Applied Radiation and Isotopes 78 (2013) 1–9

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Applied Radiation and Isotopes

0969-80http://d

n CorrE-mURL

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 .

a r t i c l e i n f o

Article history:Received 15 November 2012Received in revised form14 March 2013Accepted 16 March 2013Available online 23 March 2013

Keywords:Si-PIN photodiodesRadiation detectorsPhotodiode detectorsNatural radioactivityRadon detection

43/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.apradiso.2013.03.054

esponding author. Tel.: þ90 212 285 6990; faail addresses: cozben2000@yahoo.com, ozben: http://www.fiz.itu.edu.tr/ozbenc (C.S. Ozben

a b s t r a c t

Monitoring the 222Rn activity through the 1200 km long Northern Anatolian fault line, for the purpose ofearthquake 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 detectorcircuit of this monitor, First Sensor PS100-7-CER-2 windowless PIN photodiode and a custom madetransempedence/shaping amplifier were used. In order to collect the naturally ionized radon progeny tothe surface of the PIN photodiode, a potential of 3500 V was applied between the conductive hemi-spherical shell and the PIN photodiode. In addition to the count rate of the radon progeny, absolutepressure, humidity and temperature were logged during the measurements. A GSM modem wasintegrated to the system for transferring the measurements from the remote locations to the dataprocess center.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

There has been great interest from the scientists and theengineers in measuring the ionizing radiation since the discoveryof the radioactivity. As a result of the improvements in the solidstate technology, the new generation detectors have been used inthe field of radiation detection (Renker, 2007, 2009) more fre-quently. Replacing the conventional radiation detectors with thenew generation ones have numerous advantages including abilityto work with relatively low DC voltage, insensitivity to themagnetic fields (Wauters et al., 2009) and low cost.

There have been quite a few works on the PIN photodiodes asradiation detectors especially in the last two decades (Brivitchet al., 2006; Sziki et al., 2004; Starr et al., 1999). Since they arecompact, economical and easy to work with, they have been usedin the wide range of applications, from the detection of thecharged particles (Ramirez-Jimenez et al., 2005) to the detectionof X-rays (Ramirez-Jimenez et al., 2003; Namba et al., 2002). Si-PIN

ll rights reserved.

x: þ90 212 285 6386.@itu.edu.tr (C.S. Ozben).).

photodiodes have also been used for the alpha particles detection(Kim et al., 2009; Chambaudet et al., 1997) with relatively largeefficiency.

There are some conventional techniques and measurementsystems used for the detection of the natural radioactivity level.Most of the systems use precise measurement techniques. How-ever, there are some cases where the precision is not the priority.Monitoring the level of radon activity in the fault lines is one ofthese cases. The idea of monitoring the level of 222Rn activity inthe fault lines for earthquake precursory is not new (Ghosh et al.,2009). There have been however, many ongoing efforts to useradon 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 fromthe deep underground and move to the surface through the cracksin Earth's crust. Any anomaly in the activity level of radon, possiblycaused by immeasurable seismic activities, can be determinedwith the radon monitors located around the fault lines. Sinceradon decay products are mainly alpha active, a windowless PINphotodiode with a relatively large active area and a depletion layerof thickness a few hundred microns provides a good detectionefficiency (Gutierrez et al., 2004; Martin-Martin et al., 2006;

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–92

Voytchev et al., 1999). The contributions from the beta and thegamma lines of radon and its progeny to the detected activity isclaimed to be negligible (Voytchev et al., 2001).

We plan to establish a network for monitoring the 222Rnactivity through the 1200 km long Northern Anatolian fault line.The larger the number of radon monitors located around the faultline, the easier it is to correlate the radon anomalies with theseismic activities; so a large number of cost effective radonmonitors are needed. Since some of these monitors are going tobe located in the rural locations, the power consumption of themonitors should be small enough to be operated by a single leadacid battery which can be charged by a solar panel.

2. Radon monitor

Fig. 1 shows the drawing of the radon monitor. The monitor hasthe top and the bottom sections. The collection chamber is in thetop and the electronics is located in the bottom section. These twosections are separated from each other with a 18 mm thick MDF(Medium Density Fiber). The top section contains hemi-sphericalaluminum shell with 12 cm radius and a metal ring with a circularopening of 6 cm radius. These two pieces determine the bound-aries of the collection chamber (Fig. 1) and they are hold atþ3500 V with respect to the surface of the PIN photodiode. ThePIN photodiode is located at the center of the collection chamberas shown in Fig. 1. The ambient air is taken into the collectionchamber via an air pump. The air inlet and the outlet are located inthe opposite sides of the PIN photodiode and each one of them has4.5 cm distance to the center of the PIN photodiode.

