Sensitivity to Thoron on Passive Radon Detectors

Transcription

1 Sensitivity to Thoron on Passive Radon Detectors S. Tokonami 1, H. Yonehara 1, T. Sanada 2, M. Yang 3, M. Furukawa 1 and Y. Yamada 1 1 National Institute of Radiological Sciences, Chiba , Japan 2 Japan Chemical Analysis Center, Chiba , Japan 3 Research Institute of Uranium Mining, Hunan, China INTRODUCTION Although a lot of studies on radon have been done for a long time, there are few reports on thoron. It was considered that the presence of thoron could be negligible because of its own quantity compared to that of radon. However, recent studies have shown high thoron levels in our living environment (1-4). In particular, high thoron concentrations were occasionally observed in some areas, Japan (1,2). This fact made it clear that some of passive radon detectors were sensitive to the presence of thoron. They are commonly used for indoor radon surveys. It is possible that such detectors will give false values unless they are placed properly. Therefore, it is important to understand the detector response to thoron before practical use. A compact thoron chamber system was set up for the purpose. In this study, the followings are shown: configuration of the thoron chamber and sensitivity to thoron of several passive radon detectors. MATERIALS AND METHODS The thoron chamber system consists of two parts: an exposure chamber and a thoron gas generator. They are connected through an inline filter holder, a flow meter and an external pump. Figure 1 illustrates a sectional view of the thoron chamber system when thoron exposure tests are carried out for passive radon detectors. The exposure chamber is a 150 liters cylindrical vessel (565 mm diameter and 600 mm height), made of stainless steel. Four metal tubes are attached to the lid of the chamber. They are used to supply/exhaust thoron/radon gas and to take air samples. A fan is mounted on the lid to make the internal concentration uniform. The generator is a 10 liters stainless steel cylindrical vessel (240 mm diameter and 240 mm height). The vessel is filled with thorium-rich ceramics as the solid source. The ceramics were crushed into small pieces so as to promote thoron exhalation. After connecting the exposure chamber and generator, thoron gas circulates through the system with the external pump. The thoron concentration in the exposure chamber depends on the flow rate of the circulation. The flow rate was set at around 20 L/min in this system. Thoron and radon concentrations are measured with scintillation cells after taking an air sample through the metal tube. The measurement technique is based on two measurements using the difference of the half life between radon and thoron. Details on the technique are described in another paper (5). 1

2 sampling tube fan to scintillation cell pump flow meter 20 L/min O-ring inline filter holder air outlet thoron + radon gas air inlet 600 mm 565 mm passive device thorium-rich ceramics exposure chamber Figure 1. metal wire screen thoron gas generator A sectional view of the thoron chamber system when thoron exposure tests are carried out for passive radon detectors. RESULTS AND DISCUSSION In order to check the stability of the thoron chamber system, the radon concentrations in the exposure chamber were measured after radon gas was injected. Air samples were occasionally taken through the sampling tube. They were measured with the standard technique using the scintillation cell. The radon concentrations measured follow the half life of radon as shown in Figure 2. Since the exposure condition seems to be fairly stable, the configuration as the exposure chamber is satisfactory. This chamber system can also be used for the radon exposure test. As another test, thoron and radon concentrations in the exposure chamber were measured after connecting the exposure chamber with the thoron gas generator through the external pump, as shown in Figure 1. Air samples were occasionally taken while the thoron gas circulated through the system at the flow rate of 19.5 L/min. Their concentrations were shown in Figure 3. This performance test lasted 4 days. The average thoron and radon concentrations during the period were estimated to be 2530 Bq/m 3 and 230 Bq/m 3, respectively. In order to confirm the reliability of the proposed technique, the radon concentrations using the proposed technique were compared with those using the standard technique. The result was shown in Figure 4. There was a good agreement between the two. 2

