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Radiation Measurements xxx (2013) 1e9

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Radiation Measurements

journal homepage: www.elsevier .com/locate/radmeas

A new pin-hole discriminated 222Rn/220Rn passive measurementdevice with single entry face

B.K. Sahoo, B.K. Sapra*, S.D. Kanse, J.J. Gaware, Y.S. MayyaRadiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

h i g h l i g h t s

� A model is developed to discriminate 222Rn and 220Rn using pin-hole.� Model is validated against the experimental results.� A new pinhole discriminated 222Rn/220Rn passive measurement device is developed.� The new device overcomes the limitation of the conventional twin cup dosimeter.� Device is calibrated using standard sources of 222Rn and 220Rn.

a r t i c l e i n f o

Article history:Received 28 March 2012Received in revised form31 July 2013Accepted 8 August 2013

Keywords:RadonThoronPin-holeDiffusion chamberSSNTD

* Corresponding author. Environmental & Bio-dosPhysics and Advisory Division, Bhabha Atomic ResearIndia. Tel.: þ91 22 2559 2209; fax: þ91 22 2551 9209

E-mail addresses: bsapra@barc.gov.in, bijaybarc@g

1350-4487/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.radmeas.2013.08.003

Please cite this article in press as: Sahoo, B.Kface, Radiation Measurements (2013), http:/

a b s t r a c t

Solid State Nuclear Track Detector (SSNTD) based diffusion chambers have been widely used for resi-dential radon measurements due to their cost effectiveness, portability and easy-to-use feature. In India,an LR-115 track detector based twin-cup dosimeter has been in use for about a decade for indoor 222Rnand 220Rn measurements. However, the estimation of the gas concentrations using this dosimeter wasbased on the assumption of the same entry rate of the gases into the two cups of the dosimeter, whichmay not be valid for dosimeters deployed in turbulent environmental conditions. To overcome thislimitation, a new pin-hole based 222Rn/220Rn discriminating measurement device has been developed.The underlying discrimination technique has been established by modelling 222Rn and 220Rn diffusioninto a pin-hole chamber and validating the same by carrying out experiments in a test chamber. Thedevice has been calibrated at Bhabha Atomic Research Centre, Mumbai following the standard pro-cedures to correlate the number of tracks registered in the LR-115 detector placed in the two chambers tothe 222Rn and 220Rn concentration in the environment. Salient features of the device include (i) the pin-holes act as 222Rn/220Rn discriminator and eliminate the requirement of membrane filter used in theearlier twin cup design (ii) the single entrance design for gas transmission and (iii) use of multiple pin-holes of reasonably small radius minimises effect of turbulence on 222Rn/220Rn transmission factors sothat the calibration factor is independent of indoor turbulence.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Radon (222Rn) and thoron (220Rn) gases enter into the indoor airthrough exhalation from soil and building materials used in walls,floors and ceilings (Nazaroff and Nero, 1988). Poor indoor ventila-tion conditions result in an increase in the concentration of thesegases and their decay products in rooms. It has been observed that

imetry Section, Radiologicalch Centre, Mumbai 400 085,.mail.com (B.K. Sapra).

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., et al., A new pin-hole discr/dx.doi.org/10.1016/j.radmea

radon is the second most important cause of lung cancer, aftersmoking (WHO, 2009). Epidemiological studies have providedconvincing evidence of an association between indoor radonexposure and lung cancer, even at relatively low radon levelscommonly found in residential buildings (Darby et al., 2005;Krewski et al., 2005). Due to the increasing concern about therisk associated with indoor radon, projects for monitoring of indoorradon are being carried out in several countries (Dudney et al.,1990; Miles, 1998; Yu et al., 1999; Srivastava, 2005; Zhang et al.,2007; Ramachandran and Sahoo, 2009).

For indoor radon survey, passive detectors (such as CR-39, LR-115) have been widely used because of their cost effectiveness,portability and easy-to-use feature. Most importantly, unlike the

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Fig. 1

C0

C(t)

Fig. 1. Schematic of a pin-hole chamber system.

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e92

case of active instruments, these detectors do not require powersupply and provide time integrated radon concentration encom-passing both diurnal and seasonal variations. Initially SSNTD in thebare mode was used for passive measurements of radon (222Rn)(Stranden et al., 1979; Ramachandran et al., 1986; Andriamanatenaand Enge, 1995; Ramola et al., 1996); however, the responseincluded the tracks from the decay products and 220Rn (thoron) aswell. In order to remove interference from decay products in 222Rngas measurements, a system with Solid State Nuclear Track De-tector (SSNTD) enclosed in a diffusion chamber was developed(Nikezic and Baixeras, 1995; Nikezic et al., 1996; Nikezic andStevanovic, 2007). In this system, the particulate decay productsare filtered out using a suitable filter paper at the entry face,through which gases diffuse easily. However, these systems cannotdistinguish 222Rn and 220Rn, which is a major issue in environmentswith elevated 220Rn concentrations. This necessitates developmentof a discrimination technique for 222Rn and 220Rn so as to accuratelyquantify individual gas concentrations using passive detectorsystems.

