Applied Radiation and Isotopes 78 (2013) 1–9
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A complete low cost radon detection system A. Bayrak, E. Barlas, E. Emirhan, Ç. Kutlu, C.S. Ozben n İstanbul Technical University, Faculty of Science and Letters, Department of Physics Engineering, 34469 Maslak, Sarıyer, İstanbul, Turkey
A U T H O R - H I G H L I G H T S
Low cost radon detection. Integrated GSM modem for early warning of radon anomalies. Radon detection in environment .
art ic l e i nf o
a b s t r a c t
Article history: Received 15 November 2012 Received in revised form 14 March 2013 Accepted 16 March 2013 Available online 23 March 2013
Monitoring the 222Rn activity through the 1200 km long Northern Anatolian fault line, for the purpose of earthquake precursory, requires large number of cost effective radon detectors. We have designed, produced and successfully tested a low cost radon detection system (a radon monitor). In the detector circuit of this monitor, First Sensor PS100-7-CER-2 windowless PIN photodiode and a custom made transempedence/shaping amplifier were used. In order to collect the naturally ionized radon progeny to the surface of the PIN photodiode, a potential of 3500 V was applied between the conductive hemispherical shell and the PIN photodiode. In addition to the count rate of the radon progeny, absolute pressure, humidity and temperature were logged during the measurements. A GSM modem was integrated to the system for transferring the measurements from the remote locations to the data process center. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Si-PIN photodiodes Radiation detectors Photodiode detectors Natural radioactivity Radon detection
1. Introduction There has been great interest from the scientists and the engineers in measuring the ionizing radiation since the discovery of the radioactivity. As a result of the improvements in the solid state technology, the new generation detectors have been used in the field of radiation detection (Renker, 2007, 2009) more frequently. Replacing the conventional radiation detectors with the new generation ones have numerous advantages including ability to work with relatively low DC voltage, insensitivity to the magnetic fields (Wauters et al., 2009) and low cost. There have been quite a few works on the PIN photodiodes as radiation detectors especially in the last two decades (Brivitch et al., 2006; Sziki et al., 2004; Starr et al., 1999). Since they are compact, economical and easy to work with, they have been used in the wide range of applications, from the detection of the charged particles (Ramirez-Jimenez et al., 2005) to the detection of X-rays (Ramirez-Jimenez et al., 2003; Namba et al., 2002). Si-PIN
n
Corresponding author. Tel.: þ90 212 285 6990; fax: þ90 212 285 6386. E-mail addresses:
[email protected],
[email protected] (C.S. Ozben). URL: http://www.fiz.itu.edu.tr/ozbenc (C.S. Ozben).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.03.054
photodiodes have also been used for the alpha particles detection (Kim et al., 2009; Chambaudet et al., 1997) with relatively large efficiency. There are some conventional techniques and measurement systems used for the detection of the natural radioactivity level. Most of the systems use precise measurement techniques. However, there are some cases where the precision is not the priority. Monitoring the level of radon activity in the fault lines is one of these cases. The idea of monitoring the level of 222Rn activity in the fault lines for earthquake precursory is not new (Ghosh et al., 2009). There have been however, many ongoing efforts to use radon concentration measurements as earthquake precursory (Nevinsky et al., 2012; Papastefanou, 2002; Zoran et al., 2012; Friedmann, 2012; Papastefanou, 2010; Tsvetkova et al., 2005). Because of the seismic activities, radon gas may be released from the deep underground and move to the surface through the cracks in Earth's crust. Any anomaly in the activity level of radon, possibly caused by immeasurable seismic activities, can be determined with the radon monitors located around the fault lines. Since radon decay products are mainly alpha active, a windowless PIN photodiode with a relatively large active area and a depletion layer of thickness a few hundred microns provides a good detection efficiency (Gutierrez et al., 2004; Martin-Martin et al., 2006;
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Voytchev et al., 1999). The contributions from the beta and the gamma lines of radon and its progeny to the detected activity is claimed to be negligible (Voytchev et al., 2001). We plan to establish a network for monitoring the 222Rn activity through the 1200 km long Northern Anatolian fault line. The larger the number of radon monitors located around the fault line, the easier it is to correlate the radon anomalies with the seismic activities; so a large number of cost effective radon monitors are needed. Since some of these monitors are going to be located in the rural locations, the power consumption of the monitors should be small enough to be operated by a single lead acid battery which can be charged by a solar panel.
parameters for the choice of a proper PIN photodiode. First Sensor PS100-7-CER-2 PIN photodiode was used in the detector. It comes with a ceramic package, has an active area of 100 mm2, can handle reverse bias voltage up to 50 V and has 5 nA dark current at 12 V reverse bias operation (First Sensor Corporation, 2012).
