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Highly sensitive radon monitor and radon emanation rates for detector components
Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
Highly sensitive radon monitor and radon emanation rates for detector components E. Choi *, M. Komori, K. Takahisa , N. Kudomi , K. Kume , K. Hayashi , S. Yoshida , H. Ohsumi, H. Ejiri , T. Kishimoto, K. Matsuoka, S. Tasaka Research Center for Nuclear Physics, Ibaraki, Osaka 0047, Japan Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Culture and Education, Saga University, Saga, Saga 840-8502, Japan Department of Physics, Faculty Education, Gifu University, Gifu, Gifu 501-11, Japan Received 17 August 1999; received in revised form 27 May 2000; accepted 12 July 2000
Abstract The radon emanation rates for materials were measured by using the electrostatic precipitation method as a radon monitor. It was found that a low level of radon was emanated from several material components in ELEGANT V. The radon monitor has been developed for the highly sensitive measurements of low-level radon concentration. The system was shown to have a sensitivity to radon concentrations as low as 1.6 mBq/m for one day measurement. The system was also used as the radon concentration monitor for the gas inside the airtight box of the ELEGANT V at Oto Cosmo Observatory. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Ym Keywords: High sensitive radon monitor; Radon concentration; Detector component
1. Introduction Recently, the measurements of ultra-rare nuclear processes, such as double beta decays, dark matter search and so on, have been playing an important role in nuclear and particle physics. In such measurements, Rn in the air is one of the most serious origins of background, because the daughters of Rn, such as Bi, successively decay with b-rays and c-rays. In order to measure such ultra-
* Corresponding author. E-mail address: [email protected] (E. Choi).
rare processes, Rn concentration in the air and the emanation of Rn from detector component must be small. Thus, we have to select ultra-low background materials for the detector components which contain little U-series contamination. The required level of Rn concentration in open spaces (about 600l) inside ELEGANT V[1] is of the order of 10\ Bq/m so as to improve the neutrino mass sensitivity. This paper aims at reporting the design principle of the radon detection system, the measurement of radon emanation from detector components in ELEGANT V and the monitoring system of the radon concentrator which extracted gas from the airtight box of ELEGANT V.
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 0 4 - 4
178
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
2. Design principle The radon monitor with an electrostatic concentrator is developed for the measurement of radon emanation rate of the materials used in the detector components for ultra rare nuclear processes. The system consists of a cylindrical vessel which acts as an electrostatic concentrator and an a particle detector with high-voltage divider and ampli"er. The positively charged daughter nuclei of radon in the vessel are attracted to the PIN photodiode by an electric "eld applied between the photodiode and the vessel. The energy of a particles, emitted from the attracted daughter nuclei, is measured by the PIN photodiode. The decays of Po, Po, Po(daughter of Rn in the U-series), and Po(daughter of Rn in the Th-series) are distinguished by monoenergetic peaks in the energy spectrum (Fig. 1). The PIN photodiode (HAMAMATSU S3204-06 SPL) with a surface area of 18;18 mm and a thickness of 0.5 mm is operated at a bias voltage of 75 V. A high voltage of !1500 V at the surface of the PIN photodiode generates the electric "eld needed to electrostatically concentrate the daughter nuclei. The cylindrical vessel (SUS 304,68.7l) can make
a measurement of the radon concentration by isolating an atmosphere of gas via an O-ring inside the vessel, polished to reject the sticking of U series nuclei. The vessel is also grounded to prevent sticking of daughter nuclei of radon. The radon concentration is derived as R"r/e, where e is the detection e$ciency and r is the counting rate of the peak of Po. It is also likely to detect the a particles emitted from Po which is ionized equally as Po. Po is oxidized in the air as PoO , and its ionization energy(E &10 eV) is ' higher than that of NO(E "9.25 eV), NO (E " ' ' 9.79 eV), naphthalene (E "8.12 eV) and so on, ' which are likely to be contained in the measured gas. Therefore, the Po ionization is unstable and thus not always concentrated on the PIN photodiode. The lower ionization energies Pb(E "7.415 eV) and Bi(E "7.287 eV) are ' ' stably concentrated on the photodiode; therefore, the a particle counting rate peaks from Po are electrically stable. The detection e$ciency of the a particle peaks emitted from Po depends on the dew point of the measured gas. This is not due to neutralization (by charge exchange between daughter nuclei of radon and H O) but to an unknown chemical mechanism [2] because the ionization energy of H O(E "12.56 eV) is higher than that of daughter ' nuclei of Rn. The background for the radon monitor is thus only the radon emanated from the vessel itself. The radon concentration due to the emanation from vessel of 14.0$0.9 mBq/m was obtained by measuring for 120 days. The sensitivity for measurement of the radon gas concentration in the vessel depends on the radon emanation rate from the vessel. The sensitivity (S ) to be detected L by the radon monitor is obtained from the condition >"e is determined by the #ucL tuation of yield from background signal. It is given as >"(> . Then one gets % (> % S " L e
Fig. 1. The energy spectrum of alpha particles measured by radon monitor system. The decay of Po, Po, Po and Po is identi"ed with the energy of alpha particle.
