimental uncertainty is due to either random errors or systematic errors. For any radiation standard system, both random and systematic errors should be estimated accurately. Random errors are statistical fluctuations in the measured data due to the precision limits of the measurement instrument. On the other hand, systematic errors are reproducible errors that present consistently in the same direction. Systematic errors are often due to a problem that persists throughout the entire experiment. In general, systematic errors are related to the errors introduced by instruments and measurement technique. For the radon standard, random error is generally associated with the estimation of the count rate of measured radon decay products (218Po and 214Po for alpha measurements or 214Pb and 214Bi for gamma measurements) or error in activity concentration using radon monitors; but systematic errors are associated with the errors introduced by the instrumentation and measurement technique. In our case, random error was generally associated with the estimation of the count rate for the measured radon decay products (214Bi for gamma measurements) in addition to errors in the estimated activity concentration with the AlphaGUARD monitor. The systematic errors included errors in determining the activity of nuclides using polynomial approximation of registration efficiency of the HPGe detector especially in the range of the radon progeny emission line 214Bi (Eγ=609.3 keV). In addition, errors in determining the detection efficiency of the α-radiation monitor (AlphaGUARD) and errors in the total system volume were considered to be systematic errors. The systematic and random errors that were obtained by the HPGe detector and AlphaGUARD monitor are presented in Table 1. In the gamma measurements, the average relative uncertainty of the measured net count for the 214Bi line (609.3 keV) (random error) was around Δrγ =0. 5%. On the other hand, due to the continuous measurements of radon activity concentration, the mean value of the random measurement error for the AlphaGUARD monitor ∆αr was ± 4.5%. It can
Fig. 2. The polynomial peak efficiency of HPGe is estimated using standard sources (241Am, 133Ba, and 152Eu).
Relative emanation factor
0.40
Emanation factor Average ( 0.3749 ± 0.0032
0.39
0.38
0.37
0.36
0.35
0
5
10
15
20
25
30
35
40
45
Days Fig. 3. The online measured emanation coefficient of an emanation radium source.
over time.
CRn (t)=
ARa . χ (1 − exp(−λRn t)) V
(2)
In the case of the emanation factor variation over time
A . λ .χ(t) dCRn =−λRn CRn + Ra Rn dt V
(3)
103
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M.Y.A. Mostafa et al.
Fig. 4. Radon activity concentration calculated with the mean value of the emanation factor compared with the measured value using the AlphaGUARD radon monitor and the recorded metrological parameters.
factor during the period of calibration with variance of not more 1%. It gives the ability to use the mean value of the emanation factor during the measurement period to calculate the generated activity concentration inside the volume of the closed system over time using Eq. (2). During the calibration test, the radon activity concentration was measured with the AlphaGUARD monitor in diffusion mode continuously. The AlphaGUARD results were compared with the calculated activity concentration using the measured mean emanation factor of radon, which was released from the solid 226Ra source in a certified volume of the closed system. The mean activity of the emanated radon in the volume of the closed system was 1500 ± 45 Bq. It was calculated with a mean value for the emanation factor (0.3749 ± 0.0032) and the standard activity of the radium source (3.98 ± 0.12 kBq). The total volume of the closed system was (48.1 ± 0.5) x10−3 m3. Finally, the mean radon concentration in the closed system when saturated was 31.1 ± 0.3 kBq/m3. Fig. 4 shows the comparison between the calculated values of radon activity concentration, using Eq. (2), and the mean value of the emanation factor and the radon activity concentration, which was measured with the AlphaGUARD monitor continuously. According to Eq. (2), the measured activity concentration of radon, with the continuous AlphaGUARD monitor, starts to increase over time until the equilibrium starts to achieve between 226Ra and 222Rn after nearly 26 days (99%). In the same way, the calculated activity concentration using the emanation factor, the standard 226Ra activity measured with HPGe, and the certified volume of the closed system behave in nearly the same manner (in Eq. (2)) from the start to the end of the calibration experiment. In this case, the stability of the meteorological conditions during the experiment was also observed. From Fig. 4, it could be seen that there is good agreement between the measured and the calculated values for radon activity concentration during the calibration period within the error limits. The difference between the results of the AlphaGUARD measurement and the calculated curve is in the range of 3%. The recorded metrological parameters with the AlphaGUARD are also presented in Fig. 4, with almost no influential changes in these parameters during the experiment.
