Radiation Protection Dosimetry Advance Access published April 24, 2015 Radiation Protection Dosimetry (2015), pp. 1–5
doi:10.1093/rpd/ncv221
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD FOR THE CALIBRATION OF THORON MEASUREMENT INSTRUMENTS B. Sabot1,2,*, S. Pierre1, P. Cassette1, N. Michielsen2 and S. Bondiguel2 1 CEA, LIST, LNHB, CEA Saclay, Gif-sur-Yvette F-91191, France 2 Institut de Radioprotection et de Suˆrete´ Nucle´aire (IRSN) PSN-RES, SCA, LPMA, Centre de Saclay, Gif-sur-Yvette 91192, France
The LNHB and IRSN are working on a reference atmosphere for thoron (220Rn) instrument calibration. The LNHB, as the national metrology institute for activity measurement in France, has to create a new thoron reference standard in order to estimate with accuracy the thoron concentration of a reference atmosphere. The measurement system presented in this paper is based on a reference volume using an alpha detector, which is able to measure thoron and its decay products to define the thoron concentration of a thoron reference atmosphere. This paper presents the first results with this new system using a well-known radon (222Rn) atmosphere and a thoron (220Rn) atmosphere.
INTRODUCTION 232
Thoron is a decay product of Th, which is present in the Earth’s crust. Due to its short half-life [55.8 s(1)], the thoron emanation from the ground is often assumed to be not significant compared with radon. However, measurements in Korea(2) and India(3) have shown that thoron and radon concentrations are similar in closed environments due to the building materials. Even if there is no direct thoron issue, it may have an effect on the radon measurement as shown in Canada with alpha track detectors(4). Currently, there are many devices to measure radon and thoron concentrations and evaluate radiological risks. It is necessary to calibrate the detectors with a reference atmosphere to make sure the risks can be well evaluated. LNE-LNHB already has a primary standard for radon (half-life 3.8 d) based on a solid angle method with a frozen radon source(5). This device has been recently upgraded to produce a radon standard with a relative uncertainty of 0.3 %. In the case of thoron, due to its short half-life, it is not possible to use the same technique. A new reference volume with an alpha detector is developed to precisely measure the thoron activity concentration in a reference atmosphere (cf. Figure 1). This system uses a specific geometry built to improve the quality of the alpha spectra and the detection efficiency, which has been evaluated with the Monte Carlo method. To validate the calculation, it is possible to use the radon standard since both gases can be measured in the system. In order to improve the spectra, it has been decided to use the electrical property of the radon and thoron decay products to catch them on the detector surface. These decay products can be measured in the authors’ system since
they are solid charged particles of various isotopes of polonium, bismuth and lead, decaying through alpha emission. DEVELOPMENT OF THE THORON REFERENCE MEASUREMENT SYSTEM Efficiency evaluation by the Monte Carlo method The first step of development was to simulate different models using MCNPX 2.7(6). This Monte Carlo code simulates alpha particles in a 3D geometry. Using this code, the detection efficiency, and corresponding spectra, can be calculated. The simulations were performed for the thoron and radon gases and the decay products. Since a radon measurement standard is already available(5), the simulation can be validated against experiment. In the end, the model selected and presented here is a small cylindrical volume of 11 mm high with a top surface composed of a silicon barrier detector (diameter 39 mm). The active volume is 13.0 + 0.1 cm3. With this geometry, the results show high efficiency for gas detection, but problems arise due to the decay products being deposited on all surfaces, or in all the volume, e.g. 216Po, causing the alpha peaks in the spectra to overlap. As a result, it has been decided to use the electrical properties of the radon and thoron decay products(7). Knowing these properties and using an electric field, it is possible to catch the decay products on the detector surface and to improve the spectra for the thoron gas measurement (cf. Figure 2). With such a system, the spectra can be used to define the thoron activity in the measurement volume at normal atmospheric pressure. The efficiency calculated
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
*Corresponding author: [email protected]
B. SABOT ET AL.
Figure 3. Particle path in the thoron measurement volume (13.0 + 0.1 cm3) with Comsol Multiphysics 4.4, the air flow is 1 l min21 and particles have only 1 positive charge and 1 nm of diameter. The detector surface is shown in grey.