2. Radon monitor
2.2.1. Detector The most important section of the radon monitor is the detector since it is the heart of the measurement system. Reducing the noise, choosing the best OP-AMP, obtaining the optimum shielding and determining the best design of the PCB (Printed Circuit Board) were the main issues during the design stage of the detector. The detector has three sections. A small current is induced as a result of the absorption of alpha particles in the PIN photodiode and this current should be properly amplified. This is done by the transempedence/shaping amplifier section. The second section is the comparator where the digital signals are produced above a pre-defined threshold. The last section of the detector is the amplifier for MCA (Multi Channel Analyzer) where it is used for the threshold adjustments and the tests. Fig. 2 shows the transempedence amplifier circuit. The internal capacitance of the PIN photodiode slows down the signal. Applying a DC reverse bias voltage through the large resistors Rb1 and Rb2, expands the depletion region and this increases the detection efficiency. To obtain a stable signal at the output, a feedback capacitor Cf is added to the circuit. An ultra low noise OP-AMP or a JFET must be used in the transempedence preamplifier. It is not difficult to find OP-AMPs with an excellent noise characteristics today. Input current noise density in, input voltage noise density en, input capacitance Cin and input bias current IB are the most important parameters for choosing the most suitable OP-AMP. Following the transempedence amplifier section with large gain, three identical stages take place for shaping the signal.
Fig. 1 shows the drawing of the radon monitor. The monitor has the top and the bottom sections. The collection chamber is in the top and the electronics is located in the bottom section. These two sections are separated from each other with a 18 mm thick MDF (Medium Density Fiber). The top section contains hemi-spherical aluminum shell with 12 cm radius and a metal ring with a circular opening of 6 cm radius. These two pieces determine the boundaries of the collection chamber (Fig. 1) and they are hold at þ3500 V with respect to the surface of the PIN photodiode. The PIN photodiode is located at the center of the collection chamber as shown in Fig. 1. The ambient air is taken into the collection chamber via an air pump. The air inlet and the outlet are located in the opposite sides of the PIN photodiode and each one of them has 4.5 cm distance to the center of the PIN photodiode. 2.1. Si-PIN photodiodes Alpha particles from natural radionuclides have energies between 4 and 10 MeV and loose almost all of their energies in the base material of the PIN photodiode by electronic excitation and ionization. The active area, the maximum reverse bias voltage, the thickness of the depletion layer and the dark current are some of the important
Fig. 1. The drawing of the radon monitor.
2.2. Electronics The electronics of the radon monitor consist of six sections: the detector, the high voltage power supply, the mains supply with medical filter, the power selector and distributor, the sensor and the datalogger-SMS unit.
Fig. 2. The transempedance amplifier section of the detector circuit.
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Fig. 3 shows the shaping amplifier section. The feedback capacitor values of the shaping stage are experimentally determined to have the best signal to noise ratio. Since the amplification gain is large in the transempedence amplifier, this stage has to be shielded properly. Because of the sensitivity of the PIN photodiode to the light, the detection chamber is shielded against the daylight. The output of the shaping amplifier is also buffered for MCA. The PCB of the detector is designed to minimize the signal path especially in the transempedence section in order to obtain the best signal to noise ratio possible.
2.2.2. High voltage power supply Due to very low range of alpha particles in the air, only the alpha particles near the PIN photodiode can induce a signal. It has been known that when radon decays, the decay products (218Po and 214Po) are highly ionized. Most of these ions attach to the other ionized particles in the environment (Hopke, 1989; Porstendorfer, 1984; Lehtimaki et al., 1984; Kojima et al., 1992). By means of electrostatic collection, these ions can be drifted to the PIN photodiode with the
Fig. 3. The shaping amplifier section of the detector circuit.