be observed that the total systematic errors for the alpha measurement are higher than the gamma measurement errors. In the same way, the total errors by gamma measurements are lower than the total errors from the AlphaGUARD by a factor of two. All indicators show that the prototype system developed has sufficient accuracy to be considered as radon standard system. 3.2. Radioactivity measurements using germanium detectors The polynomial dependence of the peak efficiency of the HPGe detector was determined using standard calibration sources, i.e., 241Am (26.8 ± 0.48 kBq), 133Ba (4.12 ± 0.07 kBq), and 152Eu (4.33 ± 0.086 kBq), (traceable to the national primary standard source from D.I. Mendeleyev Institute for Metrology) (P=0.99 and k=2.58). These sources were used with the same geometry as the source for the 226Ra in the emanation box. The energy-dependent polynomial efficiency curve obtained for the absolute full-energy peak of the HPGe gamma spectrometer is shown in Fig. 2. The absolute detection efficiency for the 214Bi line (609.3 keV) was found to be 2.05×10−2 ± 3% for the 95% confidence interval. The uncertainty in this detection efficiency was obtained by the least squares method using the experimental data of the energy dependence detection efficiency in the Spectra Line Ultimate Gamma Lab program (Mostafa et al., 2015, 2016). The fit quality for this polynomial efficiency was directly tested in each experiment by measuring the total absorption peaks of the Cs standard source with the activity 10.64 ± 0.19 kBq (P=0.99 and k=2.58), traceable to the same set of calibration sources, Eγ=661.66 keV (close to the energy line of 214Bi, i.e., 609.3 keV). The difference between the measured value and the reference value, corrected for radioactive decay, was 0.54%, as determined by Eq. (5). The passport systematic error of 137Cs activity was 1.8%. Therefore, without exception, the systematic error due to the 214 Bi line (609.3 keV) measurement was assumed to be 1.8%. This value is considered to be the systematic uncertainty ∆εsys of gamma measurements in the range of energies of 600–700 keV.
∆εsys = ±
A − Ap Ap
∙100%
(5) 4. Conclusions
where A is the experimentally measured activity with gamma spectroscopy and Ap is the absolute standard activity of the Cs-137 point source.
The radon standard system thus developed offers high accuracy and high reliability. Improvement in the traceability in radon concentration measurement can be expected by utilizing the proposed standard source. In a standard calibration systems working, the activity concentration control should be conducted by absolute measurement techniques. Our prototype standard system is created based on the HPGe detector and solid 226Ra point source. The results from determining
3.3. Radon standard system In Fig. 3, a measurement of the emanation factor as a function of time is presented. The measurement shows a nearly constant emanation 104
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Heidary, S., Setayeshi, S., Ghannadi-Maragheh, M., Negarestani, A., 2011. Monitoring and measurement of radon activity in a new design of radon calibration chamber. Radiat. Meas. 46, 694–700. Hutchinson, J.M.R., Cessna, J. Collé, R. Hodge, P.A., 1992. An international radon-in-air measurement intercomparison using a new transfer standard. Int. J. Radiat. Appl. Instrum. Part A Appl. Radiat. Isot. 43 (1–2), 175–189. Kim, B.C., Lee, K.B., Park, T.S., Lee, J.M., Lee, S.H., Oh, P.J., Lee, M.K., Ahn, J.K., 2012. Development of the primary measurement standard for gaseous radon-222 activity. Appl. Radiat. Isot. 70, 1934–1939. Linzmaier, D., Röttger, A., 2013. Development of a low-level radon reference atmosphere. Appl. Radiat. Isot. 81, 208–211. Lee, J.M., Lee, K.B., Lee, S.H., Oh, P.J., Park, T.S., Kim, B.C. Lee, M.S., 2013. Calibration of the KRISS reference ionization chamber for certification of 222Rn gaseous sources. Appl. Radiat. Isot. 81, 230–232. López-Coto, I., Bolivar, J.P., Mas, J.L., Garcıґa-Tenorio, R., Vargas, A., 2007. Development and operational performance of a single calibration chamber for radon detectors. Instrum. Methods Phys. Res. A 579, 1135–1140. Mostafa, Y.A.M., Vasyanovich, M., Zhukovsky, M., Zaitceva, N., 2015. Calibration system for radon EEC measurements. Radiat. Prot. Dosim. 164 (4), 587–590. Mostafa, Y.A.M., Vasyanovich, M., Zhukovsky, M., 2016. Prototype of a primary calibration system for measurement of radon activity concentration. Appl. Radiat. Isot. 107, 109–112. Nedjadi, Y., Spring, P., Bailat, C., Decombaz, M., Triscone, G., Gostely, J.J., Laedermann, J.P., Bochud, F., 2007. Primary activity measurements with 4πγ NaI(Tl) counting and Monte Carlo calculated efficiencies. Appl. Radiat. Isot. 65, 534–538. Picolo, J.L., 1996. Absolute measurement of radon 222 activity. Nucl. Instrum. Methods Phys. Res. A 369, 452–457. Röttger, A., Honig, A., Linzmaier, D., 2014. Calibration of commercial radon and thoron monitors at stable activity concentrations. Appl. Radiat. Isot. 87, 44–47. Honig, A., Röttger, A, 2011. Recent developments in radon metrology: new aspects in the calibration of radon, thoron and progeny devices. Radiat. Prot. Dosim. 