Development of radon sources with a high stability and a wide range K. Fukutsu and Y. Yamada Citation: Review of Scientific Instruments 84, 125110 (2013); doi: 10.1063/1.4847155 View online: http://dx.doi.org/10.1063/1.4847155 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Demonstration and development of control mechanism for radioactive sources in Saudi Arabia AIP Conf. Proc. 1448, 211 (2012); 10.1063/1.4725457 AAPM Task Group 108: PET and PET/CT Shielding Requirements Med. Phys. 33, 4 (2006); 10.1118/1.2135911 Accounting for high Z shields in brachytherapy using collapsed cone superposition for scatter dose calculation Med. Phys. 30, 2206 (2003); 10.1118/1.1587411 Development of Collaborative Engineering Environments for Spacecraft Design AIP Conf. Proc. 654, 907 (2003); 10.1063/1.1541384 Nuclear fragmentation cross sections for NASA database development AIP Conf. Proc. 610, 285 (2002); 10.1063/1.1469936
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 125110 (2013)
Development of radon sources with a high stability and a wide range K. Fukutsu and Y. Yamada National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan
(Received 15 July 2013; accepted 28 November 2013; published online 19 December 2013) A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. The emission rate of radon emanated from the disk depended on the content of 226 Ra and the sintering temperature. A 226 Ra sulfate (226 RaSO4 ) solution was dropped on a fibrous SiC ceramic disk (33 mmφ) of 1 mm in thickness, and sintered at 400 ◦ C. The radon concentration from a disk containing 226 Ra of 1.85 MBq was measured to be 38 kBq m−3 at a carrier airflow rate of 0.5 L min−1 . By adjusting the 226 Ra content or the sweep airflow rate, the radon concentrations were easily controlled over a wide range of over three orders of magnitude. The concentration was very stable for a long term. The compactness of the source disk made is easy for handling the source container and the shielding of gamma radiation from 226 Ra and its decay products. Such advantages in a radon generation system are desirable for experiments of high-level, large-scale radon exposure. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4847155] I. INTRODUCTION
Radon is the maximum factor of natural radiation sources, and occupies about half of the global average individual dose of 2.4 mSv y−1 .1 Many epidemiological studies have revealed that high-level radon exposure, such as in uranium mines causes lung cancer. How should low-level radon in the natural environment be considered? Various radon generation methods have been used for exposure studies. One of them is a bubbling method, which releases radon gas from a radium solution (i.e., radium chloride, 226 RaCl2 ) by bubbling. This method had been used in the exposure of rats to radon and radon progeny at ITRI (Inhalation Toxicology Research Institute, USA).2 In CEA (Atomic Energy Commission) of France, high-grade uranium ore, full of a mixture of 238 U and 232 Th, have been used in large-scale exposure studies. The radon gas was sucked out from stainless-steel underground tanks containing ore.3 In AEA (Atomic Energy Authority) of UK, 226 Ra was coprecipitated with 3 g zirconium phosphate, and enclosed in a porous stainless-steel tube. Six sources, containing different radium contents, are connected in parallel, so that a wide range of radon outputs can be selected for the exposure of animals to radon.4 On the other hand, a commercially available source for detector checks or calibration (Pylon, Model RN-1025, Canada), containing radium dry powder, was used to irradiate blood cells in vitro with radon gas.5 However, several problems concerning the use have been pointed out. The bubbling method is certainly easy to generate radon gas, but the emanated radon concentration might go up with condensing the solution. Also, some small droplets containing radium might not return to the solution. They caused instability of radon generation, and spoiled reproduction of the radon concentration. An ore method can become simple generator only with uranium ore and airflow system. But generally radium content in the ore is not high. It needs a great amount of ore, so that large space for containers and thick radiation shielding are indispensable. A dry powder method might be good for the radon emission rate. Of course, it de0034-6748/2013/84(12)/125110/5/$30.00