(1)
where > "Q e
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
179
Fig. 3. The e$ciency of the improved radon monitor. The e$ciency of the improved radon monitor is determined by using Pylon RNC calibrated radon gas source (80$3.2 Bq). The main part of error bar is the ambiguity of the source contents.
Fig. 2. The schematic view of the improved radon detector. The detector is designed to decrease the surface dimension of inner vessel (SUS 304,70l) by using "ne electrical polish and no welding structure.
The sensitivity is obtained by using e&0.3 for the dew point of !403C, and one day measurement, as S "2.8 mBq/m. L The radon monitor was improved to achieve a high sensitivity for measurement of low-level radon concentration. The radon emanation rate of the vessel depends on the condition of surface of the radon monitor. In order to improve the surface problem, the new design was considered to decrease the surface of the vessel (SUS 304,70l) by using "ne electrical polish and no welding structure. The schematic view of the improved radon detector is shown in Fig. 2. The radon concentration due to the vessel was found to be 5.36$0.76 mBq/m with 13 days measurement. The sensitivity for one day measurement was improved to be 1.6 mBq/m for the dew point of !403C. The threshold for the measurement of radon gas concentration is 43% better than the previous one. The e$ciency of the improved radon monitor (Fig. 3) was determined using the Pylon RNC Calibrated Radon Gas Source (80$3.2 Bq). The procedure is as the follows. (1) The inner gas was ventilated by liquid-nitrogen boil-o! gas, in order
to purge the radon in the vessel. (2) When the degassing was completed the recorded time was set as ¹ . (3) The RNC required at least 24 h to build up radon gas concentration. The RNC was connected to the radon monitor and pumped to it from a closed system so as to distribute the radon gas throughout the whole system for at least 10 min. (4) The RNC was disconnected at time ¹ . The radon gas concentration (R) was calculated at time ¹ by using the following formula: R"Q(1!exp(!jt)) exp(!jt )/<
(2)
t"¹ !¹ t "¹ !¹ Q"80 Bq($4%) j"0.0001258/min <"0.07 m where Q is the intensity of radon gas source, j is the half-life of Rn and < is the volume of the monitor vessel.
3. Measurement of the radon emanation rate and improvement of the material using ELEGANT V The detector system ELEGANT V has been designed for the study of ultra rare processes such as
180
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
Table 1 Emanation from materials of ELEGANT V Sample
Time (h) Rn (mBq)
PMT k-metal k-metal cable OFHC RC box Mo
37 50 100 100 50 50 200
1.91$0.13 mBq/PMT 1.70$0.12 mBq/k-metal (0.0107 mBq/k-metal (3.30$0.27);10\ mBq/m (0.454 mBq/m (2.89$0.58);10\ mBq/RC box (0.0561 mBq/m
double beta decay, dark matter search, etc. [3}6]. The stability of radon concentration in ELEGANT V is also important from the viewpoint of the study of annual modulation of dark matter spectrum. Emanation rates of Rn from several materials were measured by means of the radon monitor. Measurements were carried out as follows. At "rst, the air in the cylindrical vessel which contained a material to be measured was purged out by nitrogen gas. The valve of the vessel was closed to shut o! atmospheric radon at the start of the data taking. The emanation of radon from the material increased gradually and attained equilibrium about 3 days later. The emanation rate was calculated at the equilibrium state. Typical Rn emanation rates are shown in Table 1. It was observed that the emanation rates of the l-metal shield of the photomultiplier and the HV cable were relatively high. For the l-metal, the origin of Rn was the paint on metal which contained U-chain, and for the HV cable, the origin was the dust which adhered to the surface of the cable. Therefore we have to improve some materials as follows; E replace the l-metal of PMT without painting, E replace HV cables to reject surface contaminations, From the results in Table 1, the total emanation from materials of the detector components amounts to less than 150 mBq/m, this amount is less than that of the atmosphere(10 Bq/m). It is very important to reduce the incursion of atmosphere. Thus, we investigated an airtight box made of stainless steel.