For discriminating 222Rn and 220Rn measurements, twin cham-ber device with a diffusion barrier, which cuts off short lived 220Rnbut allows 222Rn to pass through it, have been designed. Whilesome systems use membranes (Eappen and Mayya, 2004;Tokonami et al., 2005), others use pinhole based diffusion barrier(Doi and Kobayashi, 1994; Sciochheti et al., 2010). In India, LR-115SSNTD based cylindrical twin cup dosimeterusing membrane forthoron cut-off, developed by Eappen and Mayya (2004), has beenwidely used for measurement of 222Rn and 220Rn in dwellings. Thedetector has two entrances (facing opposite to each other), bothusing glass fibre filters to cut off entries of decay products. Inaddition, one entrance uses a cellophane membrane to cut-off220Rn transmission so that only 222Rn enters into the so called‘radon’ chamber. The other entrance allows both 222Rn and 220Rninto the so called ‘radon þ thoron’ chamber. A subtraction tech-nique is used to remove the contribution of radon to the tracks inradon þ thoron chamber and obtain thoron concentration. How-ever, it was observed that in some cases the track densities ofSSNTD detector placed in ‘radon’ chamber exceeded the trackdensities of detector placed in ‘radon þ thoron’ chamber, resultingin a physically unacceptable negative 220Rn concentration. Onepossible reason for this is the different entry rates of 222Rn throughtwo entrances of the dosimeter whichmay arise from turbulence orair flow in one direction (as in case of one entrance facing a fan andother being opposite to it). This ambiguity of different 222Rn entryin two chambers can be removed by developing a twin chamberdevice having a single entrance. The conventionally used dosimeterwhich uses membrane based 222Rn e 220Rn discrimination tech-nique cannot be easily converted to a twin chamber with singleentrance. However, it is possible to achieve the required design byreplacing the membrane with a pin-hole based discriminatingdesign. Though pinhole based diffusion barrier has been used insome detectors there has been no theoretical basis to decide thepin-hole dimensions for desired 220Rn cut off and 222Rn trans-mission into the diffusion chamber.

In this paper, we discuss the development of a pin-hole based222Rn/220Rn discrimination technique, established by modelling222Rn and 220Rn diffusion into the pin-hole chamber and validatingthe model predictions with the experimental observations. Basedon this, a new pin-hole based 222Rn/220Rn discriminating devicehas been designed and developed. This LR-115 track detector baseddevice has a single face for gas entry and gives time integratedmeasurement of 222Rn and 220Rn in dwellings. The optimalconfiguration of pin-hole dimensions was decided with the help ofmodel predictions as well experimental measurements in turbulentenvironmental conditions. In order to minimise the effect of

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turbulence penetration of 222Rn and 220Rn, multiple pin-holes ofreasonably small radius (0.5 mm) were used and an arrangementwas made for deploying the device in a face-down condition. Thedevice has been calibrated in a laboratory calibration facility at theBhabha Atomic Research Centre, Mumbai to correlate the numberof tracks registered in the LR-115 detector to the 222Rn and 220Rnconcentration in the environment.

2. Model development for pin-hole based 222Rn e 220Rndiscriminator

Let us consider a closed cylindrical chamber having a pin-hole ofradius a and length d at one face (Fig. 1). It is assumed that the gasenters the chamber through pin-hole by the process of diffusion. IfCðtÞ is the average 222Rn/220Rn gas concentration in the chambervolume at time t, then, the non-steady state equation for CðtÞ maybe written as:

VvC tð Þvt

¼ JA� lC tð ÞV (1)

where V is the volume of the chamber, J is the 222Rn/220Rn trans-mission flux through the pin-hole, A (¼p a2) is the area of the holeand l is the decay constant of the gas (either 222Rn or 220Rn).

The flux J through a pin-hole is related to the difference in theconcentration of the gas between the outside and inside air, byapplying the Fick’s law of diffusion as follows:

J ¼ DCo � CðtÞ

d(2)

Where, C0 is the 222Rn/220Rn gas concentration in the outside air atthe entry face,D is the 222Rn/220Rn diffusion coefficient in air (hole).Substituting J from Eq. (2) in Eq. (1) and simplifying we arrive at

vCðtÞvt

¼ lpC0 � leCðtÞ (3)

where we denote

lp ¼ ADVd

(3a)

and

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

0

5

10

15

20

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

Rn-

222

trans

mis

sion

(%)

Rn-

220

trans

mis

sion

(%)

time (m in)

Rn-220

Rn-222

D=0.1 cm2/sn =1A = 0.2 cm2

d =0.2 cmV =100 cc

Fig. 2. Transient response of 222Rn and 220Rn transmission into pin-hole chamber.