pends on the powder size. But the powder might clog a filter pore, and damage the airtightness of the valve. Finally, Strong’s method was adopted as a base method in our study. Our targets in the development of radon source are a higher radon emission rate, a higher stability, a higher radiationresistance, and better handling. II. MATERIALS AND METHODS A. Source disk
In order to obtain a high radon concentration with stability, various materials were investigated as a source disk. Since a thick layer of radium or its compound disturbs the emanation of radon, it is desirable that the radium is coated as widely as possible. Also, heat-resistance is needed for sintering under high temperature. A ceramic material with a fibrous structure realized the ideal situation. The selected material was a silicon carbide (SiC) ceramic as shown in Figure 1. A porous SiC ceramic body was made from a cellulose filter and poly-carbo-silane.6 A body of 33 mm in diameter and 1 mm in thickness was mounted in a support ring, which was made from stainless steel with a size of 35 mm in diameter and 2 mm in thickness. To prevent any elution of coprecipitated radium, two insoluble compounds of radium sulfate and radium titanate were investigated. Radium-226 chloride solution was uniformly dropped on the disk, and the disk was dried using an infrared lamp. In addition, sulfuric or titanium acid solution was uniformly dropped, and also the disk was dried. Then, the disk was sintered in an electric oven for 1 h. The dropped 226 Ra ranged from 18.5 MBq to 0.185 kBq. Four sintering temperatures over a range of from 400 to 1000 ◦ C were selected. Figure 2 shows a SiC disk after sintering. The source disks were enclosed in a metal-seal container with valves. The source container was put in a special container made of lead for radiation shielding. The dose rate constant of 226 Ra on the committed effective dose equivalent is 0.00105 μSv m2 MBq−1 h−1 .7 However, if all decay products from 222 Rn to
84, 125110-1
© 2013 AIP Publishing LLC
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125110-2
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
FIG. 3. Schematic diagram of radon concentration measurement setup.
The activity of 222 Rn, A2 , at time t is given by A2 = A1 × 1 − (1/2)t/T , FIG. 1. Electron microscope photograph (SEM, ×270) of SiC ceramic with a fibrous structure.
206
where A1 is the activity of 226 Ra (Bq m−3 ), and T is the radiological half-life of 222 Rn (= 3.824 d). When the flow rate of sweep air is q (m3 s−1 ), CT is given by CT = A2 /q.
Pb are included in the dose, it grows with about 200 times 0.214 μSv m2 MBq−1 h−1 . Therefore, the compactness values in size and shape of our source disk were absolutely desirable for radon exposure studies.
III. RESULTS AND DISCUSSION
B. Radon concentration test
A. Relationship between radium sulfate and the radon emission rate
Basic tests on radon emanation were made using a single disk ranging from 1.85 MBq to 1.85 kBq. Figure 3 shows a schematic diagram of radon concentration measurement set up. The radon concentrations were measured with the AB5 with a scintillation cell of 300A (Pylon Electronics Inc., Canada) for a short-term test and with the AlphaGuard (Genitron Instruments GmbH, Germany) for a long-term test. Both instruments were operated in a continuous flow mode. The radon emission rate, E (%), was determined to be E = CM /CT × 100,
(1)
where CM is the measured radon concentration (Bq m−3 ) and CT is the theoretical radon concentration (Bq m−3 ).
FIG. 2. Photograph of a radon source disk made of SiC ceramic.
(2)
(3)
A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. In order to know the characteristics of our radon source, several tests were made. At first, the radon emission rates from two radium compounds in various sintering temperatures were measured using an AB5 with scintillation cell of 300A. As shown in Figure 4, the radon emission rate depended on the sintering temperature. In the case of radium sulfate, 226 RaSO4 , the rate of 42.7% ± 0.2% at 400 ◦ C decreased to 12.8% ± 0.3% at 800 ◦ C. The 226 Ra seeped into the ceramic, because SiC was quickly eroded when in contact with liquid solution at higher temperature. Therefore 226 Ra remained on the surface of SiC fiber at low temperature, and the emission rate was high. The radon emission rate, ESulfate , can be estimated with the following equation: ESulfate = 73.9 − 0.0748 TS , R2 = 0.979, where
FIG. 4. Relationship between the radon emission rate and the sintering temperature.
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125110-3
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
FIG. 5. Effects of the relative humidity on the radon emission rate.