4. Radon concentration in ELEGANT V at Oto Cosmo Observatory Oto Cosmo Observatory [7], RCNP, Osaka University has been constructed in the center of the tunnel whose location is about 100 km south of Osaka. The ELEGANT series highly sensitive detector systems are operated in this observatory. The experimental room with an area of 6 m;3 m located at 3 km from Oto entrance, has been used for ELEGANT V from April 1997. In order to further decrease the Rn concentration, radonfree air ventilates the airtight box. At the Kamioka Laboratory we used nitrogen gas boil-o! from liquid nitrogen. In this way, the Rn concentration is low, but the nitrogen gas #ow is insu$cient (0.5l/min). The Rn concentration of nitrogen boil-o! gas is known to be 8.2$0.9 mBq/m. The nitrogen gas supply system from liquid nitrogen was not reliable so a mechanical system making radon-free air was needed. A commercial system making nitrogen gas is expected to substitute the system making radon-free air. The system makes nitrogen gas by the removal of oxygen, water vapor and so on from the atmosphere by use of a charcoal "lter. We have measured the Rn concentration contained in the nitrogen gas(8l/min, 99.9%, KOFLOC-M3NT-8) with the radon monitor system. It was detected that the Rn concentration of nitrogen gas so supplied is 8.4$3.8 mBq/m with the dew point at !553C. Therefore, the radon concentration is reduced to about 1/1250 relative to atmospheric air. The Rn concentration of extracted gas from the airtight box of the ELEGANT V has also been measured by using the radon monitoring system. The average of the Rn concentration of the extracted gas from the ELEGANT V was about 125 mBq/m with the dew point at !233C. The relation between radon concentration of extracted gas from the airtight box and radon emanation inside the airtight box is as follows:
#R R" (1/q #1/q )<
(3)
where R is the radon concentration (mBq/m),
is the radon emanation (mBq/day), q is the
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
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5. Conclusion The origin of the Rn gas in the open spaces in the ELEGANT V has been studied. The radon monitor was developed to get high sensitivity for the measurement of the low-level radon concentration. The radon detector was improved to measure the radon concentration to 1.6 mBq/m/day. By introducing the airtight box and radon-free air system and replacing some materials, the Rn concentration of extracted gas from the airtight box in the ELEGANT V was measured to be 125 mBq/m, which is quite satisfactory for the study of bb decay and DM search. Acknowledgements Fig. 4. The variation of Rn concentration in ELEGANT V. The seasonal variation of Rn concentration in open space measured by the highly sensitive Rn monitor. The horizontal axis indicates the day from January 1, 1998. The quoted errors show only statistical errors, not including the systematic errors (5%).
We thank the people in Oto village and Nishiyoshino village. This work is supported by a Grant-in-Aid of Scienti"c Research, Ministry of Education, Science and Culture, Japan.
half-life of Rn(day), q is the replacement time of all gas (day), < is the air volume (m), R is the radon concentration of input gas (mBq/m). The variation of the Rn concentration in ELEGANT V measured with the radon monitor is shown in Fig. 4. From this monitoring system, the variation of Rn concentration is about $25 around 125 mBq/m. This is negligible for the measurement of DM by annual modulation.
References [1] H. Ejiri et al., Nucl. Instr. and Meth. A 302 (1991) 304. [2] A. Busigin, A.W. van der Vooren et al., Health Phys. 40 (1981) 333. [3] H. Ejiri et al., J. Phys. Soc. Japan Lett. 64 (1995) 339. [4] H. Ejiri et al., Nucl. Phys. A 611 (1996) 85. [5] K. Fushimi et al., Astropart. Phys. 12 (1999) 185. [6] N. Kudomi et al., Nucl. Phys. B (Proc. Suppl.) 87 (2000) 58. [7] H. Ohsumi et al., Proceedings of the Symposium on Frontier '96, 1996.