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e9 3

le ¼ lþ lp (3b)

Solving Eq. (3) with initial conditionCðt ¼ 0Þ ¼ 0, the timevariation of 222Rn/220Rn concentration in the chamber volume afterthe deployment is found to be

CðtÞ ¼ C0

�lple

��1� e�let

�(4)

A typical plot of 222Rn and 220Rn transmission into pin-holechamber with time is shown in Fig. 2. At large times, a steadystate is reached having a steady state concentration (Cs) in the pin-hole chamber:

Cs ¼ C0

�lple

�(5)

Using Eq. (4) and Eq. (5), we may now define the following twoquantities in order to assess the relative performance of the pin-hole chamber for the transmission of 222Rn vis’a vis’ 220Rn.

Table 1Model predicted transmission time and percentage transmission of 222Rn and 220Rninto pin-hole chambers of various pin-hole dimensions and chamber volume.Diffusion coefficient of 222Rn and 220Rn through the pin-hole is taken as 0.1 cm2 s�1.

Holediameter(mm)

Holelength(mm)

Chambervolume(cm3)

Transmissiontime, T95 (min)

Percentagetransmission,F (%)

222Rn 220Rn 222Rn 220Rn

1 1 150 919 4 96 0.42 1 150 236 4 99 1.61 2 150 1769 4 93 0.22 2 150 468 4 98 0.83 2 150 210 4 99 1.82 3 150 696 4 97 0.63 3 150 105 4 99 1.24 3 150 919 4 96 0.43 4 150 236 4 99 1.64 4 150 376 4 98 1.04 5 150 468 4 98 0.83 1 500 348 4 98 14 1 500 197 4 99 23 2 500 687 4 97 0.54 2 500 391 4 98 15 2 500 252 4 99 1.53 1 1000 687 4 97 0.54 1 1000 391 4 98 16 2 1000 348 4 98 18 3 1000 294 4 99 1

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(i) Transmission time (T95): T95 may be defined as the timerequired by 222Rn or 220Rn for attaining 95% of its final steadystate concentration (Cs) in the pin-hole chamber. Mathemati-cally, it can be expressed using Eq. (4) as

T95 ¼ 3VdlVdþ AD

(6)

(ii) Transmission factor (F): F may be defined as the ratio of finalsteady state concentration of 222Rn or 220Rn in the pin-holechamber to the concentration of 222Rn or 220Rn just outsidethe entry face of the pin-hole chamber (i.e. F ¼ Cs/C0). Math-ematically, it can be expressed using Eq. (5) as

F ¼ ADADþ lVd

(7)

Using Eqs. (6) and (7), the transmission time (T95) and trans-mission factor (F) of 222Rn and 220Rn into the pinhole chamber areestimated for various pinhole radii and lengths. The results arelisted in Table 1.

It may be noted that Eqs. (6) and (7) will also be valid for mul-tiple pin-holes but in such cases, A will be the sum of areas of in-dividual holes. Hence, the predicted values of the transmissionfactor (F) and transmission time (T95) of 222Rn and 220Rn into apinhole chamber will remain same for a given total area of the pin-holes irrespective of single or multiple pin-holes (all other pa-rameters remaining the same). For example, T95 and F for a singlehole of diameter 1 mm will be the same as that for the case of 4holes of 0.5 mm diameter each. For designing 222Rn/220Rndiscriminating device, a suitable combination of area and length ofpinhole can be used depending upon T95 and F values. Towards this,Table 1 provides the first level of guidance for selecting the same.The validation of the model has been carried out by comparisonwith experiments discussed in the following section.

3. Experimental validation

For the purpose of experimental validation, few units of cylin-drical twin chamber device with two identical chambers werefabricated (see schematic diagram of Fig. 4). Gas enters into the 1stchamber through a filter paper and then into the 2nd chamberthrough discriminating pin-hole disc which separates the twochambers. The 1st chamber is the reference chamber and the 2ndone is the pin-hole chamber. Each chamber has a length of 4.1 cmand radius of 3.1 cm (same dimensions as in the twin cup dosimeterdeveloped by Eappen and Mayya (2004)). Two LR-115 detectorfilms of size 3 cm � 3 cmwere fixed at end of the two chambers soas to register the tracks due to alphas emitted from 222Rn, 220Rngases and their decay products formed in the chamber volume. Theratio of track densities obtained in the detector belonging to pin-hole chamber and to that in reference chamber is a measure oftransmission factor of 222Rn or 220Rn. Experiment was carried outfor pin-hole diameters of 0.5 mm, 1 mm, 2 mm and 3 mm and pin-hole lengths of 2 mm and 5 mm. The experimental procedurecomprise of controlled exposure of LR-115 detector loaded twinchambers in a calibration chamber, etching of LR-115 films fordeveloping the track, and counting the developed tracks usingspark counter.