145 (2–3), 260–266. Sahagia, M., Stanga, D., Wätjen, A.C., Luca, A., Cassette, P., Ivan, C., Antohe, A., 2010. The 222Rn standard system established at IFIN HH, Romania. Appl. Radiat. Isot. 68, 1503–1506. Sakamoto, S., Ishimori, Y., Maruo, Y., 2005. Development of a radon standard source. Instrum. Methods Phys. Res. A 545, 516–523. Spring, P., Nedjadi, Y., Bailat, C., Triscone, G., Bochud, F., 2006. Absolute activity measurement of radon gas at IRA-METAS. Nucl. Instrum. Methods Phys. Res. A 568, 752–759.
radon activity concentration with gamma spectrometry are consistent with saturated radon concentration in a closed certified volume measured with alpha-radiometry (AlphaGUARD) for a long period (approximately 40 d), but have fewer sources of error. The observed systematic error between the alpha and gamma activity concentration measurements is within the total calibration uncertainty from the reference AlphaGUARD used. The prototype developed allows for obtaining levels of systematic error at ± 3.6% and a random error of ± 0.5% with a total error of 3.65%, which corresponds to the required accuracy for the national standards for radon activity concentration. The system that was developed can be recommended as a national or regional prototype standard of radon activity concentration to calibrate different working radon measurement devices not only for high activity but also for a wide range from 3 to 30 kBq/m3. Acknowledgements The work was partly supported by Act 211 of the Government of the Russian Federation (contract № 02.A03.21.0006) and the Centre of Excellence: Radiation and Nuclear Technologies. References Busch, I., Greupner, H., Keyser, U., 2002. Absolute measurement of the activity of 222Rn using a proportional counter. Nucl. Instrum. Methods Phys. Res. A 481, 330–338. Cassette, P., Sahagia, M., Grigorescu, E.L., Lepy, M.C., Picolo, J.L., 2006. Standardization of 222Rn by LSC and comparison with α- and γ-spectrometry. Appl. Radiat. Isot. 64, 1465–1470. De Felice, P., 2007. Primary standards of radon. Metrologia 44, S82–S86. Dersch, R., 2004. Primary and secondary measurement of 222Rn. Appl. Radiat. Isot. 60, 387–390. Dersch, R., 1998. Production and measurement of 222Rn standards. Appl. Radiat. Isot. 49, 1171–1174.
105
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A primary standard source of radon-222 based on the HPGe detector a,⁎
M.Y.A. Mostafa , M. Vasyanovich a b
a,b
, M. Zhukovsky
b
MARK
Ural Federal University, Ekaterinburg, Russia Institute of Industrial Ecology UB RAS, Ekaterinburg, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Radon measurement Radon standard Solid radium source Emanation
The present paper describes the prototype of a calibration standard system for radon concentrations to be used in establishing the traceability of radon concentration measurements in dwellings. Radon gas was generated with a radium-226 solid source in a certified volume as a closed system. The activity of the radon that was released in the closed system was determined from the difference between the absolute activity of the standard radium solid source and the residual radon decay products (214Bi or 214Pb). A high-purity germanium (HPGe) detector, which was calibrated using gamma reference standard sources, was used to measure the activity of a radium solid source and radon decay products (214Bi or 214Pb). The emanation factor of the 226Ra source was controlled online with the HPGe detector. Radon activity was achieved at ~1500 ± 45 Bq from the radium source at 3.95 ± 0.2 kBq under equilibrium conditions. After this activity, the radon gas was transferred into the closed system producing radon activity concentrations of 31.1 ± 0.3 kBq/m3. Systematic errors were found of less than 4% with a random error around 0.5%. The random error is generally associated with the estimation of the count rate of the measured radon progenies (214Po and 214Po for alpha measurements or 214Pb and 214Bi for gamma measurements), but systematic errors are associated with the errors introduced by the instrumentation and measurement technique. The system that was developed has a high degree of accuracy and can be recommended as a national or regional prototype standard of radon activity concentration to calibrate different working radon measurement devices.