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help of a strong electric field. For that reason, a DC high voltage power supply is required. In the circuit given in Fig. 4, a stepped up potential of about 440 V is generated at the output of the transformer. It is then cascaded to 3500 V using a Villard Cascade multiplier with four stages. A 500 MΩ resistor is connected between the last stage of the multiplier and the ground for stability. The output of the HV unit is filtered with an RC circuit before it is connected to the collection chamber. The ripple from the HV source is measured to be 0.3% of 3500 V. The electric field generated from 3500 V provides a good collection efficiency and this value is consistent with the previous works (Kiko, 2001; Wrenn et al., 1975; Mitsuda et al., 2003). 2.2.3. Mains filter A conservative power line filter is used for eliminating the current spikes in the power line which may cause fake counts. 2.2.4. Power selector and distributor This section provides an automated switching between the power sources. Power needed by the radon monitor is primarily supplied by the mains. However when the mains power is not available, the power source is switched automatically to the lead acid battery. 2.2.5. Sensor unit Continuous measurements of temperature, humidity and absolute pressure of the environment are required due to the correlations between these parameters and the detected radon level (Mitsuda et al., 2003). National's LM35 temperature, Hokuriku's HSU-07C1-NMC3A analog humidity and Motorola's MPX200A absolute pressure sensors are used in the sensor unit together with the appropriate signal conditioning circuits (Fig. 5). The humidity and the pressure sensors are calibrated using the CEM
Fig. 4. The high voltage circuit.
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DT-172 humidity–temperature logger and the Kestrel 4500 pocket weather tracker absolute pressure sensor, respectively.
2.2.6. Datalogger-SMS unit A PIC (Programmable Interface Controller) is programmed for storing the radon counts and the atmospheric parameters (humidity, pressure and temperature) to an SD card at pre-defined or dynamically adjusted sampling periods. The PIC also communicates with SIEMENS TC35 GSM module and the data is sent to the data process center.
3.1. Detector noise and temperature effect The upper part of Fig. 6 shows the snapshot of the noise at the output of the shaping amplifier. The lower part of Fig. 6 shows the projection of the snapshot noise which is a Gaussian with s ¼ 27:4 mV. Fig. 7 shows the correlation between the temperature and the RMS value of the detector noise. It should be pointed out that even with the highest temperature, the noise level of the detector was well below the radon detection threshold of 1 V. Fourier analysis of the noise data was performed and a 9.5 kHz frequency component was visible in the Fourier spectrum. This noise was due to the ripple in the HV power supply and the maximum amplitude of this noise was below the radon detection threshold.
3. Measurements The noise due to the temperature and from the high voltage power supply are studied and the detector efficiency calibration is performed.
3.1.1. Calibration The efficiency calibration of the radon monitor is performed with the Standard Reference Material SRM 4974 (NIST, 2005).
Fig. 5. The sensor circuit.
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5
200
Noise Amplitude (mV)
150 100 50 0 -50 -100 -150 -200 0
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Time (ms) RMS
14
27.35
12
AU
10 8 6 4 2 0 -200
-150
-100
-50 0 50 Noise Amplitude (mV)
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150
200
Fig. 6. The detector noise. The snapshot of the detector noise (top figure) and its projection (bottom figure).
The reference standard in the capsule is employed in a glass container larger than 20 mL. Gaseous 222Rn escapes from the capsule by diffusion into the glass accumulation volume. The activity of radon ARn at the end of an accumulation period tA is given as (NIST, 2005)
50
45
ARn ¼ A0Ra e−λRa t D f 0 ½x það1−xÞ
RMS Noise (mV)
40
Here, A0Ra is the reference activity, λRa is the decay constant of Ra, tD is the time duration from the date of certification, f0 is the 222 Rn emanation fraction at equilibrium. The parameter x ¼ 1− expð−λRn t A Þ, where λRn is the decay constant of 222Rn and a is the 222 Rn fraction in the polyethylene (0.013 70.010). In the calibration process, the accumulation time tA was set to 71.7 h and the corresponding 222Rn activity was calculated to be 17427 20 Bq. The accumulated radon was then transferred into the collection chamber of the detector by a pump with 0.4 L/m flow rate for 10 min. The acquisition time was set to 60 s and the spectra were recorded successively for about 9.5 h. Then, the area counts of 218Po and 214Po peaks were determined from each spectrum. Finally, the time dependent area counts were fitted to the exponential functions, N 218 ðtÞ and N 214 ðtÞ, respectively. With this procedure the time dependent collection efficiencies (ϵ218 and ϵ214 ) were determined. Start time of the fit was set to 180 min where the equilibrium between radon and its decay products were established. Time dependent collection efficiency were obtained from the following equations (Kiko, 2001); 226
35
30
25
20
15 15
20
25
30 35 40 Temperature (°C)
45
50
55
Fig. 7. The temperature dependence of the detector noise.