Figure 2. Shape of the spectrum with the results from MCNPX in a small volume for thoron and all its decay products caught on the detector surface. The thoron activity is considered constant; there is an equilibrium reached after some hours between thoron and its decays product.
with MCNPX is 31 + 4 % for the thoron (or radon) peak. This is very interesting in the authors’ geometry because both gases will have the same detection efficiency. The decay products will have an efficiency of ,50 + 6 % due to the alpha particles, which may backscatter from the detector surface. The uncertainty is large because it takes into account the large variations of air pressure, temperature and humidity. The variation of these parameters modifies the density, which changes the detection efficiency. A part of the uncertainty is also due to the volume estimation of the system. Use of an electric field to catch decay products on the surface of the detector The decay products from radon (218Po) and thoron (216Po) are positively charged particles(7). In the measurement volume, the air mixed with radon or thoron is filtered just at the entrance. Hence, there is only radon or thoron and air entering the authors’ volume. The decay products of radon or thoron come only from the gas disintegration in the measurement volume. In this volume, there is an electric field
(400 kV m21), which allows the decay products to be caught on the detector surface. The authors used the deterministic code Comsol Multiphysics 4.3b to build and optimise their system(8). This code allows the simulation of the flow rate, electric field and particle trajectories in a 3D geometry (cf. Figure 3). Combining these data, the authors updated their geometry and defined the appropriate air flow and electric field. In the example presented in Figure 3, the decay products are caught on the detector surface within 80 ms for the longest particle path.
FIRST RESULTS WITH THE THORON REFERENCE MEASUREMENT SYSTEM Measurement of the radon reference atmosphere The LNE-LNHB already produces a reference atmosphere with the primary radon standard mixed with air in a closed volume. Since this atmosphere is known, and since the thoron measurement system can also measure radon, the first step is to check the gas detection efficiency. The system is connected to this reference atmosphere, and measurements are made at an atmospheric pressure of 1003 + 2 hPa, with a flow rate of 1 l min21 inside the measurement. The activity concentration of radon is 1.00 + 0.01 MBq m23 mixed with filtered air (HEPA filter) and a relative humidity of 3 + 3 % RH. The measured spectrum is presented in Figure 4. With this geometry, it is possible to see the radon gas and its decay products. The resolution of the decay product peaks is such that there is no overlap with the radon peak. The tails to the left of the decay product peaks are very small compared with the counting rate in the authors’ region of interest. The 218Po has a very small effect on the 222Rn peak; the overlapping tail area is ,0.4 % of the total radon peak.
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Figure 1. Diagram of the thoron reference measurement system. The decay products are caught on the detector surface, whereas the gas fills the entire volume.
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD
Figure 6. Spectrum of a thoron measurement in the new chamber without an electric field and a flow rate of 1 l min21 (700-s measurement).
measurements are then corrected with the radon decay to a given reference time. The corrected measurements show a good agreement. The relative expanded uncertainty is 1.4 % (k ¼ 2), which is a good result for this first measurement. Measurement of a thoron atmosphere
Figure 5. Series of radon measurement in the chamber with a constant air flow of 1 l min21 and a high radon activity concentration. Diamond represents counting in radon window. Circle denotes counting in radon window with decay correction. Line represents average value. Dashed line represents standard deviation.