1. Introduction
2012; Lee et al., 2013; Röttger et al., 2014) or by using radon itself in an absolute calibration method (Busch et al., 2002; Spring et al., 2006; Cassette et al., 2006; Nedjadi et al., 2007; Sahagia et al., 2010). Critical analysis of all of these methods was done by De Felice in 2007. All these methods were accomplished through the realization of a complex system for the generation, circulation, and recovery of the radon standard source or system. Because the existence of a radon standard system for radon sources is very important, over the last year, a series of experiments on radon equilibrium equivalent concentrations (EECs) (Mostafa et al., 2015) were conducted and a radon activity concentration standard system was launched (Mostafa et al., 2016). The radon concentration standard system in open flow mode was realised by the pumping of air through an emanation chamber containing the radium source. In this situation, a 226Ra source with high activity (~33 kBq) was used and the controlling of flow rate was necessary (Mostafa et al., 2016). In this present work, we have upgraded the system with a closed system and used a relatively small activity source of ~4 kBq in the calibration. Because the system is closed, the value of the flow rate does not influence the radon concentration because the ratio of (flow rate)/
The quality assurance and state of a device must be calibrated in order to quantitatively measure radiation. Radiation monitoring devices are often calibrated using standard sources. However, in the case of radon monitoring devices, calibration is difficult because of the short half-life of radon and its status as a noble gas. Therefore, the use of the radon (222Rn) primary standards system is important because it provides an opportunity to obtain reference sources or systems to assure the traceability of radon measurements under various conditions. In 1996, Picolo reported the first absolute measurement method for radon based on the detection of alpha particles directly emitted by 222 Rn (Picolo, 1996). This method was also successfully adopted by Dersch (1998, 2004) and Spring et al. (2006). Other methods of standardization systems for 222Rn measurements in the atmosphere have been developed, either by measuring the gamma-emitting decay products of 222Rn gas traceable to a 226Ra standard reference material (SRM) (Hutchinson et al., 1992; Sakamoto et al., 2005; López-Coto et al., 2007; Röttger and Honig, 2011; Heidary et al., 2011; Kim et al.,
⁎
Corresponding author. E-mail address: [email protected] (M.Y.A. Mostafa).
http://dx.doi.org/10.1016/j.apradiso.2016.12.012 Received 24 February 2016; Received in revised form 13 September 2016; Accepted 8 December 2016 Available online 09 December 2016 0969-8043/ © 2016 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 120 (2017) 101–105
M.Y.A. Mostafa et al.