TS is the sintering temperature. The rate of radium titanate, 226 RaTiO3 , also showed a similar decreasing. The radon emission rate, ETitanate , can be estimated with the following equation: ETitanate = 87.9 − 0.0856 TS , R2 = 0.999. It was slightly higher than that of radium sulfate, but the chemical handling was more complex. In order to shorten the exposure time in the source preparation, radium sulfate was selected. Also, a sintering temperature of 400 ◦ C was selected. B. Effects of the relative humidity on the radon emission rate
The effects of the relative humidity of sweep air on the radon emission rate were examined. The test was done with increasing of the relative humidity from 10 to 73 RH% step by step, and finally the humidity was returned to the starting point. The relative humidity of the sweep air was regulated by mixing 100% humid and dry air. The radon concentration was measured after making the humidity condition for 8–12 h. As shown in Figure 5, the emission rates depended on the humidity. The rate of 29.0% ± 0.8% at dry air of 10 RH% increased up to 52.7% ± 2.0% at humid air of 73 RH%, and moved back to 28.7% ± 1.2% when the sweep
FIG. 6. Relationship between the source activity of 226 Ra and the emanated radon concentration.
FIG. 7. Response of the emanated radon concentration at a change of the source disk or the sweep flow rate. The condition of source disk and flow rate is (a) 18.5 kBq, 0.5 L min−1 , (b) 18.5 kBq, 0.05 L min−1 , (c) 1.85 MBq, 0.5 L min−1 , and (d) 1.85 MBq, 0.05 L min−1 .
air became dry. The radon emission rate, E, can be estimated with the following equation: E = 26.2 e (0.00945 H) , R2 = 0.998, where H is the relative humidity. It is well known that the radon emanation of nature is affected by moisture.8 From the ion collision theory, Sun and Furbish simulated the moisture effect that the radon emanation rate is positively correlated with the moisture saturation in a porous media.9 In a partially saturated condition, the water is retained in the pores by capillary forces. The water stops the recoil radon from embedding into another part of the pore wall. This mechanism would act well to this radon source cause of sufficient porous SiC disk. This effect is good for elevating the emission rate, but inversely it might be hard to maintain the stability of the radon concentration. The emission rate ranging from 29% to 53% is the almost the same as the dry powder source supplied by Pylon.10 However, our disk source is superior in practical uses, considering the difficulty in the handling of powder.
FIG. 8. Stability of the emanated radon concentration.
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125110-4
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
TABLE I. Daily fluctuation of the emanated radon concentration. (a) Case of high radon concentration Day
Average Std. dev. Maximum Minimum
1
2
3
4
5
6
7
8
9
10
11
12
1–12
8.32 0.10 8.60 8.15
8.25 0.09 8.56 8.07
8.26 0.12 8.68 8.07
8.18 0.08 8.35 7.99
8.12 0.09 8.27 7.95
8.09 0.04 8.19 7.95
8.27 0.23 9.17 7.99
8.14 0.06 8.5 8.07
8.09 0.07 8.31 7.99
8.05 0.08 8.35 7.95
8.15 0.05 8.35 8.03
8.21 0.05 8.35 8.07
8.18 0.13 9.17 7.95
9 1.51 0.10 1.76 1.22
10 1.51 0.11 1.78 1.18
11 1.49 0.10 1.77 1.29
12 1.48 0.10 1.67 1.27
1–12 1.47 0.11 1.79 1.11
(b) Case of low radon concentration Day
Average Std. dev. Maximum Minimum
1 1.39 0.11 1.74 1.11
2 1.40 0.10 1.62 1.13
3 1.44 0.10 1.75 1.23
4 1.49 0.10 1.77 1.25
5 1.48 0.10 1.79 1.23
6 1.44 0.10 1.70 1.24
C. Stability of radon concentration