From this measurement, since the authors know the 222Rn activity, they measured the efficiency of their system as 35 + 2 %. The uncertainty is rather high for this first measurement, mainly due to the uncertainty on the total chamber volume. In the future, the efficiency of the system should be defined with a relative uncertainty of 0.3 %. The second step is to define the system reproducibility and stability with time. Maintaining the same conditions, with the reference atmosphere circulating in the measurement volume, a series of 2000-s measurements were made with this radon atmosphere; the results are presented in Figure 5. The region of interest used to define the count rate for the radon peak can be seen in Figure 4. These measurements were made in a closed volume; hence, the radon activity decreases with time. This effect is well represented in Figure 5; the
The first measurements were made with a 228Th source from Pylonw, which can be used to create a thoron atmosphere. A constant air flow (1 l m21) is maintained at the entrance of the source. The air is filtered, radon free and relative humidity (3 + 3 % RH), and at constant atmospheric pressure (1003 + 2 hPa) and temperature (25 + 4 8C). This air is mixed with the thoron and filtered before it enters the measurement volume; the thoron activity concentration is (1.0 + 0.2 MBq m23). In this experiment, the measurements were initially done without an electric field (Figure 6). With such results, it is not possible to identify each peak with the corresponding isotope; thus, it is not possible to define which peaks are gas and which are decay products. The 216Po peak overlaps with the 220 Rn peak and, hence, it is not possible to accurately determine the peak areas. Since both behave differently in the volume, it is not possible to use the sum to define the activity. Also, the authors are not able to see the 212Bi in this spectrum since it is within the thoron peak. The second measurement is done under the same conditions, but with an applied electric field. It is now possible to identify each decay product and the gas in the spectrum. The same region of interest is selected in both spectra. It corresponds to the 216Po peak, highlighted in Figures 6 and 7. In this region, for the measurement without an electric field, the counting rate is 50 s21, whereas with the electric field it becomes 550 s21. These first results show the efficiency of the electric field and the overlap between
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Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
Figure 4. Spectrum of a radon measurement in the new chamber with an electric field and a flow rate of 1 l min21. This 2000-s measurement has been done after 17 h of exposure to the radon atmosphere. The equilibrium is reached between the 218Po caught on detector and 214Po build up from 218Po decay.
B. SABOT ET AL.
Figure 8. Spectrum measured after the measurement presented in Figure 7 and once the volume has been cleaned of thoron.
the thoron and its first decay products disappears, as the decay products are well trapped on the detector surface. The resolution of the 216Po peak is still not very good due to the detector surface and the electronics used. The tail of the 216Po peak overlaps with the 220 Rn peak, but this effect is very small (0.4 % of the peak area). Concerning the 212Po peak, there is a tail on the right of the peak, which is unusual for an alpha peak. It is due to the coincidence between the beta emitted from 212Bi and the alpha produced by its daughter 212Po, which decays too rapidly [300 ns(1)] to be discriminated by the system electronics (time constant of 2 ms). For this reason, it is not possible to use this peak for the measurement, as the efficiency is too difficult to determine. There remains one decay product (212Bi) that may interfere significantly with the 220Rn peak after a long measurement. By comparing the measurement without an electric field, one can see its contribution. It is very low in Figure 7, but for a longer measurement time, a larger peak appears. There are two possibilities to remove this peak from the spectrum: The first method is by calculation; since the 216Po amount deposited on the detector surface is known from the spectrum, it is possible to deduce the 212Bi activity, assuming all of the daughters of 216Po stays on the detector surface. The second approach is to directly measure using alpha spectrometry: once the initial measurement is finished, the measurement volume is flushed with clean air to remove the thoron. The following measurement is then sensitive to only the decay products remaining from the previous measurement (212Bi and 212 Po, cf. Figure 8). As a result, the system allows a useful spectrum to be measured directly of the thoron gas. It is now necessary to accurately determine the efficiency for a thoron measurement. Since the spectrum obtained with thoron present is a similar form to the radon one, it is possible to use an equivalent analysis to
define each gas activity. However, there is the effect of the 212Bi within the thoron peak that must also be addressed, since this does not appear with radon. CONCLUSION Using Monte Carlo and Comsol simulations, the authors have defined an appropriate geometry to measure a thoron atmosphere under specific conditions. This system allows to trap the decay products on the detector surface and has proved to be efficient for measuring thoron. As a result, the authors have obtained useful spectra for the measurement of a thoron atmosphere, since they can directly measure the gas and also the decay products. The gas peak and the decay product peaks can be both used to evaluate the thoron concentration of a thoron reference atmosphere. The first experiment, with thoron and radon, allowed to define a gas detection efficiency and also to see how the response of the authors’ system changes through time. Nevertheless, it is still desirable to improve the system further; a new prototype is planned with new electronics, which should increase the resolution. Finally, the authors will also improve the spectrum analysis and their measurement technique to have a small and well defined uncertainty on the thoron measurement. REFERENCES 1. Be´, M. -M., Chiste´, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Scho¨nfeld, E. and Dersch, R. Table of Radionuclides, Monographie BIPM-5 (2004). http://www.nucleide.org/Publications/ monographies_bipm.htm (30 March 2015, date last accessed). 2. Kim, Y. J., Lee, H. Y., Kim, C. S., Chang, B. U., Rho, B. H., Kim, C. K. and Tokonami, S. Indoor radon, thoron and thoron daughter concentrations in Korea. Int. Congr. Ser. 1276, 46–49 (2005).