AlphaGUARD monitor in a diffusion mode and the results stored in the AlphaGUARD memory. At the same time, the gamma spectrum of the 226 Ra solid source was measured continuously every 24 h during the calibration period. The environmental parameters for pressure, temperature, and humidity were measured with the AlphaGUARD sensors in the closed measurement circuit during the whole experiment. In this experiment, an AlphaGUARD (PQ2000Pro) radon monitor was used as a reference device (a secondary standard to test the radon activity concentration). A 60-min diffusion mode was standard for our version of the AlphaGUARD PQ2000Pro equipment. This AlphaGUARD was calibrated using a NIST calibration source (Mostafa et al., 2016). Using the NIST source, the calibration error (including source activity error, random errors during measurements, etc.) was approximately 8– 9%. As in Fig. 1, the solid 226Ra source was assembled in the emanation box and the box was installed on the HPGe detector for online γ-ray spectrometry. The emanation box included a slot for the volume flow and the transport of the radon gas to the reference volume. The emanation factor of the 222Rn gas from the solid 226Ra source was controlled (measured continually) online with the HPGe detector by measuring the residual activity of 214Bi (Eγ=609.3 keV) activity in the solid radium source and comparing this value with the absolute activity of the 226Ra source. The stability of gamma spectrometry was controlled with standard 137Cs source installed in the emanation box. The calibration process with reference atmosphere was performed over a time period of almost 40 days. During this period, the reference volume had to be completely isolated from any surrounding influences. Thus, the components of the radon reference chamber are made of stainless steel. Therefore, there is no chance for any additional source of radon to enter (Linzmaier and Röttger 2013). For this procedure, if the radon gas emanation is constant, the emanation factor χ is calculated using:
(emanation chamber volume) is considerably greater than the half-life of radon. In this case, we can neglect the error from flow rate fluctuations, which improve the quality of the calibration. In general, the calibration of the 222Rn activity concentration standard for further transfer is possible using a radium standard source with the release of an activity concentration C by means of a reference atmosphere. In this case, the released radon activity concentration (C) equals A/V Bq/m3, where A is the activity (Bq) of released radon from a solid radium standard source, and V is the calibration volume (m3). The radon gas is emanated from a solid 226Ra standard source and the emanation factor was controlled online with a gamma spectrometer. The radon activity, which was emanated and released in the closed system, was calculated absolutely from the residual activity in the solid 226Ra standard source. A high-purity germanium (HPGe) detector was used to measure the absolute activity of the 226Ra with online controlling of the radon emanation (by measuring the 214Bi activity, the progeny of 226Ra) to produce the released absolute activity of 222Rn. An alternative approach to 222Rn metrology, based on the use of this prototype of the 222Rn concentration standard was proposed. In this approach, no reference measurements for the 222Rn are needed in the calibration process and the uncertainty of the reference 222Rn activity concentrations in the standard radon test system was kept to around 3% (P=0.95). Using this prototype of the radon concentration standard source, the calibration of radon concentration measurement devices is greatly simplified and the measurement accuracy is improved. 2. Experimental set-up and procedure A schematic drawing of the calibration system is shown in Fig. 1. The traceable volume for the generation of the reference radon atmosphere consists of a standard chamber with a certified volume of (5.04×10−2 ± 5×10−4 m3) 50.4 l and an emanation box (2.1×10−4 ± 1×10−6 m3). The total volume of the complete system including all experimental components is (48.1 ± 0.5)×10−3 m3 with a systematic volume uncertainty ΔVsys= ± 1%. The emanation box was installed on the HPGe detector for online gamma measurement. A 226Ra standard source (national primary standard source from D.I. Mendeleyev Institute for Metrology) was placed inside this box. The certified activity according to original certification was (3.95 ± 0.2 kBq (K=2)). The activity of the 226Ra measured using the 186-keV line directly with our HPGe detector was (3.98 ± 0.12 kBq). The emanated radon from the solid radium source was cycled and accumulated through the radon chamber in a closed system with a flow rate of (10−3 m3/min). An AlphaGUARD (PQ2000Pro) radon monitor was located inside the radon chamber. Radon concentration in the chamber was measured directly with the
χ222Rn=
A (222Rn) =1 − A (226Ra)
A (214Bi) A (226Ra)
(1)
where A is the activity. In general, for the emanation source, the emanation factor is fulfilled by the equation 0≤χ≤1. The velocity of the generated radon activity in the closed radon system is (ARn= NRn.λRn) and λRn is the radon decay constant. At the constant χ, the radon activity concentration CRn at time (t) is calculated by Eq. (2). The radon activity concentration in the reference volume (total volume of the whole closed system) starts to increase according to this equation until the equilibrium is achieved (99% after ~600 h). If the emanation factor changes with time, in this case, Eq. (2) cannot describe the activity concentration of radon and the source cannot be used in the radon standard system. Eqs. (3) and (4) present the dependence of the radon activity concentration on the variance of the emanation factor
Fig. 1. Schematic drawing of the calibration system.
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Table 1 Systematic and random error values for the gamma spectrometry (HPGe) and (AlphaGUARD). Type of measurement
AlphaGUARD
HPGe detector
Random errors Systematic errors
Δrα = ± 4. 5% Δεsys= ± 6%
Δrγ = ± 0. 5% Δεsys= ± 1. 8%
ΔVsys= ± 1%
ΔVsys= ± 1%
—
Δεsys (Cs−137) = ± 0. 54%
—
Δεsys (Ra−226) = ± 3%
Total systematic error (HPGe)
∆γsys = ± K√ (Δεsys (214Bi))2+ (ΔVsys )2+ (Δεsys (137Cs))2+ (Δεsys (226Ra))2= ± 3.6%
Total systematic error (AlphaGUARD)
A ∆sys = ± K√ (Δεsys) 2 +(ΔVsys) = ± 6.7%
Total errors
ΔTα = (∆αsys )2+(Δrα )2 = ± 8.8%
2
ΔTγ = (∆γsys )2+(Δrγ )2 = ± 3.65%
*K=1.1.
