Development of high sensitivity radon detectors

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1 Nuclear Instruments and Methods in Physics Research A 421 (1999) Development of high sensitivity radon detectors Y. Takeuchi *, K. Okumura, T. Kajita, S. Tasaka, H. Hori, M. Nemoto, H. Okazawa Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Higashi-mozumi, Kamioka-cho, Gifu , Japan Department of Physics, Faculty of Education, Gifu University, Gifu , Japan Department of Physics, Faculty of Science, Tokai University, Kanagawa , Japan Department of Physics, Faculty of Science, Niigata University, Niigata , Japan Received 3 August 1998 Abstract High sensitivity detectors for radon in air and in water have been developed. We use electrostatic collection and a PIN photodiode for these detectors. Calibration systems have been also constructed to obtain collection factors. As a result of the calibration study, the absolute humidity dependence of the radon detector for air is clearly observed in the region less than about 1.6 g/m. The calibration factors of the radon detector for air are 2.2$0.2 (counts/day)/(mbq/m ) at 0.08 g/m and 0.86$0.06 (counts/day)/(mbq/m ) at 11 g/m. The calibration factor of the radon detector for water is 3.6$0.5 (counts/day)/(mbq/m ). The background level of the radon detector for air is 2.4$1.3 counts/day. As a result, one standard deviation excess of the signal above the background of the radon detector for air should be possible for 1.4 mbq/m in a one-day measurement at 0.08 g/m Elsevier Science B.V. All rights reserved. PACS: n Keywords: Radon; Super-Kamiokande; Electrostatic collection; Photodiode 1. Introduction KAMIOKA Nucleon Decay Experiment (Kamiokande) was an imaging water Cherenkov detector with 4500 tons of purified water. It was located about 1000 m underground (2700 m water equivalent) in the Mozumi mine, in Kamioka town in Gifu Prefecture, central Japan. * Corresponding author. Tel.: # ; fax: # ; takeuchi@icrr.u-tokyo.ac.jp. It is also called KAMIOKA Neutrino Detection Experiment). One of the main purposes of Kamiokande was to observe solar neutrinos. It observed solar neutrinos between December 1986 and February 1995 [1]. The energy of solar neutrinos is distributed from 0 to 18 MeV. The analysis at Kamiokande used the energy region above 7.0 MeV. The background events in the lower energy region were dominated by the beta decay of radon daughters remaining in the water. The radon concentration in the Kamiokande water was 0.5 Bq/m [2]. In April 1996, Super-Kamiokande began taking data. Super-Kamiokande is a large water Cherenkov detector with tons of highly purified /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 )

2 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) water which makes it possible for us to observe nucleon decay, solar neutrinos, atmospheric neutrinos and so on with high statistics. One of the main purposes of Super-Kamiokande is to lower the energy threshold of the solar neutrino analysis from 7.0 MeV (at Kamiokande) to 5.0 MeV, then measure the energy spectrum of solar neutrinos precisely. The current analysis at Super-Kamiokande uses the energy region above 6.5 MeV [3]. To achieve the 5 MeV threshold, the radon concentration in the water should be less than about 1/100th that of Kamiokande. Therefore, we upgraded the water purification system, upgraded the air purification system, and made the whole water tank airtight. For a radon detector, the detection limit should be less than 5 mbq/m, but there was no commercial radon detector which had such a high sensitivity. As a first step to monitor the radon concentration in water, a high sensitivity detector for radon in air has been developed. Next, a high sensitivity detector for radon in water has been developed. In that detector, the radon detector for air and a shower-type diffuser of radon gas in water are used. Fig. 1. A schematic view of the high sensitivity radon detector for air. 2. Radon detector for air The principle of radon detection which is used in this detector is the electrostatic collection of the daughter nuclei of Rn, and the energy measurement of the alpha decay with a PIN photodiode [4,5]. It was reported that more than 90% of Po atoms, which was one of the daughter nuclei of Rn, tended to become positively charged [6]. Fig. 1 shows a schematic view of the high sensitivity radon detector for air. It consists of a cylindrical stainless steel vessel, a PIN photodiode, a high-voltage divider and amplifier circuit, and a feed through. A negative high voltage is supplied to the p-layer of the photodiode. The vessel is grounded. Then, an electric field is produced in the vessel. The daughter nuclei which ionized positively are collected on the surface of the photodiode, then the energy of alpha decays are measured. The dimensions of the stainless steel vessel are 50 cm in diameter and 35 cm in height. The volume is 68.7 l. In order to achieve a low background level, Fig. 2. A typical pulse height spectrum for alpha particles from radon daughter products. The radon concentration is about 3 Bq/m for this plot. The signal region of Po decays is from 80 to 112 ADC channel for this detector. the inside of the vessel is electropolished. The PIN photodiode is electrically isolated from the stainless steel vessel with an acrylic plate and a ceramic feed through. The PIN photodiode is a Hamamatsu Photonics S with passivation finish. The glass cover plate is removed to measure the energy of alpha particles. The detection area, sensitive thickness, and capacity are 16 mm 16 mm, 500 μm, and 80 pf, respectively. A typical pulse height spectrum is shown in Fig. 2. In order to measure radon concentration, only the Po peak is used because that energy region has fewer noise events, no other alpha peak overlapped, and it has a higher