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Figure 7. Spectrum of a thoron measurement in the new chamber with an electric field and a flow rate of 1 l min21 (100-s measurement after 1 h exposed to thoron atmosphere).
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD 3. Baciu, A. Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania). J. Environ. Radioact. 83, 171– 189 (2005). 4. Chen, J. and Moir, D. A study on the thoron sensitivity of radon detectors available to Canadians. J. Radiol. Prot. 32, 419– 425 (2012). 5. Picolo, J. L. Absolute measurement of radon 222 activity. Nucl. Instrum. Methods Phys. Res. A. 369, 452–457 (1996).
6. Pelowitz, D. B. et al. MCNPX 2.7.0 Extensions, LA-UR02295 (2011). https://mcnp.lanl.gov/ (30 March 2015, date last accessed). 7. Dua, S. K. and Kotrappa, P. Comment on the charge on decay products of thoron and radon. Am. Ind. Hyg. Assoc. 42, 242 –243 (1981). 8. Comsol. Comsol Multiphysics Version 4.3b release notes. www.comsol.com (30 March 2015, date last accessed) (2014).
Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
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doi:10.1093/rpd/ncv221
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD FOR THE CALIBRATION OF THORON MEASUREMENT INSTRUMENTS B. Sabot1,2,*, S. Pierre1, P. Cassette1, N. Michielsen2 and S. Bondiguel2 1 CEA, LIST, LNHB, CEA Saclay, Gif-sur-Yvette F-91191, France 2 Institut de Radioprotection et de Suˆrete´ Nucle´aire (IRSN) PSN-RES, SCA, LPMA, Centre de Saclay, Gif-sur-Yvette 91192, France
The LNHB and IRSN are working on a reference atmosphere for thoron (220Rn) instrument calibration. The LNHB, as the national metrology institute for activity measurement in France, has to create a new thoron reference standard in order to estimate with accuracy the thoron concentration of a reference atmosphere. The measurement system presented in this paper is based on a reference volume using an alpha detector, which is able to measure thoron and its decay products to define the thoron concentration of a thoron reference atmosphere. This paper presents the first results with this new system using a well-known radon (222Rn) atmosphere and a thoron (220Rn) atmosphere.