The emanated radon concentrations from various source disks containing 226 Ra were measured with the AlphaGuard. The flow rates of sweep air were 0.5 or 0.05 L min−1 . Figure 6 shows the measurement results. The concentration of radon emanated from 1.85 kBq-226 Ra disk was (3.26 ± 0.59) × 102 Bq m−3 at an airflow rate of 0.5 L min−1 . In the case of 1000-times higher activity disk, it was (3.81 ± 0.13) × 104 Bq m−3 . The radon concentration of C0.5 at a sweep airflow rate of 0.5 L min−1 can be estimated with the following equation: C0.5 = 1.94 ARa 0.669 , R2 = 0.991, where ARa is the 226 Ra activity in the source disk. And the radon concentration of C0.05 at the sweep airflow rate of 0.05 L min−1 can be estimated with the following equation: C0.05 = 21.3 ARa 0.660 , R2 = 0.996. The radon concentrations in both sweep airflow rates increased linearly with the source activity, but the slope was less than 45◦ . The emanation ability decreased relatively with increasing the source activity. This might be caused by a depression of radon emanation due to the thick layer of the sintered radium sulfate. Figure 7 shows the response of the radon concentration upon changes of the source disk and the sweep airflow rate. When the sweep airflow rate was decreased from 0.5 to 0.05 L min−1 , the radon concentration increased by a factor of about 10 for 30 min. It took more 10 or 20 min to become steady. AlphaGuard is an ionization chamber with an effective volume of 0.56 L. Even if sample air were to be drawn by 0.05 L min−1 , it would take 30 min for the exchange rate to reach 93% (= (1-e−0.05×30/0.56 ) × 100). The response time in the measurements was almost the same as that in the ideal air exchange. This indicated that radon emanation, itself, from the source disk is stable, and its concentration is easy to control by the sweep airflow rate. In the case of the source disk exchange, the sweep airflow rate once returned to 0.5 L min−1 . At a higher airflow rate by ten times, it was estimated to be reduced in only 3 min for the exchange rate to reach 93%. This quick response was observed in the second change of the response curve.
7 1.47 0.10 1.74 1.25
8 1.49 0.10 1.72 1.25
Figure 8 shows the results of two long-period measurements. The concentrations were measured with AlphaGuard in the flow mode. Daily changes for 12 days are summarized in Table I. The concentrations changed at random. In the low-concentration example, the average concentration for 1 day ranged from 1.39 × 103 to 1.51 × 103 Bq m−3 . The scattering was small, and there was no tendency of increasing or decreasing. In the high-concentration example, the scattering range became smaller and more stable. Thus, the stability of the emanated radon concentration for a long term was validated. IV. CONCLUSIONS
A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. The radon concentration emanated from a disk containing 226 Ra of 1.85 MBq was measured to be 38 kBq m−3 at a sweep airflow rate of 0.5 L min−1 . By adjusting the 226 Ra content or the sweep airflow rate, the radon concentrations were easily controlled over a wide range of over three orders of magnitude. The concentration was very stable for a long time. However, since it depended on the relative humidity of the sweep air, it was important to keep air conditioning for radon stability. The disk was protected by a guard-ring of 35 mm in diameter and 2 mm in thickness, so that plural disks could be compactly stacked in a cylindrical way with an appropriate opening. The compactness made it easy for handling the source container and the shielding of gamma radiation from 226 Ra and its decay products. Such advantages in a radon generation system are desirable for experiments involving highlevel and large-scale radon exposure. ACKNOWLEDGMENTS
The authors wish to thank K. Tatenuma, K. Ishikawa from KAKEN, Co. and A. Koizumi from NIRS for their excellent support in this study. 1 United
Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), UNSCEAR 2008 Report (United Nations, New York, 2010), Vol. I, p. 6.
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125110-5 2 D.
K. Fukutsu and Y. Yamada
G. Thomassen, G. J. Newton, R. A. Guilmette, and N. F. Johnson, Radiat. Prot. Dosim. 38, 65 (1991). 3 G. Monchaux, J. P. Morlier, M. Morin, J. Chameaud, J. Lafuma, and R. Masse, Environ. Health Perspect. 102, 64 (1994). 4 J. C. Strong and M. Walsh, in Proceedings of 29th Hanford Symposium on Health and the Environment, Indoor Radon and Lung Cancer: Reality or Myth?, edited by F. T. Cross (Battelle Press, Columbus, 1990), p. 731. 5 V. Z. Hamza, P. R. V. Kumar, R. K. Jeevanram, R. Santanam, and B. Danalaksmi, Radiat. Prot. Dosim. 130, 343 (2008).
Rev. Sci. Instrum. 84, 125110 (2013) 6 Y.