0.1
t
CRn (t)=
∫
Relative Efficiency
0
dCRn dt dt
(4)
Because the radon gas was removed very quickly from the emanation box by airflow, there were no effects observed of wall attachment of radon decay products in the volume of the emanation box. Wall attachment of radon decay products can be checked at the end of the experiment when the emanation source is quickly removed from the emanation box and the empty emanation box is measured with γspectrometry. Within the limits of the measurement error and the detection limit, there were no additional background counts in the emanation box.
0.01
1E-3
3. Results and discussion 100
3.1. Systematic and random errors
1000
Energy (KeV)
In general, experimental uncertainty is due to either random errors or systematic errors. For any radiation standard system, both random and systematic errors should be estimated accurately. Random errors are statistical fluctuations in the measured data due to the precision limits of the measurement instrument. On the other hand, systematic errors are reproducible errors that present consistently in the same direction. Systematic errors are often due to a problem that persists throughout the entire experiment. In general, systematic errors are related to the errors introduced by instruments and measurement technique. For the radon standard, random error is generally associated with the estimation of the count rate of measured radon decay products (218Po and 214Po for alpha measurements or 214Pb and 214Bi for gamma measurements) or error in activity concentration using radon monitors; but systematic errors are associated with the errors introduced by the instrumentation and measurement technique. In our case, random error was generally associated with the estimation of the count rate for the measured radon decay products (214Bi for gamma measurements) in addition to errors in the estimated activity concentration with the AlphaGUARD monitor. The systematic errors included errors in determining the activity of nuclides using polynomial approximation of registration efficiency of the HPGe detector especially in the range of the radon progeny emission line 214Bi (Eγ=609.3 keV). In addition, errors in determining the detection efficiency of the α-radiation monitor (AlphaGUARD) and errors in the total system volume were considered to be systematic errors. The systematic and random errors that were obtained by the HPGe detector and AlphaGUARD monitor are presented in Table 1. In the gamma measurements, the average relative uncertainty of the measured net count for the 214Bi line (609.3 keV) (random error) was around Δrγ =0. 5%. On the other hand, due to the continuous measurements of radon activity concentration, the mean value of the random measurement error for the AlphaGUARD monitor ∆αr was ± 4.5%. It can
Fig. 2. The polynomial peak efficiency of HPGe is estimated using standard sources (241Am, 133Ba, and 152Eu).
Relative emanation factor
0.40
Emanation factor Average ( 0.3749 ± 0.0032
0.39
0.38
0.37
0.36
0.35
0
5
10
15
20
25
30
35
40
45
Days Fig. 3. The online measured emanation coefficient of an emanation radium source.
over time.
CRn (t)=
ARa . χ (1 − exp(−λRn t)) V
(2)
In the case of the emanation factor variation over time
A . λ .χ(t) dCRn =−λRn CRn + Ra Rn dt V
(3)
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Fig. 4. Radon activity concentration calculated with the mean value of the emanation factor compared with the measured value using the AlphaGUARD radon monitor and the recorded metrological parameters.