INTRODUCTION 232
Thoron is a decay product of Th, which is present in the Earth’s crust. Due to its short half-life [55.8 s(1)], the thoron emanation from the ground is often assumed to be not significant compared with radon. However, measurements in Korea(2) and India(3) have shown that thoron and radon concentrations are similar in closed environments due to the building materials. Even if there is no direct thoron issue, it may have an effect on the radon measurement as shown in Canada with alpha track detectors(4). Currently, there are many devices to measure radon and thoron concentrations and evaluate radiological risks. It is necessary to calibrate the detectors with a reference atmosphere to make sure the risks can be well evaluated. LNE-LNHB already has a primary standard for radon (half-life 3.8 d) based on a solid angle method with a frozen radon source(5). This device has been recently upgraded to produce a radon standard with a relative uncertainty of 0.3 %. In the case of thoron, due to its short half-life, it is not possible to use the same technique. A new reference volume with an alpha detector is developed to precisely measure the thoron activity concentration in a reference atmosphere (cf. Figure 1). This system uses a specific geometry built to improve the quality of the alpha spectra and the detection efficiency, which has been evaluated with the Monte Carlo method. To validate the calculation, it is possible to use the radon standard since both gases can be measured in the system. In order to improve the spectra, it has been decided to use the electrical property of the radon and thoron decay products to catch them on the detector surface. These decay products can be measured in the authors’ system since
they are solid charged particles of various isotopes of polonium, bismuth and lead, decaying through alpha emission. DEVELOPMENT OF THE THORON REFERENCE MEASUREMENT SYSTEM Efficiency evaluation by the Monte Carlo method The first step of development was to simulate different models using MCNPX 2.7(6). This Monte Carlo code simulates alpha particles in a 3D geometry. Using this code, the detection efficiency, and corresponding spectra, can be calculated. The simulations were performed for the thoron and radon gases and the decay products. Since a radon measurement standard is already available(5), the simulation can be validated against experiment. In the end, the model selected and presented here is a small cylindrical volume of 11 mm high with a top surface composed of a silicon barrier detector (diameter 39 mm). The active volume is 13.0 + 0.1 cm3. With this geometry, the results show high efficiency for gas detection, but problems arise due to the decay products being deposited on all surfaces, or in all the volume, e.g. 216Po, causing the alpha peaks in the spectra to overlap. As a result, it has been decided to use the electrical properties of the radon and thoron decay products(7). Knowing these properties and using an electric field, it is possible to catch the decay products on the detector surface and to improve the spectra for the thoron gas measurement (cf. Figure 2). With such a system, the spectra can be used to define the thoron activity in the measurement volume at normal atmospheric pressure. The efficiency calculated
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
*Corresponding author: [email protected]
B. SABOT ET AL.
Figure 3. Particle path in the thoron measurement volume (13.0 + 0.1 cm3) with Comsol Multiphysics 4.4, the air flow is 1 l min21 and particles have only 1 positive charge and 1 nm of diameter. The detector surface is shown in grey.
Figure 2. Shape of the spectrum with the results from MCNPX in a small volume for thoron and all its decay products caught on the detector surface. The thoron activity is considered constant; there is an equilibrium reached after some hours between thoron and its decays product.
with MCNPX is 31 + 4 % for the thoron (or radon) peak. This is very interesting in the authors’ geometry because both gases will have the same detection efficiency. The decay products will have an efficiency of ,50 + 6 % due to the alpha particles, which may backscatter from the detector surface. The uncertainty is large because it takes into account the large variations of air pressure, temperature and humidity. The variation of these parameters modifies the density, which changes the detection efficiency. A part of the uncertainty is also due to the volume estimation of the system. Use of an electric field to catch decay products on the surface of the detector The decay products from radon (218Po) and thoron (216Po) are positively charged particles(7). In the measurement volume, the air mixed with radon or thoron is filtered just at the entrance. Hence, there is only radon or thoron and air entering the authors’ volume. The decay products of radon or thoron come only from the gas disintegration in the measurement volume. In this volume, there is an electric field
(400 kV m21), which allows the decay products to be caught on the detector surface. The authors used the deterministic code Comsol Multiphysics 4.3b to build and optimise their system(8). This code allows the simulation of the flow rate, electric field and particle trajectories in a 3D geometry (cf. Figure 3). Combining these data, the authors updated their geometry and defined the appropriate air flow and electric field. In the example presented in Figure 3, the decay products are caught on the detector surface within 80 ms for the longest particle path.