Hasegawa and K. Okamura, J. Mater. Sci. Lett. 4, 356 (1985). Radioisotope Association, Radioisotope Data Book, 10th ed. (Maruzen, Tokyo, 2002), p. 71. 8 J. Porstendörfer, J. Aerosol. Sci. 25, 219 (1994). 9 H. Sun and D. J. Furbish, J. Contam. Hydrol. 18, 239 (1995). 10 R. Rolle, M. Grundel, R. Schulz, and J. Porstendörfer, “Radioactivity in the Environment,” in Proceedings of 7th International Symposium on Natural Radiation Environment (NRE-VII), Rhodes, Greece, 2002, edited by J. P. McLughlin, S. E. Simopoulos, and F. Steinhäusler (Elsevier, Amsterdam, 2005), Vol. 7, p. 404. 7 Japan
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 125110 (2013)
Development of radon sources with a high stability and a wide range K. Fukutsu and Y. Yamada National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan
(Received 15 July 2013; accepted 28 November 2013; published online 19 December 2013) A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. The emission rate of radon emanated from the disk depended on the content of 226 Ra and the sintering temperature. A 226 Ra sulfate (226 RaSO4 ) solution was dropped on a fibrous SiC ceramic disk (33 mmφ) of 1 mm in thickness, and sintered at 400 ◦ C. The radon concentration from a disk containing 226 Ra of 1.85 MBq was measured to be 38 kBq m−3 at a carrier airflow rate of 0.5 L min−1 . By adjusting the 226 Ra content or the sweep airflow rate, the radon concentrations were easily controlled over a wide range of over three orders of magnitude. The concentration was very stable for a long term. The compactness of the source disk made is easy for handling the source container and the shielding of gamma radiation from 226 Ra and its decay products. Such advantages in a radon generation system are desirable for experiments of high-level, large-scale radon exposure. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4847155] I. INTRODUCTION
Radon is the maximum factor of natural radiation sources, and occupies about half of the global average individual dose of 2.4 mSv y−1 .1 Many epidemiological studies have revealed that high-level radon exposure, such as in uranium mines causes lung cancer. How should low-level radon in the natural environment be considered? Various radon generation methods have been used for exposure studies. One of them is a bubbling method, which releases radon gas from a radium solution (i.e., radium chloride, 226 RaCl2 ) by bubbling. This method had been used in the exposure of rats to radon and radon progeny at ITRI (Inhalation Toxicology Research Institute, USA).2 In CEA (Atomic Energy Commission) of France, high-grade uranium ore, full of a mixture of 238 U and 232 Th, have been used in large-scale exposure studies. The radon gas was sucked out from stainless-steel underground tanks containing ore.3 In AEA (Atomic Energy Authority) of UK, 226 Ra was coprecipitated with 3 g zirconium phosphate, and enclosed in a porous stainless-steel tube. Six sources, containing different radium contents, are connected in parallel, so that a wide range of radon outputs can be selected for the exposure of animals to radon.4 On the other hand, a commercially available source for detector checks or calibration (Pylon, Model RN-1025, Canada), containing radium dry powder, was used to irradiate blood cells in vitro with radon gas.5 However, several problems concerning the use have been pointed out. The bubbling method is certainly easy to generate radon gas, but the emanated radon concentration might go up with condensing the solution. Also, some small droplets containing radium might not return to the solution. They caused instability of radon generation, and spoiled reproduction of the radon concentration. An ore method can become simple generator only with uranium ore and airflow system. But generally radium content in the ore is not high. It needs a great amount of ore, so that large space for containers and thick radiation shielding are indispensable. A dry powder method might be good for the radon emission rate. Of course, it de0034-6748/2013/84(12)/125110/5/$30.00
pends on the powder size. But the powder might clog a filter pore, and damage the airtightness of the valve. Finally, Strong’s method was adopted as a base method in our study. Our targets in the development of radon source are a higher radon emission rate, a higher stability, a higher radiationresistance, and better handling. II. MATERIALS AND METHODS A. Source disk
In order to obtain a high radon concentration with stability, various materials were investigated as a source disk. Since a thick layer of radium or its compound disturbs the emanation of radon, it is desirable that the radium is coated as widely as possible. Also, heat-resistance is needed for sintering under high temperature. A ceramic material with a fibrous structure realized the ideal situation. The selected material was a silicon carbide (SiC) ceramic as shown in Figure 1. A porous SiC ceramic body was made from a cellulose filter and poly-carbo-silane.6 A body of 33 mm in diameter and 1 mm in thickness was mounted in a support ring, which was made from stainless steel with a size of 35 mm in diameter and 2 mm in thickness. To prevent any elution of coprecipitated radium, two insoluble compounds of radium sulfate and radium titanate were investigated. Radium-226 chloride solution was uniformly dropped on the disk, and the disk was dried using an infrared lamp. In addition, sulfuric or titanium acid solution was uniformly dropped, and also the disk was dried. Then, the disk was sintered in an electric oven for 1 h. The dropped 226 Ra ranged from 18.5 MBq to 0.185 kBq. Four sintering temperatures over a range of from 400 to 1000 ◦ C were selected. Figure 2 shows a SiC disk after sintering. The source disks were enclosed in a metal-seal container with valves. The source container was put in a special container made of lead for radiation shielding. The dose rate constant of 226 Ra on the committed effective dose equivalent is 0.00105 μSv m2 MBq−1 h−1 .7 However, if all decay products from 222 Rn to
84, 125110-1
© 2013 AIP Publishing LLC
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125110-2
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
FIG. 3. Schematic diagram of radon concentration measurement setup.