3 336 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) collection efficiency than Po. The signal region of Po decays is from 80 to 112 ADC channel for this case. It takes a few hours to reach radiative equilibrium of Po. However, it is not a problem in the case of radon monitoring in Super-Kamiokande. The typical period of the time variation of radon concentration at Super-Kamiokande is longer than a month. Concerning the electrostatic collection method, the neutralization effect of Po atoms was pointed out [7,8]. Hence, the humidity dependence of the calibration factor is carefully studied. 3. Radon detector for water Fig. 3 shows a schematic view of the high sensitivity radon detector for water. There are two sections. One is the diffuser of radon gas to air. The other is the electrostatic collection part for radon in air. The diffuser is a cylindrical stainless steel vessel attached to the bottom of the electrostatic collection vessel. They are separated by a stainless steel mesh to prevent the electric field from entering the diffuser vessel. In the diffuser vessel, there are many small plastic balls to enlarge the surface area. The sample water is put into the diffuser vessel through a in stainless steel pipe. The flow rate of the water is adjusted to 1 l/min. There are many small holes at the end of the water inlet pipe in order to make the water shower on the plastic balls. Water falls down along the plastic balls, and the radon gas dissolved in the water is diffused into the air during the fall. The radon gas diffused in air goes through the stainless steel mesh, then enters the electrostatic collection vessel. The electrostatic collection part is almost the same as the radon detector for air. We use the same PIN photodiode, electronics circuit, and feed through. The only difference is the shape of the electrostatic collection vessel and its volume. 4. Calibration system Fig. 3. A schematic view of the high sensitivity radon detector for water. Fig. 4 shows a schematic diagram of the calibration system for the radon detector for air. The high sensitivity radon detector for air was connected to a radon source, an ionization chamber, a vapor trap, and a syringe. The radon source consisted of small mine rocks and a 3 l glass vessel. The mine rocks were put in the vessel and covered with a membrane filter to keep dust out. The ionization chamber was used as the standard device to measure the radon concentration in this system. We used a 1500 cm ionization chamber made by Okura Electric Co. Ltd. The vapor trap consisted of a cooler, a vacuum bottle, and ethanol. The humidity in this system was controlled by setting the cooler temperature. The others were a barometer, a dew point meter, a flow meter, and a circulation pump. They were connected with in stainless steel tubes and rigid nylon tubes. Class A pure air made by Nihon Sanso Co. Ltd. was used as the air in this system. The circulation rate was 3 l/min. Before