The SRM 4974 is hermetically sealed with polyethylene capsule containing 226Ra solution. The SRM 4974 is suitable to use both with the accumulation mode and with in-flow calibrations. The radium activity of 4863 Bq and the emanation fraction of 0.844 (at 21 1C) are the certified values (Volkovitsky, 2006) of the SRM 4974.
ð1Þ
N 218 ðtÞ AG N 214 ðtÞ−N218 ðtÞ ϵ214 ðtÞ ¼ AGð1−ϵ218 ðtÞÞ ϵ218 ðtÞ ¼
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300
0.12
214Po
250
150
6.00 MeV
0.06 218Po 214Po
0.04
100
0.02
7.69 MeV
218Po
200
0.08 Counts
Collection Efficieny (%)
0.1
50
0 200
300 Time (m)
400
0
500
Fig. 8. The time dependent collection efficiency functions of the detector for radon progeny.
Here, G is the geometry factor and A is the 222Rn activity in Bq. The geometry factor G is determined from the ratio between the surface area of the PIN photodiode and the area of the collection region. The area of the collection region is equal to the area of the grounded plate around the photodiode (including the area of the photodiode). It is assumed that only half of the alpha particles from the decay products can be detected on the PIN photodiode since the alpha decay happens isotropically. Fig. 8 shows the collection efficiency functions of 218Po and 214Po, obtained from the fit of the ϵ218 and ϵ214 to the exponential functions after the equilibrium is established. The collection efficiency depends on the collection voltage, the humidity, the temperature and the pressure of the environment. These are not the only parameters altering the detection efficiency. Since the attachment and the neutralization processes work different for 218Po and 214Po ions, the counting ratio between the two ions differs from 1, even in equilibrium. In our radon detection system, this ratio is measured to be 0.62 in equilibrium.
3.2. Radon measurements The first tests of the detector were performed with a 0:1 μCi 226 Ra source (LD Didactic GmBH, 1999). The source was located just above the surface of the PIN photodiode and Amptek MCA8000A Pocket MCA recorded the alpha spectrum for about 5 min. A 4.78 MeV alpha peak from the decay of radon was immediately observed in the spectrum and it was well separated from the noise. The gain and the threshold adjustments of the detector circuit were performed using this radium source. Fig. 9 shows the MCA spectrum of radon obtained from the SRM 4974 222Rn source. This spectrum was taken about 6.5 h after radon was sniffed into the collection chamber with 0.4 L/m air flow rate. The time dependent 218Po and 214Po concentrations can be derived from well known decay equations (Maiello and Hoover, 2010). Fig. 10 shows the theoretical time dependence of 218Po and 214 Po concentrations. As can be seen in Fig. 10, approximately 3 h is required to establish the equilibrium between the two nuclides.
2
3
4
5
6
7
8
Energy (MeV) Fig. 9. The recorded MCA spectrum of the NIST 4974 radon standard reference source.
1 218
Po
0.8
Relative Activity
100
214
Po
0.6
0.4
0.2
0 0
50
100 150 Time (m)
Fig. 10. The calculated time dependence of
218
Po and
200 214
250
Po concentrations.
The time dependence of the measured rate of the radon decay products was also studied to determine the influence of the air flow. In the first measurement, the radon was sniffed into the collection chamber and measurement was performed with continuing air flow (Fig. 11). In the second measurement, the same procedure was performed but this time the pump was turned off right after radon was taken into the collection chamber (Fig. 12). The half life of the detected 218Po was measured to be 6.6 70.3 m with the 0.4 L/m flow rate. This value is much longer compared to the one with no air flow. This half life is not the radiological half life. It includes many dynamic parameters, influencing the detection efficiency, such as ionization and neutralization rates (Kiko, 2001).