FIRST RESULTS WITH THE THORON REFERENCE MEASUREMENT SYSTEM Measurement of the radon reference atmosphere The LNE-LNHB already produces a reference atmosphere with the primary radon standard mixed with air in a closed volume. Since this atmosphere is known, and since the thoron measurement system can also measure radon, the first step is to check the gas detection efficiency. The system is connected to this reference atmosphere, and measurements are made at an atmospheric pressure of 1003 + 2 hPa, with a flow rate of 1 l min21 inside the measurement. The activity concentration of radon is 1.00 + 0.01 MBq m23 mixed with filtered air (HEPA filter) and a relative humidity of 3 + 3 % RH. The measured spectrum is presented in Figure 4. With this geometry, it is possible to see the radon gas and its decay products. The resolution of the decay product peaks is such that there is no overlap with the radon peak. The tails to the left of the decay product peaks are very small compared with the counting rate in the authors’ region of interest. The 218Po has a very small effect on the 222Rn peak; the overlapping tail area is ,0.4 % of the total radon peak.
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Figure 1. Diagram of the thoron reference measurement system. The decay products are caught on the detector surface, whereas the gas fills the entire volume.
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD
Figure 6. Spectrum of a thoron measurement in the new chamber without an electric field and a flow rate of 1 l min21 (700-s measurement).
measurements are then corrected with the radon decay to a given reference time. The corrected measurements show a good agreement. The relative expanded uncertainty is 1.4 % (k ¼ 2), which is a good result for this first measurement. Measurement of a thoron atmosphere
Figure 5. Series of radon measurement in the chamber with a constant air flow of 1 l min21 and a high radon activity concentration. Diamond represents counting in radon window. Circle denotes counting in radon window with decay correction. Line represents average value. Dashed line represents standard deviation.
From this measurement, since the authors know the 222Rn activity, they measured the efficiency of their system as 35 + 2 %. The uncertainty is rather high for this first measurement, mainly due to the uncertainty on the total chamber volume. In the future, the efficiency of the system should be defined with a relative uncertainty of 0.3 %. The second step is to define the system reproducibility and stability with time. Maintaining the same conditions, with the reference atmosphere circulating in the measurement volume, a series of 2000-s measurements were made with this radon atmosphere; the results are presented in Figure 5. The region of interest used to define the count rate for the radon peak can be seen in Figure 4. These measurements were made in a closed volume; hence, the radon activity decreases with time. This effect is well represented in Figure 5; the
The first measurements were made with a 228Th source from Pylonw, which can be used to create a thoron atmosphere. A constant air flow (1 l m21) is maintained at the entrance of the source. The air is filtered, radon free and relative humidity (3 + 3 % RH), and at constant atmospheric pressure (1003 + 2 hPa) and temperature (25 + 4 8C). This air is mixed with the thoron and filtered before it enters the measurement volume; the thoron activity concentration is (1.0 + 0.2 MBq m23). In this experiment, the measurements were initially done without an electric field (Figure 6). With such results, it is not possible to identify each peak with the corresponding isotope; thus, it is not possible to define which peaks are gas and which are decay products. The 216Po peak overlaps with the 220 Rn peak and, hence, it is not possible to accurately determine the peak areas. Since both behave differently in the volume, it is not possible to use the sum to define the activity. Also, the authors are not able to see the 212Bi in this spectrum since it is within the thoron peak. The second measurement is done under the same conditions, but with an applied electric field. It is now possible to identify each decay product and the gas in the spectrum. The same region of interest is selected in both spectra. It corresponds to the 216Po peak, highlighted in Figures 6 and 7. In this region, for the measurement without an electric field, the counting rate is 50 s21, whereas with the electric field it becomes 550 s21. These first results show the efficiency of the electric field and the overlap between
Page 3 of 5
Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
Figure 4. Spectrum of a radon measurement in the new chamber with an electric field and a flow rate of 1 l min21. This 2000-s measurement has been done after 17 h of exposure to the radon atmosphere. The equilibrium is reached between the 218Po caught on detector and 214Po build up from 218Po decay.