3 10 6 Rn conc. (Bq/m 3 ) 10 5 Actual Rn Estimated Rn Elapsed time (day) Figure 2. Time variation of the radon concentration in the exposure chamber after radon gas injection. In the present study, the following passive radon detectors were actually examined: German monitors (CR-39 installed) (6), RADTRAK, R-T discriminative dosimeters (7) and electrostatic integrating radon monitors (EIRM) (8). The result of the conversion factor on the radon and thoron concentration is shown in Table 1. The radon conversion factors on the RADTRAK and R-T discriminative dosimeter were determined with the authorized facilities in advance. In terms of the RADTRAK, the conversion factors have changed compared to the previous result (9). The German monitor and the RADTRAK were highly sensitive to thoron. The R-T discriminative dosimeter consists of two radon detectors with two different ventilation rates. Since the upper hemisphere has a high ventilation rate, thoron concentrations are mainly measured. The lower hemisphere is designed to detect radon only. However, it was found that even this lower hemisphere was affected by thoron in this exposure test. On the other hand, the EIRM had no sensitivity to thoron. Although most of passive radon detectors are designed to minimize the entry of thoron, it can be realized that they are sensitive to the presence of thoron except one device in the present study. Table 1 Conversion factors on radon and thoron concentrations. Measuring device Radon Thoron German monitor (CR-39) x x 10-4 RADTRAK 6.74 x x 10-4 Radon-Thoron discriminative dosimeter (upper hemisphere) 2.40 x x 10-4 Radon-Thoron discriminative dosimeter (lower hemisphere) 2.82 x x 10-4 Electrostatic integrating radon-222 monitor x 10-4 No sensitivity unit: tracks/mm 2 /(Bq/m 3 -d) 3

4 Concentration (Bq/m 3 ) 10 3 Thoron Radon Elapsed time (day) Figure 3. Thoron and radon concentrations while the thoron gas circulates through the chamber system. 3 ) Rn conc. with standard tech. (Bq/m Rn conc. with prompt tech. (Bq/m 3 ) Figure 4. Comparison of the radon concentration with two different techniques. CONCLUSION In order to check the sensitivity to thoron on passive radon detectors, the thoron chamber system has been set up. Subsequently, performance tests of the chamber system were carried out so as to confirm its stability and reliability. Although some passive radon detectors were examined, most of them were sensitive to thoron. Although the presence of thoron can be negligible in the environment, it is important to check the sensitivity to thoron with the proposed test before practical use. 4

5 ACKNOWLEDGEMENTS The authors thank Ms. Ikumi Ishikawa, National Institute of Radiological Sciences, for her technical assistance on the exposure test. REFERENCES 1. M. Doi and S. Kobayashi, Characterization of Japanese Wooden Houses with Enhanced Radon and Thoron Concentrations. Health Phys., 66, (1994). 2. J. Ma, H. Yonehara, T. Aoyama, M. Doi, et al., Influence of Air Flow on the Behavior of Thoron and its Progeny in a Traditional Japanese House. Health Phys., 72, (1997). 3. W.H. Chung, S. Tokonami and M. Furukawa, Preliminary Survey on Radon and Thoron Concentrations in Korea. Radiat. Prot. Dosim. 80, (1998). 4. L.F. Toussaint, S. Tokonami, M. Doi, et al., The Measurement of Thoron Concentrations in Australia Using the Japanese Passive R-T Dosimeter. In: Proceedings of the 7 th Tohwa University International Symposium, , Fukuoka (1998). 5. S. Tokonami, T. Sanada, H. Yonehara, et al., Detection response to the presence of thoron on passive radon detectors. Radioisotopes, 49, 1-8 (2000) [in Japanese]. 6. M. Urban and E. Piesch, Low level environmental radon dosimetry with a passive track etch detector device. Radiat. Prot. Dosim., 1, (1981). 7. M. Doi and S. Kobayashi, The passive radon-thoron discriminative dosimeter for practical use. Hoken Butsuri, 29, (1994). 8. T. Iida, Y. Ikebe, T. Hattori, et al., An electrostatic integrating 222 Rn monitor with cellulose nitrate film for environmental monitoring. Health Phys., 54, (1988). 9. M.D. Pearson and R.R. Spangler, Calibration of alpha-track monitors for measurement of thoron ( 220 Rn). Health Phys., 60, (1991). 5