4 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 4. A schematic diagram of the calibration system for the humidity dependence measurement. Fig. 5. A schematic diagram of the calibration system for the radon detector for water. starting calibration, the leak rate was checked, and it was less than 1 10 cm /s. The radon concentration was set to Bq/m during the calibration. Fig. 5 shows a schematic diagram of the calibration system for the radon detector for water. There are two parts; the radon water making system and the radon water circulation system. In order to calibrate the radon detector for water, water with a known amount of radon (radon water) is necessary. This radon water was made by using the radon water making system. At first, pure air was circulated through a radon source, an ionization chamber, and a 4.7 l buffer tank. The flow rate was adjusted to 1 l/min. It took about 6 h to fill the buffer tank with enough radon gas. At the end

5 338 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Table 1 Summary of the Rn water making. A liquid scintillation counter and an ionization chamber are used for measuring radon concentration in water and in air, respectively Water sample Rn con. in water (Bq/m ) Rn con. in air (Bq/m ) Ratio (water/air) Sample 1a Sample 1b Sample 2a Sample 2b Sample 2c Average 0.138$0.006 of this circulation, the radon concentration was measured by the ionization chamber. Next, the radon gas was circulated between the buffer tank and a 2 l water tank at a flow rate of 1 l/min. The water tank was filled with purified water before circulation. The water was bubbled with radon gas during circulation, then radon water was made. The radon concentration in this radon water was measured by a Liquid Scintillation Counter (LSC) system, and compared with the radon concentration in the gas before the circulation measured by the ionization chamber. Table 1 shows a summary of the Rn water making. There was a good correlation between them; therefore, the ionization chamber was mainly used to determine the radon concentration in the radon water. The typical radon concentration in the radon water was 10 Bq/m. In the radon water circulation system, the high sensitivity radon detector for water and a 20 l water buffer tank were connected with in rigid nylon tubes. The radon water was added to this system by using a constant flow rate pump. The flow rate of the pump was variable and we set it to 20 cc/6 min. In a typical case, we added 20 cc of 10 Bq/m radon water to 20 l water. This corresponded to a 0.5 Bq/m level calibration. 5. Results Fig. 6 shows the high voltage dependence of the calibration factor for the high sensitivity radon detector for air. The absolute humidity was kept at 1.2$0.2 g/m during this measurement. The radon source was introduced into the circulation system in order to keep the radon concentration constant. From this measurement, we chose!1500 V for the other calibrations because the calibration factor was almost saturated and electric noise became significant below!2000 V. Fig. 7 shows a typical response of the high sensitivity radon detector for air after radon gas injection. The humidity dependence of the count rate is seen in this figure. The data points were fit by using the following exponential function; N(¹)"N(0) exp!ln2 ) ¹ #Constant, (1) 3.8 then N(0) was obtained for each absolute humidity. In this equation, ¹ is the elapsed time since radon injection in the day, and N is the count rate at time ¹, and N(0) is the obtained count rate at the injection time. Comparing N(0) and the radon concentration measured by the ionization chamber, the calibration factor at each absolute humidity was obtained. Fig. 8 shows the humidity dependence of the calibration factor for the high sensitivity radon detector in air. Three kinds of methods of controlling the absolute humidity were used for this measurement. A syringe was used in Method 1. A cooler and a vapor trap were used in Methods 2 and 2. These results agree well. From Fig. 8, the absolute humidity dependence is clearly observed in the region less than about 1.6 g/m. Above about 2 g/m, it becomes constant. This behavior is qualitatively consistent with Ref. [9]. The averaged calibration factors obtained in these measurements