The activity of 222 Rn, A2 , at time t is given by A2 = A1 × 1 − (1/2)t/T , FIG. 1. Electron microscope photograph (SEM, ×270) of SiC ceramic with a fibrous structure.
206
where A1 is the activity of 226 Ra (Bq m−3 ), and T is the radiological half-life of 222 Rn (= 3.824 d). When the flow rate of sweep air is q (m3 s−1 ), CT is given by CT = A2 /q.
Pb are included in the dose, it grows with about 200 times 0.214 μSv m2 MBq−1 h−1 . Therefore, the compactness values in size and shape of our source disk were absolutely desirable for radon exposure studies.
III. RESULTS AND DISCUSSION
B. Radon concentration test
A. Relationship between radium sulfate and the radon emission rate
Basic tests on radon emanation were made using a single disk ranging from 1.85 MBq to 1.85 kBq. Figure 3 shows a schematic diagram of radon concentration measurement set up. The radon concentrations were measured with the AB5 with a scintillation cell of 300A (Pylon Electronics Inc., Canada) for a short-term test and with the AlphaGuard (Genitron Instruments GmbH, Germany) for a long-term test. Both instruments were operated in a continuous flow mode. The radon emission rate, E (%), was determined to be E = CM /CT × 100,
(1)
where CM is the measured radon concentration (Bq m−3 ) and CT is the theoretical radon concentration (Bq m−3 ).
FIG. 2. Photograph of a radon source disk made of SiC ceramic.
(2)
(3)
A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. In order to know the characteristics of our radon source, several tests were made. At first, the radon emission rates from two radium compounds in various sintering temperatures were measured using an AB5 with scintillation cell of 300A. As shown in Figure 4, the radon emission rate depended on the sintering temperature. In the case of radium sulfate, 226 RaSO4 , the rate of 42.7% ± 0.2% at 400 ◦ C decreased to 12.8% ± 0.3% at 800 ◦ C. The 226 Ra seeped into the ceramic, because SiC was quickly eroded when in contact with liquid solution at higher temperature. Therefore 226 Ra remained on the surface of SiC fiber at low temperature, and the emission rate was high. The radon emission rate, ESulfate , can be estimated with the following equation: ESulfate = 73.9 − 0.0748 TS , R2 = 0.979, where
FIG. 4. Relationship between the radon emission rate and the sintering temperature.
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125110-3
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
FIG. 5. Effects of the relative humidity on the radon emission rate.
TS is the sintering temperature. The rate of radium titanate, 226 RaTiO3 , also showed a similar decreasing. The radon emission rate, ETitanate , can be estimated with the following equation: ETitanate = 87.9 − 0.0856 TS , R2 = 0.999. It was slightly higher than that of radium sulfate, but the chemical handling was more complex. In order to shorten the exposure time in the source preparation, radium sulfate was selected. Also, a sintering temperature of 400 ◦ C was selected. B. Effects of the relative humidity on the radon emission rate
The effects of the relative humidity of sweep air on the radon emission rate were examined. The test was done with increasing of the relative humidity from 10 to 73 RH% step by step, and finally the humidity was returned to the starting point. The relative humidity of the sweep air was regulated by mixing 100% humid and dry air. The radon concentration was measured after making the humidity condition for 8–12 h. As shown in Figure 5, the emission rates depended on the humidity. The rate of 29.0% ± 0.8% at dry air of 10 RH% increased up to 52.7% ± 2.0% at humid air of 73 RH%, and moved back to 28.7% ± 1.2% when the sweep
FIG. 6. Relationship between the source activity of 226 Ra and the emanated radon concentration.