2.1. Si-PIN photodiodes

Alpha particles from natural radionuclides have energiesbetween 4 and 10 MeV and loose almost all of their energies inthe base material of the PIN photodiode by electronic excitationand ionization.

The active area, the maximum reverse bias voltage, the thickness ofthe depletion layer and the dark current are some of the important

Fig. 1. The drawing of the radon monitor.

parameters for the choice of a proper PIN photodiode. First SensorPS100-7-CER-2 PIN photodiode was used in the detector. It comeswith a ceramic package, has an active area of 100 mm2, can handlereverse bias voltage up to 50 V and has 5 nA dark current at 12 Vreverse bias operation (First Sensor Corporation, 2012).

2.2. Electronics

The electronics of the radon monitor consist of six sections: thedetector, the high voltage power supply, the mains supply withmedical filter, the power selector and distributor, the sensor andthe datalogger-SMS unit.

2.2.1. DetectorThe most important section of the radon monitor is the

detector since it is the heart of the measurement system. Reducingthe noise, choosing the best OP-AMP, obtaining the optimumshielding and determining the best design of the PCB (PrintedCircuit Board) were the main issues during the design stage of thedetector.

The detector has three sections. A small current is induced as aresult of the absorption of alpha particles in the PIN photodiodeand this current should be properly amplified. This is done by thetransempedence/shaping amplifier section. The second section isthe comparator where the digital signals are produced above apre-defined threshold. The last section of the detector is theamplifier for MCA (Multi Channel Analyzer) where it is used forthe threshold adjustments and the tests.

Fig. 2 shows the transempedence amplifier circuit. The internalcapacitance of the PIN photodiode slows down the signal. Apply-ing a DC reverse bias voltage through the large resistors Rb1 andRb2, expands the depletion region and this increases the detectionefficiency. To obtain a stable signal at the output, a feedbackcapacitor Cf is added to the circuit.

An ultra low noise OP-AMP or a JFET must be used in thetransempedence preamplifier. It is not difficult to find OP-AMPswith an excellent noise characteristics today. Input current noisedensity in, input voltage noise density en, input capacitance Cin andinput bias current IB are the most important parameters forchoosing the most suitable OP-AMP.

Following the transempedence amplifier section with largegain, three identical stages take place for shaping the signal.

Fig. 2. The transempedance amplifier section of the detector circuit.

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9 3

Fig. 3 shows the shaping amplifier section. The feedback capacitorvalues of the shaping stage are experimentally determined to havethe best signal to noise ratio.

Since the amplification gain is large in the transempedenceamplifier, this stage has to be shielded properly. Because of thesensitivity of the PIN photodiode to the light, the detectionchamber is shielded against the daylight. The output of theshaping amplifier is also buffered for MCA.

The PCB of the detector is designed to minimize the signal pathespecially in the transempedence section in order to obtain thebest signal to noise ratio possible.

2.2.2. High voltage power supplyDue to very low range of alpha particles in the air, only the alpha

particles near the PIN photodiode can induce a signal. It has beenknown that when radon decays, the decay products (218Po and 214Po)are highly ionized. Most of these ions attach to the other ionizedparticles in the environment (Hopke, 1989; Porstendorfer, 1984;Lehtimaki et al., 1984; Kojima et al., 1992). By means of electrostaticcollection, these ions can be drifted to the PIN photodiode with the

Fig. 3. The shaping amplifier section of the detector circuit.

Fig. 4. The high v

help of a strong electric field. For that reason, a DC high voltage powersupply is required. In the circuit given in Fig. 4, a stepped up potentialof about 440 V is generated at the output of the transformer. It is thencascaded to 3500 V using a Villard Cascade multiplier with four stages.A 500 MΩ resistor is connected between the last stage of the multi-plier and the ground for stability. The output of the HV unit is filteredwith an RC circuit before it is connected to the collection chamber. Theripple from the HV source is measured to be 0.3% of 3500 V.

The electric field generated from 3500 V provides a goodcollection efficiency and this value is consistent with the previousworks (Kiko, 2001; Wrenn et al., 1975; Mitsuda et al., 2003).

2.2.3. Mains filterA 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 distributorThis section provides an automated switching between the

power sources. Power needed by the radon monitor is primarilysupplied by the mains. However when the mains power is notavailable, the power source is switched automatically to the leadacid battery.