B. SABOT ET AL.
Figure 8. Spectrum measured after the measurement presented in Figure 7 and once the volume has been cleaned of thoron.
the thoron and its first decay products disappears, as the decay products are well trapped on the detector surface. The resolution of the 216Po peak is still not very good due to the detector surface and the electronics used. The tail of the 216Po peak overlaps with the 220 Rn peak, but this effect is very small (0.4 % of the peak area). Concerning the 212Po peak, there is a tail on the right of the peak, which is unusual for an alpha peak. It is due to the coincidence between the beta emitted from 212Bi and the alpha produced by its daughter 212Po, which decays too rapidly [300 ns(1)] to be discriminated by the system electronics (time constant of 2 ms). For this reason, it is not possible to use this peak for the measurement, as the efficiency is too difficult to determine. There remains one decay product (212Bi) that may interfere significantly with the 220Rn peak after a long measurement. By comparing the measurement without an electric field, one can see its contribution. It is very low in Figure 7, but for a longer measurement time, a larger peak appears. There are two possibilities to remove this peak from the spectrum: The first method is by calculation; since the 216Po amount deposited on the detector surface is known from the spectrum, it is possible to deduce the 212Bi activity, assuming all of the daughters of 216Po stays on the detector surface. The second approach is to directly measure using alpha spectrometry: once the initial measurement is finished, the measurement volume is flushed with clean air to remove the thoron. The following measurement is then sensitive to only the decay products remaining from the previous measurement (212Bi and 212 Po, cf. Figure 8). As a result, the system allows a useful spectrum to be measured directly of the thoron gas. It is now necessary to accurately determine the efficiency for a thoron measurement. Since the spectrum obtained with thoron present is a similar form to the radon one, it is possible to use an equivalent analysis to
define each gas activity. However, there is the effect of the 212Bi within the thoron peak that must also be addressed, since this does not appear with radon. CONCLUSION Using Monte Carlo and Comsol simulations, the authors have defined an appropriate geometry to measure a thoron atmosphere under specific conditions. This system allows to trap the decay products on the detector surface and has proved to be efficient for measuring thoron. As a result, the authors have obtained useful spectra for the measurement of a thoron atmosphere, since they can directly measure the gas and also the decay products. The gas peak and the decay product peaks can be both used to evaluate the thoron concentration of a thoron reference atmosphere. The first experiment, with thoron and radon, allowed to define a gas detection efficiency and also to see how the response of the authors’ system changes through time. Nevertheless, it is still desirable to improve the system further; a new prototype is planned with new electronics, which should increase the resolution. Finally, the authors will also improve the spectrum analysis and their measurement technique to have a small and well defined uncertainty on the thoron measurement. REFERENCES 1. Be´, M. -M., Chiste´, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Scho¨nfeld, E. and Dersch, R. Table of Radionuclides, Monographie BIPM-5 (2004). http://www.nucleide.org/Publications/ monographies_bipm.htm (30 March 2015, date last accessed). 2. Kim, Y. J., Lee, H. Y., Kim, C. S., Chang, B. U., Rho, B. H., Kim, C. K. and Tokonami, S. Indoor radon, thoron and thoron daughter concentrations in Korea. Int. Congr. Ser. 1276, 46–49 (2005).
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Figure 7. Spectrum of a thoron measurement in the new chamber with an electric field and a flow rate of 1 l min21 (100-s measurement after 1 h exposed to thoron atmosphere).
DEVELOPMENT OF A PRIMARY THORON ACTIVITY STANDARD 3. Baciu, A. Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania). J. Environ. Radioact. 83, 171– 189 (2005). 4. Chen, J. and Moir, D. A study on the thoron sensitivity of radon detectors available to Canadians. J. Radiol. Prot. 32, 419– 425 (2012). 5. Picolo, J. L. Absolute measurement of radon 222 activity. Nucl. Instrum. Methods Phys. Res. A. 369, 452–457 (1996).
6. Pelowitz, D. B. et al. MCNPX 2.7.0 Extensions, LA-UR02295 (2011). https://mcnp.lanl.gov/ (30 March 2015, date last accessed). 7. Dua, S. K. and Kotrappa, P. Comment on the charge on decay products of thoron and radon. Am. Ind. Hyg. Assoc. 42, 242 –243 (1981). 8. Comsol. Comsol Multiphysics Version 4.3b release notes. www.comsol.com (30 March 2015, date last accessed) (2014).
Downloaded from http://rpd.oxfordjournals.org/ at University of Tasmania Library on April 29, 2015
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