A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9
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Pump Rate 0.4 l/m 300
P1 P2
250
384.5 6.604
25.97 0.2736
Peak Area
200 218 Po
150 100 50 0 0
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91.03 49.95
200 9.144 3.189
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214 Po
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Time (m) Fig. 11. The time evolution of 218Po and 214Po counts when the flow rate is 0.4 l/m. The experimental data is fitted to P 1 expð−t=P 2 Þ function and the fit parameters are given on the upper right corner of the figure.
300
P1 P2
2 min. intervals 5 min. intervals
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155.3 450.1
3.131 12.10
Peak Area
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Po
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320.6 388.6
3.484 5.191
200 214
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Po
100 50 0 0
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400
600
800
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Time (m) Fig. 12. The time evolution of 218Po and 214Po counts when there is no air flow. The experimental data is fitted to P 1 expð−t=P 2 Þ function and the fit parameters are given on the upper right corner of the figure.
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After the energy calibration was performed with the radon source, a spectrum of the soil sampled from Geyikli Beach of Dardanels was taken. Fig. 13 shows the alpha spectrum of this soil sample. In addition to radon, the peaks of thoron (220Rn) are visible in the spectrum.
4. Results and discussions
2250
The performance tests and the measurements showed that the radon monitor can detect the radon level down to the desired level. Since monitoring the anomalies in the concentration level is more crucial than determining the absolute radon level, the radon monitors do not have to be calibrated if they are going to be used for the purpose of earthquake precursory. However, we performed the calibration for our system.
2000 1750 1500 1250
Radon Concentration (Bq/m3)
Counts
The minimum detectable activity measurement was also performed with the standard radon source. 2556 Bq/m3 222Rn activity provides approximately one net count rate in the detector. This result can be translated to 4 Bq/m3 minimum detectable activity for 1 h measurement.
1000 750 500 250 0 2
3
4
5 6 7 Energy (MeV)
8
9
100 80 60 40 20 0 2
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Fig. 13. The recorded MCA spectrum of the sand sample from Geyikli Beach, Dardanels.
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6 8 Time (Days)
Radon Concentration (Bq/m3)
80
60
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0 6.5
7
7.5
8
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0 8.5
9
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Fig. 15. 12 days of radon measurement by the monitors located in the basement of the Faculty of Science and Letters building of ITU.
100
6
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9.5
10
10.5
Time (Months of 2012) Fig. 14. 152 days of radon measurement in the basement of the Faculty of Mines building of ITU.
A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9
We performed continuous test measurements with the radon monitors on the campus of our university. Nine radon monitors have been located throughout the campus buildings. The measurement locations were chosen to be the locations with possibly high concentrations of radon due to the deficient air circulation. Fig. 14 shows the 152 days of radon measurement in the basement of the Faculty of Mines building of ITU. As one can see from the figure, the detected radon level oscillates with daily periods especially in the summer months. This is due to the changes in humidity and temperature on a daily basis. Another five radon monitors were located in the basement of the Faculty of Science and Letters building of ITU to observe if their measurements were consistent with each other. Fig. 15 shows the 12 day measurements. One can see in the figure that the count rates from each station are statistically consistent with each other most of the time. However, there are few points with larger deviations than statistics may tolerate. This might be due to the local fluctuations in the radon concentration. One of the concerns about this monitor is its maintenance. Air is not filtered in our system. It is known that the accumulation of the dust on the surface of the PIN photodiode gradually decreases the detection efficiency. Not to mention, corrosive gases may spoil the surface of the photodiode since there is no protective window. However, there is a thin anti-reflective layer on it. The manufacturer claims that the surface of the photodiode can be gently cleaned with a soft tissue using alcohol. Just to be on the safe side, we plan to coat the surface of the PIN photodiode with 50–100 nm thick diamond like carbon. This coating may serve as a protective layer against the corrosive atmospheric gases. Then the collection chamber will be rotated by 901 and a tiny sprinkle system will be installed to wash the surface of the photodiode periodically. No water drop will stay on the coated surface of the photodiode since diamond like carbon is super hydrophobic. The absorption due to the effect of 50–100 nm carbon layer is calculated to be negligible for the alpha particles. An air filter will also be integrated into the air intake of the monitor in the future. The SMS modem is the only commercial device in the radon monitor. This is the reason why the cost of the monitor was significantly reduced. The cost of the radon monitor has been reduced down to the price range of a commercial charge sensitive preamplifier. Since large number of units are needed, the cost of the device could be reduced even more with the mass production in the future. With these low cost monitors, radon maps of wider regions can be produced in shorter times and seasonal changes of radon level can be tracked with larger statistics in some locations which helps the modeling of radon transport. These monitors can be used for the purpose of monitoring radon activities in the basements of the buildings, the caves, the tunnels, the mines and the metro stations without any modification. They can also be integrated into the air circulation system of the buildings to track the air quality.