FIG. 7. Response of the emanated radon concentration at a change of the source disk or the sweep flow rate. The condition of source disk and flow rate is (a) 18.5 kBq, 0.5 L min−1 , (b) 18.5 kBq, 0.05 L min−1 , (c) 1.85 MBq, 0.5 L min−1 , and (d) 1.85 MBq, 0.05 L min−1 .
air became dry. The radon emission rate, E, can be estimated with the following equation: E = 26.2 e (0.00945 H) , R2 = 0.998, where H is the relative humidity. It is well known that the radon emanation of nature is affected by moisture.8 From the ion collision theory, Sun and Furbish simulated the moisture effect that the radon emanation rate is positively correlated with the moisture saturation in a porous media.9 In a partially saturated condition, the water is retained in the pores by capillary forces. The water stops the recoil radon from embedding into another part of the pore wall. This mechanism would act well to this radon source cause of sufficient porous SiC disk. This effect is good for elevating the emission rate, but inversely it might be hard to maintain the stability of the radon concentration. The emission rate ranging from 29% to 53% is the almost the same as the dry powder source supplied by Pylon.10 However, our disk source is superior in practical uses, considering the difficulty in the handling of powder.
FIG. 8. Stability of the emanated radon concentration.
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125110-4
K. Fukutsu and Y. Yamada
Rev. Sci. Instrum. 84, 125110 (2013)
TABLE I. Daily fluctuation of the emanated radon concentration. (a) Case of high radon concentration Day
Average Std. dev. Maximum Minimum
1
2
3
4
5
6
7
8
9
10
11
12
1–12
8.32 0.10 8.60 8.15
8.25 0.09 8.56 8.07
8.26 0.12 8.68 8.07
8.18 0.08 8.35 7.99
8.12 0.09 8.27 7.95
8.09 0.04 8.19 7.95
8.27 0.23 9.17 7.99
8.14 0.06 8.5 8.07
8.09 0.07 8.31 7.99
8.05 0.08 8.35 7.95
8.15 0.05 8.35 8.03
8.21 0.05 8.35 8.07
8.18 0.13 9.17 7.95
9 1.51 0.10 1.76 1.22
10 1.51 0.11 1.78 1.18
11 1.49 0.10 1.77 1.29
12 1.48 0.10 1.67 1.27
1–12 1.47 0.11 1.79 1.11
(b) Case of low radon concentration Day
Average Std. dev. Maximum Minimum
1 1.39 0.11 1.74 1.11
2 1.40 0.10 1.62 1.13
3 1.44 0.10 1.75 1.23
4 1.49 0.10 1.77 1.25
5 1.48 0.10 1.79 1.23
6 1.44 0.10 1.70 1.24
C. Stability of radon concentration
The emanated radon concentrations from various source disks containing 226 Ra were measured with the AlphaGuard. The flow rates of sweep air were 0.5 or 0.05 L min−1 . Figure 6 shows the measurement results. The concentration of radon emanated from 1.85 kBq-226 Ra disk was (3.26 ± 0.59) × 102 Bq m−3 at an airflow rate of 0.5 L min−1 . In the case of 1000-times higher activity disk, it was (3.81 ± 0.13) × 104 Bq m−3 . The radon concentration of C0.5 at a sweep airflow rate of 0.5 L min−1 can be estimated with the following equation: C0.5 = 1.94 ARa 0.669 , R2 = 0.991, where ARa is the 226 Ra activity in the source disk. And the radon concentration of C0.05 at the sweep airflow rate of 0.05 L min−1 can be estimated with the following equation: C0.05 = 21.3 ARa 0.660 , R2 = 0.996. The radon concentrations in both sweep airflow rates increased linearly with the source activity, but the slope was less than 45◦ . The emanation ability decreased relatively with increasing the source activity. This might be caused by a depression of radon emanation due to the thick layer of the sintered radium sulfate. Figure 7 shows the response of the radon concentration upon changes of the source disk and the sweep airflow rate. When the sweep airflow rate was decreased from 0.5 to 0.05 L min−1 , the radon concentration increased by a factor of about 10 for 30 min. It took more 10 or 20 min to become steady. AlphaGuard is an ionization chamber with an effective volume of 0.56 L. Even if sample air were to be drawn by 0.05 L min−1 , it would take 30 min for the exchange rate to reach 93% (= (1-e−0.05×30/0.56 ) × 100). The response time in the measurements was almost the same as that in the ideal air exchange. This indicated that radon emanation, itself, from the source disk is stable, and its concentration is easy to control by the sweep airflow rate. In the case of the source disk exchange, the sweep airflow rate once returned to 0.5 L min−1 . At a higher airflow rate by ten times, it was estimated to be reduced in only 3 min for the exchange rate to reach 93%. This quick response was observed in the second change of the response curve.