Highly sensitive radon monitor and radon emanation rates for detector components E. Choi *, M. Komori, K. Takahisa , N. Kudomi , K. Kume , K. Hayashi , S. Yoshida , H. Ohsumi, H. Ejiri , T. Kishimoto, K. Matsuoka, S. Tasaka Research Center for Nuclear Physics, Ibaraki, Osaka 0047, Japan Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Culture and Education, Saga University, Saga, Saga 840-8502, Japan Department of Physics, Faculty Education, Gifu University, Gifu, Gifu 501-11, Japan Received 17 August 1999; received in revised form 27 May 2000; accepted 12 July 2000
Abstract The radon emanation rates for materials were measured by using the electrostatic precipitation method as a radon monitor. It was found that a low level of radon was emanated from several material components in ELEGANT V. The radon monitor has been developed for the highly sensitive measurements of low-level radon concentration. The system was shown to have a sensitivity to radon concentrations as low as 1.6 mBq/m for one day measurement. The system was also used as the radon concentration monitor for the gas inside the airtight box of the ELEGANT V at Oto Cosmo Observatory. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Ym Keywords: High sensitive radon monitor; Radon concentration; Detector component
1. Introduction Recently, the measurements of ultra-rare nuclear processes, such as double beta decays, dark matter search and so on, have been playing an important role in nuclear and particle physics. In such measurements, Rn in the air is one of the most serious origins of background, because the daughters of Rn, such as Bi, successively decay with b-rays and c-rays. In order to measure such ultra-
* Corresponding author. E-mail address: [email protected] (E. Choi).
rare processes, Rn concentration in the air and the emanation of Rn from detector component must be small. Thus, we have to select ultra-low background materials for the detector components which contain little U-series contamination. The required level of Rn concentration in open spaces (about 600l) inside ELEGANT V[1] is of the order of 10\ Bq/m so as to improve the neutrino mass sensitivity. This paper aims at reporting the design principle of the radon detection system, the measurement of radon emanation from detector components in ELEGANT V and the monitoring system of the radon concentrator which extracted gas from the airtight box of ELEGANT V.
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 0 4 - 4
178
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
2. Design principle The radon monitor with an electrostatic concentrator is developed for the measurement of radon emanation rate of the materials used in the detector components for ultra rare nuclear processes. The system consists of a cylindrical vessel which acts as an electrostatic concentrator and an a particle detector with high-voltage divider and ampli"er. The positively charged daughter nuclei of radon in the vessel are attracted to the PIN photodiode by an electric "eld applied between the photodiode and the vessel. The energy of a particles, emitted from the attracted daughter nuclei, is measured by the PIN photodiode. The decays of Po, Po, Po(daughter of Rn in the U-series), and Po(daughter of Rn in the Th-series) are distinguished by monoenergetic peaks in the energy spectrum (Fig. 1). The PIN photodiode (HAMAMATSU S3204-06 SPL) with a surface area of 18;18 mm and a thickness of 0.5 mm is operated at a bias voltage of 75 V. A high voltage of !1500 V at the surface of the PIN photodiode generates the electric "eld needed to electrostatically concentrate the daughter nuclei. The cylindrical vessel (SUS 304,68.7l) can make
a measurement of the radon concentration by isolating an atmosphere of gas via an O-ring inside the vessel, polished to reject the sticking of U series nuclei. The vessel is also grounded to prevent sticking of daughter nuclei of radon. The radon concentration is derived as R"r/e, where e is the detection e$ciency and r is the counting rate of the peak of Po. It is also likely to detect the a particles emitted from Po which is ionized equally as Po. Po is oxidized in the air as PoO , and its ionization energy(E &10 eV) is ' higher than that of NO(E "9.25 eV), NO (E " ' ' 9.79 eV), naphthalene (E "8.12 eV) and so on, ' which are likely to be contained in the measured gas. Therefore, the Po ionization is unstable and thus not always concentrated on the PIN photodiode. The lower ionization energies Pb(E "7.415 eV) and Bi(E "7.287 eV) are ' ' stably concentrated on the photodiode; therefore, the a particle counting rate peaks from Po are electrically stable. The detection e$ciency of the a particle peaks emitted from Po depends on the dew point of the measured gas. This is not due to neutralization (by charge exchange between daughter nuclei of radon and H O) but to an unknown chemical mechanism [2] because the ionization energy of H O(E "12.56 eV) is higher than that of daughter ' nuclei of Rn. The background for the radon monitor is thus only the radon emanated from the vessel itself. The radon concentration due to the emanation from vessel of 14.0$0.9 mBq/m was obtained by measuring for 120 days. The sensitivity for measurement of the radon gas concentration in the vessel depends on the radon emanation rate from the vessel. The sensitivity (S ) to be detected L by the radon monitor is obtained from the condition >"e
Fig. 1. The energy spectrum of alpha particles measured by radon monitor system. The decay of Po, Po, Po and Po is identi"ed with the energy of alpha particle.