6 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 6. High voltage dependence of the calibration factor for a high sensitivity radon detector in air. The absolute humidity is 1.2$0.2 g/m. Fig. 7. A typical variation after Rn gas injection. are 2.2$0.2(stat.#syst.) (counts/day)/(mbq/m ) at 0.08 g/m and 0.86$0.06(stat.#syst.) (counts/day)/(mbq/m ) at 11 g/m. Systematic errors estimated in these calibration factors are the followings: accuracy of the dew point meter #1%!2% at 0.08 g/m and &0% at 11 g/m on the calibration factor, accuracy of the current measurement at the ionization chamber $4.2%, and accuracy of the conversion factor from the current value to the radon concentration at the ionization chamber $5%. We have calibrated three detectors. The deviation of the calibration factors among those three detectors is $20% at maximum. This deviation is not included in the above errors because it can be neglected when the calibration for each detector is carried out. The background level of this radon detector was measured by using air for 2 months. The result of this background run is 2.4$1.3 counts/day. As a result, one standard deviation excess of the signal above the background should be possible for 1.4 mbq/m in a one-day measurement at 0.08 g/m. Fig. 9 shows a typical response of the high sensitivity radon detector for water after radon water injection. The data points were fit by using the Eq. (1). We added radon water several times, and the results were listed in Table 2. The averaged calibration factor for the high sensitivity radon detector for water is 3.6$0.5(stat.#syst.). Systematic errors in this calibration factor are the following: accuracy of the dew point meter #1%!2% at 0.08 g/m and &0% at 11 g/m on the calibration factor, accuracy of the current measurement

7 340 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 8. Humidity dependence of the calibration factor for a high sensitivity radon detector in air. The supplied high voltage is!1500 V. Table 2 Results of the calibration of the high sensitivity radon detector for water Experiment Total radon amount (mbq) Calibration factor [(counts/day)/(mbq/m )] Exp $0.4 Exp $0.5 Exp $0.4 Exp $0.4 Average 3.6$0.5 Fig. 9. A typical variation after Rn water injection. at the ionization chamber $4.2%, accuracy of the conversion factor from the current value to the radon concentration at the ionization chamber $5%, accuracy of the LSC measurement $10%, and radon solubility at the equilibrium state #3.7%!4.5% (20$1 C). The detection limit of the high sensitivity radon detector for water is at a similar level as that for air. 6. Conclusions Using electrostatic collection and the PIN photodiode, a high sensitivity radon detector for air and for water have been developed. The calibration system has also been developed. During the calibration study, a technique to make radon water has been established. The calibration factors of the high sensitivity radon detector for air are 2.2$0.2 (counts/day)/(mbq/m ) at 0.08 g/m and 0.86$0.06 (counts/day)/(mbq/m ) at 11 g/m. The calibration factor of the radon detector for

8 Y. Takeuchi et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) wateris3.6$0.5 (counts/day)/(mbq/m ). The background level of the radon detector for air is 2.4$1.3 counts/day. As a result, one standard deviation excess of the signal above the background of the radon detector for air should be possible for 1.4 mbq/m in a one-day measurement at 0.08 g/m. The detection limit of the high sensitivity radon detector for water is similar to that of the high sensitivity radon detector for air and it is under study. The radon concentration in the Super- Kamiokande detector is stable now; therefore, we can obtain the average radon concentration in the SK-detector from long-term measurement. The average radon concentration in the SK-detector in January 1998 is less than 5.7 mbq/m. The current radon concentration in the SK-detector is almost at the detection limit of the high sensitivity radon detector for water. We are now developing a more sensitive radon detector. The calibration factor of that super sensitive radon detector is expected to be better than the current high sensitivity radon detector by a factor of 10. Acknowledgements The authors would like to thank Kamiokande and Super-Kamiokande collaborators for their help with this work. They acknowledge the cooperation of the Kamioka Mining and Smelting Company. This work was supported in part by a Grant-in-Aid for Scientific Research (B) of the Japanese Ministry of Education, Science and Culture. References [1] Y. Fukuda et al., Phys. Rev. Lett. 77 (1996) [2] S. Tasaka et al., Radioisotopes 43 (1994) 125. [3] The Super-Kamiokande Collaboration, Phys. Rev. Lett., 81 (1998) [4] S. Tasaka, ICRR Annual Report (April 1994 March 1995), ICRR, University of Tokyo (1996) 36. [5] M. Nemoto et al., Radioisotopes 46 (1997) 710. [6] P. Kotrappa et al., Health Phys. 46 (1981) 35. [7] T. Iida et al., Health Phys. 54 (1988) 139. [8] A.J. Howard et al., Am. J. Phys. 59 (1991) 544. [9] K.D. Chu, P.K. Hopke, Environ. Sci. Tecnol. 22 (1988) 711.