Acknowledgments We would like to thank to the Scientific and Technological Council of Turkey (TUBITAK) for supporting this project (110T261). We also thank to the Support Unit of Research Projects of ITU (BAP). References Brivitch, I., Lorenz, E., Olshevski, A., Renker, D., Sadygov, Z., Scheuermann, R., Stoykov, A., Werner, A., Zheleznykh, I., Zhuk, V., 2006. Study of Avalanche microchannel photodiodes for use in scintillation detectors. J. Instrum. 1, P08002. Chambaudet, A., Klein, D., Voytchev, M., 1997. Study of the response of silicon detectors for alpha particles. Radiat. Meas. 28 (1–6), 127–132.
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First Sensor Corporation. Datasheet of PS100-7-CER-2 PIN photodiode. 〈www. pacific-sensor.com/en/produts/optical-sensors〉. Friedmann, H., 2012. Radon in earthquake prediction research. Radiat. Prot. Dosim. 149 (2), 177–184. Ghosh, D., Deb, A., Sengupta, R., 2009. Anomalous radon emission as precursor of earthquake. J. Appl. Geophys. 69 (2), 67–81. Gutierrez, J.L., Garcia-Talavera, M., Pena, V., Nalda, J.C., Voytchev, M., Lopez, R., 2004. Radon emanation measurements using silicon photodiode detectors. Appl. Radiat. Isot. 60 (2–4), 583–587. Hopke, P.K., 1989. The initial behavior of 218Po in indoor air. Environ. Int. 15 (1–6), 299–308. Kiko, J., 2001. Detector for Rn-222 measurements in air at the 1 mBq/m(3) level. Nucl. Instrum. Methods Phys. Res., Sect. A 460 (2–3), 272–277. Kim, H.S., Park, S.H., Ha, J.H., Cho, S.Y., Park, K.S., 2009. Design and fabrication of a Si PIN-type radiation detector for alpha spectroscopy. J. Korean Phys. Soc. 55 (2), 454–458. Kojima, H., Abe, S., Fujitaka, K., 1992. Detailed behavior of indoor radon daughters focused on effectiveness of an air cleaner. Radiat. Prot. Dosim. 45 (1–4), 589–592. LD Didactic GmbH Radium Source for Wilson Cloud Chamber. Model number 559 59, 3.3 kBq source, Germany. 〈http://www.leybold-didactic.com/phk/a.asp? a=55959&L=2〉. Lehtimaki, M., Graeffe, G., Janka, K., Kulmala, V., Rajala, M., 1984. On the behavior of radon daughters in indoor air. Radiat. Prot. Dosim. 7 (1–4), 165–168. Maiello, M.L., Hoover, M.D., 2010. Radioactive Air Sampling Methods. Taylor & Francis Inc, CRC Press, Nevada. Martin-Martin, A., Gutierrez-Villanueva, J.L., Munoz, J.M., Garcia-Talavera, M., Adamiec, G., Iniguez, M.P., 2006. Radon measurements with a PIN photodiode. Appl. Radiat. Isot. 64 (10–11), 1287–1290. Mitsuda, C., Kajita, T., Miyano, K., Moriyama, S., Nakahata, M., Takeuchi, Y., Tasaka, S., 2003. Development of super-high sensitivity radon detector for the SuperKamiokande detector. Nucl. Instrum. Methods Phys. Res., Sect. A 497 (2–3), 414–428. Namba, T., Inoue, Y., Moriyama, S., Minowa, M., 2002. An X-ray detector using PIN photodiodes for the axion helioscope. Nucl. Instrum. Methods Phys. Res., Sect. A 489 (1–3), 224–229. National Institute of Standards & Technology Certificate, 2005. Standard Reference Material 4974 222Rn Emanation Standard. Gaithersburg, Maryland. Nevinsky, V., Nevinsk, I., Tsvetkova, T., 2012. Measurements of soil radon in south Russia for seismological application: methodological aspects. Radiat. Meas. 47 (4), 281–291. Papastefanou, C., 2002. An overview of instrumentation for measuring radon in soil gas and groundwaters. J. Environ. Radioact. 