(1)
where > "Q e
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
179
Fig. 3. The e$ciency of the improved radon monitor. The e$ciency of the improved radon monitor is determined by using Pylon RNC calibrated radon gas source (80$3.2 Bq). The main part of error bar is the ambiguity of the source contents.
Fig. 2. The schematic view of the improved radon detector. The detector is designed to decrease the surface dimension of inner vessel (SUS 304,70l) by using "ne electrical polish and no welding structure.
The sensitivity is obtained by using e&0.3 for the dew point of !403C, and one day measurement, as S "2.8 mBq/m. L The radon monitor was improved to achieve a high sensitivity for measurement of low-level radon concentration. The radon emanation rate of the vessel depends on the condition of surface of the radon monitor. In order to improve the surface problem, the new design was considered to decrease the surface of the vessel (SUS 304,70l) by using "ne electrical polish and no welding structure. The schematic view of the improved radon detector is shown in Fig. 2. The radon concentration due to the vessel was found to be 5.36$0.76 mBq/m with 13 days measurement. The sensitivity for one day measurement was improved to be 1.6 mBq/m for the dew point of !403C. The threshold for the measurement of radon gas concentration is 43% better than the previous one. The e$ciency of the improved radon monitor (Fig. 3) was determined using the Pylon RNC Calibrated Radon Gas Source (80$3.2 Bq). The procedure is as the follows. (1) The inner gas was ventilated by liquid-nitrogen boil-o! gas, in order
to purge the radon in the vessel. (2) When the degassing was completed the recorded time was set as ¹ . (3) The RNC required at least 24 h to build up radon gas concentration. The RNC was connected to the radon monitor and pumped to it from a closed system so as to distribute the radon gas throughout the whole system for at least 10 min. (4) The RNC was disconnected at time ¹ . The radon gas concentration (R) was calculated at time ¹ by using the following formula: R"Q(1!exp(!jt)) exp(!jt )/<
(2)
t"¹ !¹ t "¹ !¹ Q"80 Bq($4%) j"0.0001258/min <"0.07 m where Q is the intensity of radon gas source, j is the half-life of Rn and < is the volume of the monitor vessel.
3. Measurement of the radon emanation rate and improvement of the material using ELEGANT V The detector system ELEGANT V has been designed for the study of ultra rare processes such as
180
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
Table 1 Emanation from materials of ELEGANT V Sample
Time (h) Rn (mBq)
PMT k-metal k-metal cable OFHC RC box Mo
37 50 100 100 50 50 200
1.91$0.13 mBq/PMT 1.70$0.12 mBq/k-metal (0.0107 mBq/k-metal (3.30$0.27);10\ mBq/m (0.454 mBq/m (2.89$0.58);10\ mBq/RC box (0.0561 mBq/m
double beta decay, dark matter search, etc. [3}6]. The stability of radon concentration in ELEGANT V is also important from the viewpoint of the study of annual modulation of dark matter spectrum. Emanation rates of Rn from several materials were measured by means of the radon monitor. Measurements were carried out as follows. At "rst, the air in the cylindrical vessel which contained a material to be measured was purged out by nitrogen gas. The valve of the vessel was closed to shut o! atmospheric radon at the start of the data taking. The emanation of radon from the material increased gradually and attained equilibrium about 3 days later. The emanation rate was calculated at the equilibrium state. Typical Rn emanation rates are shown in Table 1. It was observed that the emanation rates of the l-metal shield of the photomultiplier and the HV cable were relatively high. For the l-metal, the origin of Rn was the paint on metal which contained U-chain, and for the HV cable, the origin was the dust which adhered to the surface of the cable. Therefore we have to improve some materials as follows; E replace the l-metal of PMT without painting, E replace HV cables to reject surface contaminations, From the results in Table 1, the total emanation from materials of the detector components amounts to less than 150 mBq/m, this amount is less than that of the atmosphere(10 Bq/m). It is very important to reduce the incursion of atmosphere. Thus, we investigated an airtight box made of stainless steel.