factor during the period of calibration with variance of not more 1%. It gives the ability to use the mean value of the emanation factor during the measurement period to calculate the generated activity concentration inside the volume of the closed system over time using Eq. (2). During the calibration test, the radon activity concentration was measured with the AlphaGUARD monitor in diffusion mode continuously. The AlphaGUARD results were compared with the calculated activity concentration using the measured mean emanation factor of radon, which was released from the solid 226Ra source in a certified volume of the closed system. The mean activity of the emanated radon in the volume of the closed system was 1500 ± 45 Bq. It was calculated with a mean value for the emanation factor (0.3749 ± 0.0032) and the standard activity of the radium source (3.98 ± 0.12 kBq). The total volume of the closed system was (48.1 ± 0.5) x10−3 m3. Finally, the mean radon concentration in the closed system when saturated was 31.1 ± 0.3 kBq/m3. Fig. 4 shows the comparison between the calculated values of radon activity concentration, using Eq. (2), and the mean value of the emanation factor and the radon activity concentration, which was measured with the AlphaGUARD monitor continuously. According to Eq. (2), the measured activity concentration of radon, with the continuous AlphaGUARD monitor, starts to increase over time until the equilibrium starts to achieve between 226Ra and 222Rn after nearly 26 days (99%). In the same way, the calculated activity concentration using the emanation factor, the standard 226Ra activity measured with HPGe, and the certified volume of the closed system behave in nearly the same manner (in Eq. (2)) from the start to the end of the calibration experiment. In this case, the stability of the meteorological conditions during the experiment was also observed. From Fig. 4, it could be seen that there is good agreement between the measured and the calculated values for radon activity concentration during the calibration period within the error limits. The difference between the results of the AlphaGUARD measurement and the calculated curve is in the range of 3%. The recorded metrological parameters with the AlphaGUARD are also presented in Fig. 4, with almost no influential changes in these parameters during the experiment.
be observed that the total systematic errors for the alpha measurement are higher than the gamma measurement errors. In the same way, the total errors by gamma measurements are lower than the total errors from the AlphaGUARD by a factor of two. All indicators show that the prototype system developed has sufficient accuracy to be considered as radon standard system. 3.2. Radioactivity measurements using germanium detectors The polynomial dependence of the peak efficiency of the HPGe detector was determined using standard calibration sources, i.e., 241Am (26.8 ± 0.48 kBq), 133Ba (4.12 ± 0.07 kBq), and 152Eu (4.33 ± 0.086 kBq), (traceable to the national primary standard source from D.I. Mendeleyev Institute for Metrology) (P=0.99 and k=2.58). These sources were used with the same geometry as the source for the 226Ra in the emanation box. The energy-dependent polynomial efficiency curve obtained for the absolute full-energy peak of the HPGe gamma spectrometer is shown in Fig. 2. The absolute detection efficiency for the 214Bi line (609.3 keV) was found to be 2.05×10−2 ± 3% for the 95% confidence interval. The uncertainty in this detection efficiency was obtained by the least squares method using the experimental data of the energy dependence detection efficiency in the Spectra Line Ultimate Gamma Lab program (Mostafa et al., 2015, 2016). The fit quality for this polynomial efficiency was directly tested in each experiment by measuring the total absorption peaks of the Cs standard source with the activity 10.64 ± 0.19 kBq (P=0.99 and k=2.58), traceable to the same set of calibration sources, Eγ=661.66 keV (close to the energy line of 214Bi, i.e., 609.3 keV). The difference between the measured value and the reference value, corrected for radioactive decay, was 0.54%, as determined by Eq. (5). The passport systematic error of 137Cs activity was 1.8%. Therefore, without exception, the systematic error due to the 214 Bi line (609.3 keV) measurement was assumed to be 1.8%. This value is considered to be the systematic uncertainty ∆εsys of gamma measurements in the range of energies of 600–700 keV.
∆εsys = ±
A − Ap Ap
∙100%
(5) 4. Conclusions
where A is the experimentally measured activity with gamma spectroscopy and Ap is the absolute standard activity of the Cs-137 point source.