2.2.5. Sensor unitContinuous measurements of temperature, humidity and abso-

lute pressure of the environment are required due to the correla-tions between these parameters and the detected radon level(Mitsuda et al., 2003). National's LM35 temperature, Hokuriku'sHSU-07C1-NMC3A analog humidity and Motorola's MPX200Aabsolute pressure sensors are used in the sensor unit togetherwith the appropriate signal conditioning circuits (Fig. 5). Thehumidity and the pressure sensors are calibrated using the CEM

oltage circuit.

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–94

DT-172 humidity–temperature logger and the Kestrel 4500 pocketweather tracker absolute pressure sensor, respectively.

2.2.6. Datalogger-SMS unitA PIC (Programmable Interface Controller) is programmed for

storing the radon counts and the atmospheric parameters (humid-ity, pressure and temperature) to an SD card at pre-defined ordynamically adjusted sampling periods. The PIC also communi-cates with SIEMENS TC35 GSM module and the data is sent to thedata process center.

3. Measurements

The noise due to the temperature and from the high voltagepower supply are studied and the detector efficiency calibration isperformed.

Fig. 5. The sen

3.1. Detector noise and temperature effect

The upper part of Fig. 6 shows the snapshot of the noise at theoutput of the shaping amplifier. The lower part of Fig. 6 shows theprojection of the snapshot noise which is a Gaussian withs¼ 27:4 mV.

Fig. 7 shows the correlation between the temperature and theRMS value of the detector noise. It should be pointed out that evenwith the highest temperature, the noise level of the detector waswell below the radon detection threshold of 1 V.

Fourier analysis of the noise data was performed and a 9.5 kHzfrequency component was visible in the Fourier spectrum. Thisnoise was due to the ripple in the HV power supply and themaximum amplitude of this noise was below the radon detectionthreshold.

3.1.1. CalibrationThe efficiency calibration of the radon monitor is performed

with the Standard Reference Material SRM 4974 (NIST, 2005).

sor circuit.

-200

-150

-100

-50

0

50

100

150

200

0 2 4 6 8 10Time (ms)

Noi

se A

mpl

itude

(mV

)

0

2

4

6

8

10

12

14

-200 -150 -100 -50 0 50 100 150 200

RMS 27.35

Noise Amplitude (mV)

AU

Fig. 6. The detector noise. The snapshot of the detector noise (top figure) and its projection (bottom figure).

15

20

25

30

35

40

45

50

15 20 25 30 35 40 45 50 55Temperature (°C)

RM

S N

oise

(mV)

Fig. 7. The temperature dependence of the detector noise.

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9 5

The SRM 4974 is hermetically sealed with polyethylene capsulecontaining 226Ra solution. The SRM 4974 is suitable to use bothwith the accumulation mode and with in-flow calibrations. Theradium activity of 4863 Bq and the emanation fraction of 0.844 (at21 1C) are the certified values (Volkovitsky, 2006) of the SRM 4974.

The reference standard in the capsule is employed in a glasscontainer larger than 20 mL. Gaseous 222Rn escapes from thecapsule by diffusion into the glass accumulation volume. Theactivity of radon ARn at the end of an accumulation period tA isgiven as (NIST, 2005)

ARn ¼ A0Rae

−λRatD f 0½xþað1−xÞ� ð1Þ

Here, A0Ra is the reference activity, λRa is the decay constant of

226Ra, tD is the time duration from the date of certification, f0 is the222Rn emanation fraction at equilibrium. The parameter x¼ 1−expð−λRntAÞ, where λRn is the decay constant of 222Rn and a is the222Rn fraction in the polyethylene (0.01370.010).

In the calibration process, the accumulation time tA was set to71.7 h and the corresponding 222Rn activity was calculated to be1742720 Bq. The accumulated radon was then transferred intothe collection chamber of the detector by a pump with 0.4 L/mflow rate for 10 min. The acquisition time was set to 60 s and thespectra were recorded successively for about 9.5 h. Then, the areacounts of 218Po and 214Po peaks were determined from eachspectrum. Finally, the time dependent area counts were fitted tothe exponential functions, N218ðtÞ and N214ðtÞ, respectively. Withthis procedure the time dependent collection efficiencies (ϵ218 andϵ214) were determined. Start time of the fit was set to 180 minwhere the equilibrium between radon and its decay products wereestablished.