4. Radon concentration in ELEGANT V at Oto Cosmo Observatory Oto Cosmo Observatory [7], RCNP, Osaka University has been constructed in the center of the tunnel whose location is about 100 km south of Osaka. The ELEGANT series highly sensitive detector systems are operated in this observatory. The experimental room with an area of 6 m;3 m located at 3 km from Oto entrance, has been used for ELEGANT V from April 1997. In order to further decrease the Rn concentration, radonfree air ventilates the airtight box. At the Kamioka Laboratory we used nitrogen gas boil-o! from liquid nitrogen. In this way, the Rn concentration is low, but the nitrogen gas #ow is insu$cient (0.5l/min). The Rn concentration of nitrogen boil-o! gas is known to be 8.2$0.9 mBq/m. The nitrogen gas supply system from liquid nitrogen was not reliable so a mechanical system making radon-free air was needed. A commercial system making nitrogen gas is expected to substitute the system making radon-free air. The system makes nitrogen gas by the removal of oxygen, water vapor and so on from the atmosphere by use of a charcoal "lter. We have measured the Rn concentration contained in the nitrogen gas(8l/min, 99.9%, KOFLOC-M3NT-8) with the radon monitor system. It was detected that the Rn concentration of nitrogen gas so supplied is 8.4$3.8 mBq/m with the dew point at !553C. Therefore, the radon concentration is reduced to about 1/1250 relative to atmospheric air. The Rn concentration of extracted gas from the airtight box of the ELEGANT V has also been measured by using the radon monitoring system. The average of the Rn concentration of the extracted gas from the ELEGANT V was about 125 mBq/m with the dew point at !233C. The relation between radon concentration of extracted gas from the airtight box and radon emanation inside the airtight box is as follows:
#R R" (1/q #1/q )<
(3)
where R is the radon concentration (mBq/m),
is the radon emanation (mBq/day), q is the
E. Choi et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 177}181
181
5. Conclusion The origin of the Rn gas in the open spaces in the ELEGANT V has been studied. The radon monitor was developed to get high sensitivity for the measurement of the low-level radon concentration. The radon detector was improved to measure the radon concentration to 1.6 mBq/m/day. By introducing the airtight box and radon-free air system and replacing some materials, the Rn concentration of extracted gas from the airtight box in the ELEGANT V was measured to be 125 mBq/m, which is quite satisfactory for the study of bb decay and DM search. Acknowledgements Fig. 4. The variation of Rn concentration in ELEGANT V. The seasonal variation of Rn concentration in open space measured by the highly sensitive Rn monitor. The horizontal axis indicates the day from January 1, 1998. The quoted errors show only statistical errors, not including the systematic errors (5%).
We thank the people in Oto village and Nishiyoshino village. This work is supported by a Grant-in-Aid of Scienti"c Research, Ministry of Education, Science and Culture, Japan.
half-life of Rn(day), q is the replacement time of all gas (day), < is the air volume (m), R is the radon concentration of input gas (mBq/m). The variation of the Rn concentration in ELEGANT V measured with the radon monitor is shown in Fig. 4. From this monitoring system, the variation of Rn concentration is about $25 around 125 mBq/m. This is negligible for the measurement of DM by annual modulation.
References [1] H. Ejiri et al., Nucl. Instr. and Meth. A 302 (1991) 304. [2] A. Busigin, A.W. van der Vooren et al., Health Phys. 40 (1981) 333. [3] H. Ejiri et al., J. Phys. Soc. Japan Lett. 64 (1995) 339. [4] H. Ejiri et al., Nucl. Phys. A 611 (1996) 85. [5] K. Fushimi et al., Astropart. Phys. 12 (1999) 185. [6] N. Kudomi et al., Nucl. Phys. B (Proc. Suppl.) 87 (2000) 58. [7] H. Ohsumi et al., Proceedings of the Symposium on Frontier '96, 1996.