The radon standard system thus developed offers high accuracy and high reliability. Improvement in the traceability in radon concentration measurement can be expected by utilizing the proposed standard source. In a standard calibration systems working, the activity concentration control should be conducted by absolute measurement techniques. Our prototype standard system is created based on the HPGe detector and solid 226Ra point source. The results from determining
3.3. Radon standard system In Fig. 3, a measurement of the emanation factor as a function of time is presented. The measurement shows a nearly constant emanation 104
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Heidary, S., Setayeshi, S., Ghannadi-Maragheh, M., Negarestani, A., 2011. Monitoring and measurement of radon activity in a new design of radon calibration chamber. Radiat. Meas. 46, 694–700. Hutchinson, J.M.R., Cessna, J. Collé, R. Hodge, P.A., 1992. An international radon-in-air measurement intercomparison using a new transfer standard. Int. J. Radiat. Appl. Instrum. Part A Appl. Radiat. Isot. 43 (1–2), 175–189. Kim, B.C., Lee, K.B., Park, T.S., Lee, J.M., Lee, S.H., Oh, P.J., Lee, M.K., Ahn, J.K., 2012. Development of the primary measurement standard for gaseous radon-222 activity. Appl. Radiat. Isot. 70, 1934–1939. Linzmaier, D., Röttger, A., 2013. Development of a low-level radon reference atmosphere. Appl. Radiat. Isot. 81, 208–211. Lee, J.M., Lee, K.B., Lee, S.H., Oh, P.J., Park, T.S., Kim, B.C. Lee, M.S., 2013. Calibration of the KRISS reference ionization chamber for certification of 222Rn gaseous sources. Appl. Radiat. Isot. 81, 230–232. López-Coto, I., Bolivar, J.P., Mas, J.L., Garcıґa-Tenorio, R., Vargas, A., 2007. Development and operational performance of a single calibration chamber for radon detectors. Instrum. Methods Phys. Res. A 579, 1135–1140. Mostafa, Y.A.M., Vasyanovich, M., Zhukovsky, M., Zaitceva, N., 2015. Calibration system for radon EEC measurements. Radiat. Prot. Dosim. 164 (4), 587–590. Mostafa, Y.A.M., Vasyanovich, M., Zhukovsky, M., 2016. Prototype of a primary calibration system for measurement of radon activity concentration. Appl. Radiat. Isot. 107, 109–112. Nedjadi, Y., Spring, P., Bailat, C., Decombaz, M., Triscone, G., Gostely, J.J., Laedermann, J.P., Bochud, F., 2007. Primary activity measurements with 4πγ NaI(Tl) counting and Monte Carlo calculated efficiencies. Appl. Radiat. Isot. 65, 534–538. Picolo, J.L., 1996. Absolute measurement of radon 222 activity. Nucl. Instrum. Methods Phys. Res. A 369, 452–457. Röttger, A., Honig, A., Linzmaier, D., 2014. Calibration of commercial radon and thoron monitors at stable activity concentrations. Appl. Radiat. Isot. 87, 44–47. Honig, A., Röttger, A, 2011. Recent developments in radon metrology: new aspects in the calibration of radon, thoron and progeny devices. Radiat. Prot. Dosim. 145 (2–3), 260–266. Sahagia, M., Stanga, D., Wätjen, A.C., Luca, A., Cassette, P., Ivan, C., Antohe, A., 2010. The 222Rn standard system established at IFIN HH, Romania. Appl. Radiat. Isot. 68, 1503–1506. Sakamoto, S., Ishimori, Y., Maruo, Y., 2005. Development of a radon standard source. Instrum. Methods Phys. Res. A 545, 516–523. Spring, P., Nedjadi, Y., Bailat, C., Triscone, G., Bochud, F., 2006. Absolute activity measurement of radon gas at IRA-METAS. Nucl. Instrum. Methods Phys. Res. A 568, 752–759.
radon activity concentration with gamma spectrometry are consistent with saturated radon concentration in a closed certified volume measured with alpha-radiometry (AlphaGUARD) for a long period (approximately 40 d), but have fewer sources of error. The observed systematic error between the alpha and gamma activity concentration measurements is within the total calibration uncertainty from the reference AlphaGUARD used. The prototype developed allows for obtaining levels of systematic error at ± 3.6% and a random error of ± 0.5% with a total error of 3.65%, which corresponds to the required accuracy for the national standards for radon activity concentration. The system that was developed can be recommended as a national or regional prototype standard of radon activity concentration to calibrate different working radon measurement devices not only for high activity but also for a wide range from 3 to 30 kBq/m3. Acknowledgements The work was partly supported by Act 211 of the Government of the Russian Federation (contract № 02.A03.21.0006) and the Centre of Excellence: Radiation and Nuclear Technologies. References Busch, I., Greupner, H., Keyser, U., 2002. Absolute measurement of the activity of 222Rn using a proportional counter. Nucl. Instrum. Methods Phys. Res. A 481, 330–338. Cassette, P., Sahagia, M., Grigorescu, E.L., Lepy, M.C., Picolo, J.L., 2006. Standardization of 222Rn by LSC and comparison with α- and γ-spectrometry. Appl. Radiat. Isot. 64, 1465–1470. De Felice, P., 2007. Primary standards of radon. Metrologia 44, S82–S86. Dersch, R., 2004. Primary and secondary measurement of 222Rn. Appl. Radiat. Isot. 60, 387–390. Dersch, R., 1998. Production and measurement of 222Rn standards. Appl. Radiat. Isot. 49, 1171–1174.
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