Time dependent collection efficiency were obtained from thefollowing equations (Kiko, 2001);

ϵ218ðtÞ ¼N218ðtÞAG

ϵ214ðtÞ ¼N214ðtÞ−N218ðtÞAGð1−ϵ218ðtÞÞ

0

0.02

0.04

0.06

0.08

0.1

0.12

100 200 300 400 500

218Po214Po

Time (m)

Col

lect

ion

Effic

ieny

(%)

Fig. 8. The time dependent collection efficiency functions of the detector for radon

0

50

100

150

200

250

300

2 3 4 5 6 7 8

6.00

MeV

218 P

o

7.69

MeV

214 P

o

Energy (MeV)C

ount

sFig. 9. The recorded MCA spectrum of the NIST 4974 radon standard referencesource.

0.2

0.4

0.6

0.8

1218Po

214Po

Rel

ativ

e A

ctiv

ity

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–96

Here, G is the geometry factor and A is the 222Rn activity in Bq.The geometry factor G is determined from the ratio between thesurface area of the PIN photodiode and the area of the collectionregion. The area of the collection region is equal to the area of thegrounded plate around the photodiode (including the area of thephotodiode). It is assumed that only half of the alpha particlesfrom the decay products can be detected on the PIN photodiodesince the alpha decay happens isotropically. Fig. 8 shows thecollection efficiency functions of 218Po and 214Po, obtained fromthe fit of the ϵ218 and ϵ214 to the exponential functions after theequilibrium is established.

The collection efficiency depends on the collection voltage, thehumidity, 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 workdifferent for 218Po and 214Po ions, the counting ratio between thetwo ions differs from 1, even in equilibrium. In our radon detectionsystem, this ratio is measured to be 0.62 in equilibrium.

progeny.

00 50 100 150 200 250

Time (m)

Fig. 10. The calculated time dependence of 218Po and 214Po concentrations.

3.2. Radon measurements

The first tests of the detector were performed with a 0:1 μCi226Ra source (LD Didactic GmBH, 1999). The source was locatedjust above the surface of the PIN photodiode and Amptek MCA-8000A Pocket MCA recorded the alpha spectrum for about 5 min.A 4.78 MeV alpha peak from the decay of radon was immediatelyobserved in the spectrum and it was well separated from thenoise. The gain and the threshold adjustments of the detectorcircuit were performed using this radium source.

Fig. 9 shows the MCA spectrum of radon obtained from theSRM 4974 222Rn source. This spectrum was taken about 6.5 h afterradon was sniffed into the collection chamber with 0.4 L/m airflow rate.

The time dependent 218Po and 214Po concentrations can bederived from well known decay equations (Maiello and Hoover,2010). Fig. 10 shows the theoretical time dependence of 218Po and214Po concentrations. As can be seen in Fig. 10, approximately 3 h isrequired to establish the equilibrium between the two nuclides.

The time dependence of the measured rate of the radon decayproducts was also studied to determine the influence of the airflow. In the first measurement, the radon was sniffed into thecollection chamber and measurement was performed with con-tinuing air flow (Fig. 11). In the second measurement, the sameprocedure was performed but this time the pump was turned offright after radon was taken into the collection chamber (Fig. 12).The half life of the detected 218Po was measured to be 6.670.3 mwith the 0.4 L/m flow rate. This value is much longer compared tothe one with no air flow. This half life is not the radiological halflife. It includes many dynamic parameters, influencing the detec-tion efficiency, such as ionization and neutralization rates(Kiko, 2001).

Pump Rate 0.4 l/m

0

50

100

150

200

250

300

0 25 50 75 100 125 150 175 200

P1 384.5 25.97P2 6.604 0.2736

Peak

Are

a218

Po

0

20

40

60

80

100

0 25 50 75 100 125 150 175 200

P1 91.03 9.144P2 49.95 3.189

214Po

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 P1expð−t=P2Þ function and the fit parameters are givenon the upper right corner of the figure.

0

50

100

150

200

250

300

0 200 400 600 800 1000

P1 155.3 3.131P2 450.1 12.10

Peak

Are

a

218Po

2 min. intervals5 min. intervals

0

50

100

150

200

250

300

0 200 400 600 800 1000

P1 320.6 3.484P2 388.6 5.191

214Po

Time (m)

Fig. 12. The time evolution of 218Po and 214Po counts when there is no air flow. The experimental data is fitted to P1expð−t=P2Þ function and the fit parameters are given onthe upper right corner of the figure.