63 (3), 271–283. Papastefanou, C., 2010. Variation of radon flux along active fault zones in association with earthquake occurrence. Radiat. Meas. 45 (8), 943–951. Porstendorfer, J., 1984. Behavior of radon daughter products in indoor air. Radiat. Prot. Dosim. 7 (1–4), 107–113. Ramirez-Jimenez, F.J., Aguilera, E.F., Lopez-Callejas, R., Benitez-Read, J.S., PachecoSotelo, J., 2005. A novel application of a PIN diode-preamplifier set for the measurement of charged particles. Nucl. Instrum. Methods Phys. Res., Sect. A 545 (3), 721–726. Ramirez-Jimenez, F.J., Lopez-Callejas, R., Cerdeira-Altuzarra, A., Benitez-Read, J.S., Estrada-Cueto, M., Pacheco-Sotelo, J.O., 2003. PIN diode-preamplifier set for the measurement of low-energy gamma and X-rays. Nucl. Instrum. Methods Phys. Res., Sect. A 497 (2–3), 577–583. Renker, D., 2007. New trends on photodetectors. Nucl. Instrum. Methods Phys. Res., Sect. A 571 (1–2), 1–6. Renker, D., 2009. New developments on photosensors for particle physics. Nucl. Instrum. Methods Phys. Res., Sect. A 598 (1), 207–212. Starr, R., Clark, P.E., Evans, L.G., Floyd, S.R., McClanahan, T.P., Trombka Jr., J.I., Goldsten, J.O., Maurer, R.H., McNutt, R.L., Roth, D.R., 1999. Radiation effects in the Si-PIN detector on the near earth asteroid rendezvous mission. Nucl. Instrum. Methods Phys. Res., Sect. A 428 (1), 209–215. Sziki, G., Dobos, E., Kertesz, Z., Szikszai, Z., Uzonyi, I., Kiss, A.Z., 2004. A PIN detector array for the determination of boron using nuclear reaction analysis at a nuclear microprobe. Nucl. Instrum. Methods Phys. Res., Sect. B 219-220, 420–424. Tsvetkova, T., Przylibski, T.A., Nevinsk, I., Nevinsky, V., 2005. Measurement of radon in the East Europe under the ground. Radiat. Meas. 40 (1), 98–105. Volkovitsky, P., 2006. NIST Rn-222 emission standards. Appl. Radiat. Isot. 64 (10– 11), 1249–1252. Voytchev, M., Klein, D., Chambaudet, A., Georgiev, G., Iovtchev, M., 1999. Applications of a silicon photodiode detector for radon progeny measurements. Radiat. Meas. 31 (1–6), 375–378. Voytchev, M., Klein, D., Chambaudet, A., Georgiev, G., 2001. The use of silicon photodiodes for radon and progeny measurements. Health Phys. 80 (6), 590–596. Wauters, F., Kraev, I.S., Tandecki, M., Traykov, E., Van Gorp, S., Zakoucky, D., Severijns, N., 2009. Performance of silicon PIN photodiodes at low temperatures and in high magnetic fields. Nucl. Instrum. Methods Phys. Res., Sect. A 604 (3), 563–567. Wrenn, M.E., Spitz, H., Cohen, N., 1975. Design of a continuous digital-output environmental radon monitor. IEEE Trans. Nucl. Sci. NS-22 (1), 645–648. Zoran, M., Savastru, R., Savastru, D., Chitaru, C., Baschir, L., Tautan, M., 2012. Monitoring of radon anomalies in South-Eastern part of Romania for earthquake surveillance. J. Radioanal. Nucl. Chem. 293 (3), 769–781.