7 1.47 0.10 1.74 1.25
8 1.49 0.10 1.72 1.25
Figure 8 shows the results of two long-period measurements. The concentrations were measured with AlphaGuard in the flow mode. Daily changes for 12 days are summarized in Table I. The concentrations changed at random. In the low-concentration example, the average concentration for 1 day ranged from 1.39 × 103 to 1.51 × 103 Bq m−3 . The scattering was small, and there was no tendency of increasing or decreasing. In the high-concentration example, the scattering range became smaller and more stable. Thus, the stability of the emanated radon concentration for a long term was validated. IV. CONCLUSIONS
A solid 222 Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. The radon concentration emanated from a disk containing 226 Ra of 1.85 MBq was measured to be 38 kBq m−3 at a sweep airflow rate of 0.5 L min−1 . By adjusting the 226 Ra content or the sweep airflow rate, the radon concentrations were easily controlled over a wide range of over three orders of magnitude. The concentration was very stable for a long time. However, since it depended on the relative humidity of the sweep air, it was important to keep air conditioning for radon stability. The disk was protected by a guard-ring of 35 mm in diameter and 2 mm in thickness, so that plural disks could be compactly stacked in a cylindrical way with an appropriate opening. The compactness made it easy for handling the source container and the shielding of gamma radiation from 226 Ra and its decay products. Such advantages in a radon generation system are desirable for experiments involving highlevel and large-scale radon exposure. ACKNOWLEDGMENTS
The authors wish to thank K. Tatenuma, K. Ishikawa from KAKEN, Co. and A. Koizumi from NIRS for their excellent support in this study. 1 United
Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), UNSCEAR 2008 Report (United Nations, New York, 2010), Vol. I, p. 6.
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125110-5 2 D.
K. Fukutsu and Y. Yamada
G. Thomassen, G. J. Newton, R. A. Guilmette, and N. F. Johnson, Radiat. Prot. Dosim. 38, 65 (1991). 3 G. Monchaux, J. P. Morlier, M. Morin, J. Chameaud, J. Lafuma, and R. Masse, Environ. Health Perspect. 102, 64 (1994). 4 J. C. Strong and M. Walsh, in Proceedings of 29th Hanford Symposium on Health and the Environment, Indoor Radon and Lung Cancer: Reality or Myth?, edited by F. T. Cross (Battelle Press, Columbus, 1990), p. 731. 5 V. Z. Hamza, P. R. V. Kumar, R. K. Jeevanram, R. Santanam, and B. Danalaksmi, Radiat. Prot. Dosim. 130, 343 (2008).
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Hasegawa and K. Okamura, J. Mater. Sci. Lett. 4, 356 (1985). Radioisotope Association, Radioisotope Data Book, 10th ed. (Maruzen, Tokyo, 2002), p. 71. 8 J. Porstendörfer, J. Aerosol. Sci. 25, 219 (1994). 9 H. Sun and D. J. Furbish, J. Contam. Hydrol. 18, 239 (1995). 10 R. Rolle, M. Grundel, R. Schulz, and J. Porstendörfer, “Radioactivity in the Environment,” in Proceedings of 7th International Symposium on Natural Radiation Environment (NRE-VII), Rhodes, Greece, 2002, edited by J. P. McLughlin, S. E. Simopoulos, and F. Steinhäusler (Elsevier, Amsterdam, 2005), Vol. 7, p. 404. 7 Japan
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