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9 7

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–98

After the energy calibration was performed with the radonsource, a spectrum of the soil sampled from Geyikli Beach ofDardanels was taken. Fig. 13 shows the alpha spectrum of this soilsample. In addition to radon, the peaks of thoron (220Rn) arevisible in the spectrum.

0

250

500

750

1000

1250

1500

1750

2000

2250

2 3 4 5 6 7 8 9 10Energy (MeV)

Cou

nts

Fig. 13. The recorded MCA spectrum of the sand sample from Geyikli Beach,Dardanels.

0

20

40

60

80

100

6 6.5

Rad

on C

once

ntra

tion

(Bq/

m3 )

0

20

40

60

80

100

8.5 9

Time (Mo

Fig. 14. 152 days of radon measurement in the bas

The minimum detectable activity measurement was also per-formed with the standard radon source. 2556 Bq/m3 222Rn activityprovides approximately one net count rate in the detector. Thisresult can be translated to 4 Bq/m3 minimum detectable activityfor 1 h measurement.

4. Results and discussions

The performance tests and the measurements showed that theradon monitor can detect the radon level down to the desiredlevel. Since monitoring the anomalies in the concentration level ismore crucial than determining the absolute radon level, the radonmonitors do not have to be calibrated if they are going to be usedfor the purpose of earthquake precursory. However, we performedthe calibration for our system.

7 7.5 8

9.5 10 10.5

nths of 2012)

ement of the Faculty of Mines building of ITU.

0

20

40

60

80

100

2 4 6 8 10 12Time (Days)

Rad

on C

once

ntra

tion

(Bq/

m3 )

Fig. 15. 12 days of radon measurement by the monitors located in the basement ofthe Faculty of Science and Letters building of ITU.

A. Bayrak et al. / Applied Radiation and Isotopes 78 (2013) 1–9 9

We performed continuous test measurements with the radonmonitors on the campus of our university. Nine radon monitorshave been located throughout the campus buildings. The mea-surement locations were chosen to be the locations with possiblyhigh concentrations of radon due to the deficient air circulation.Fig. 14 shows the 152 days of radon measurement in the basementof the Faculty of Mines building of ITU. As one can see from thefigure, the detected radon level oscillates with daily periodsespecially in the summer months. This is due to the changes inhumidity and temperature on a daily basis. Another five radonmonitors were located in the basement of the Faculty of Scienceand Letters building of ITU to observe if their measurements wereconsistent with each other. Fig. 15 shows the 12 day measure-ments. One can see in the figure that the count rates from eachstation are statistically consistent with each other most of thetime. However, there are few points with larger deviations thanstatistics may tolerate. This might be due to the local fluctuationsin the radon concentration.

One of the concerns about this monitor is its maintenance. Air isnot filtered in our system. It is known that the accumulation of thedust on the surface of the PIN photodiode gradually decreases thedetection efficiency. Not to mention, corrosive gases may spoil thesurface of the photodiode since there is no protective window.However, there is a thin anti-reflective layer on it. The manufacturerclaims that the surface of the photodiode can be gently cleaned with asoft tissue using alcohol. Just to be on the safe side, we plan to coat thesurface of the PIN photodiode with 50–100 nm thick diamond likecarbon. This coating may serve as a protective layer against thecorrosive atmospheric gases. Then the collection chamber will berotated by 901 and a tiny sprinkle systemwill be installed to wash thesurface of the photodiode periodically. No water drop will stay on thecoated surface of the photodiode since diamond like carbon is superhydrophobic. The absorption due to the effect of 50–100 nm carbonlayer is calculated to be negligible for the alpha particles. An air filterwill also be integrated into the air intake of the monitor in the future.

The SMS modem is the only commercial device in the radonmonitor. This is the reason why the cost of the monitor was signi-ficantly reduced. The cost of the radon monitor has been reduceddown to the price range of a commercial charge sensitive pre-amplifier. Since large number of units are needed, the cost of thedevice could be reduced even more with the mass production inthe future.

With these low cost monitors, radon maps of wider regions canbe produced in shorter times and seasonal changes of radon levelcan be tracked with larger statistics in some locations which helpsthe modeling of radon transport.

These monitors can be used for the purpose of monitoringradon activities in the basements of the buildings, the caves, thetunnels, the mines and the metro stations without any modifica-tion. They can also be integrated into the air circulation system ofthe buildings to track the air quality.

Acknowledgments

Wewould like to thank to the Scientific and Technological Councilof Turkey (TUBITAK) for supporting this project (110T261). We alsothank to the Support Unit of Research Projects of ITU (BAP).

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