The exposure of twin chamber device to 222Rn/220Rnwas carriedout in a cubical stainless steel calibration chamber of volume of0.5 m3 in ‘fan off’ condition as the model is based on diffusiontheory. The schematic diagram of the experimental arrangement isshown in Fig. 3. The chamber has arrangement for deploying the

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Moisture absorber

Calibration chamber

Radon-Thoronmeasurement device

RAD 7

Fan

Radon source

Fig. 3. Schematic diagram of the calibration chamber and the experimental set upused for the model validation and 222Rn calibration of the measurement device.

Table 2Results of

222Rn transmission into the pin-hole chamber through a pin-hole of various dimensions in “Fan

off” condition.

Hole dia(mm)

Holelength(mm)

No ofsets

Track density(tr. cm-2)

222Rn transmissionpercentage FRn (%)

Pin-holechamber, Tp

Referencechamber, Tr

Experiment Model(Eq. (7))

0.5 5 3 156.7 � 8.5 283.7 � 4.5 55 � 3 601 5 3 258.3 � 5.1 329.7 � 4.7 78 � 2 862 5 3 329 � 6.2 341.7 � 4.2 96 � 2 963 5 3 340 � 23.9 353.3 � 29 96 � 10 982 2 3 295 � 8.7 305 � 8.9 97 � 4 98

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e94

twin chamber devices to be exposed. Separate experiments werecarried out in 222Rn and 220Rn environment to determine theirrespective transmission factors. Radon (222Rn)/thoron (220Rn) wasintroduced into the calibration chamber from a standard source.The source used for 222Rn was Model RN -1025 from Pylon Elec-tronics Inc., Ottawa, Canada with activity of 110.6 kBq as on March1996; whereas source used for 220Rn was Model TH-1025 fromPylon Electronics Inc., Ottawa, Canada with activity of 117.1 kBq ason 8 November 1996. Initially, the fan inside the chamber was kepton for 5 min for uniform mixing and later on it was switched offthroughout the exposure period. The exposure period was decidedto be at least one day for 222Rn and 30 min for 220Rn on the basis ofmodel predicted value of T95 (Eq. (6)).

After the completion of exposure, the LR-115 films wereremoved from the twin chamber device and were chemicallyetched using 2.5N NaOH solutions at a temperature of 60 �C for90 minwithout stirring (Eappen andMayya, 2004). The counting ofetched tracks in LR-115 film was carried out using an automatedspark counting technique. Each filmwas pre-sparked at 900 V priorto counting and then counted at the operating sparking voltage of500 V. The background tracks in LR-115 detector were alsomeasured by following the same etching and counting procedureusing the unexposed LR-115 detector. The experimental trans-mission percentage (Fexp) was then calculated using the followingformula

Fexp ¼ Tp � BTr � B

� 100 (8a)

Where Tp and Tr are the total track densities in LR-115 detectorsplaced in pinhole chamber and reference chamber respectively, B isthe average background track density in unexposed LR-115 de-tectors which was found to be 4 � 2 tr. cm�2 for control samplesfrom the same batch of LR-115 films.

The uncertainty (sF ) associated with each experimentallydetermined transmission factor was derived by applying errorpropagation formula (Bevington and Robinson, 2003) to Eq. (8 a).The overall uncertainty was estimated using the expression:

sF ¼ 100

ðTr � BÞ2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðTr � BÞ2s2p þ

�Tp � B

�2s2r þ

�Tp � Tr

�2s2B

q

(8b)

Where sp and sr are the uncertainties associated with the trackdensities in LR-115 detectors placed in pinhole and reference

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chamber respectively, sB is the uncertainty associated with averagebackground track density in unexposed LR-115 detectors from thesame batch of LR-115 films (taken as 2 tr.cm�2)

The results of transmission factors for 222Rn and 220Rn arepresented in Table 2 and Table 3 respectively. The results for 222Rntransmission factors obtained from experiments were comparedwith values obtained from Model (Eq. (7)) wherein the diffusioncoefficient of 222Rn in air was taken as 0.1 cm2 s�1 (Rogers andNeilson, 1991). A fairly good matching between the experimen-tally estimated and theoretically predicted transfer factors for 222Rnis evident from Table 2. In case of 220Rn, the observed tracks den-sities in pinhole chamber were found to be very close to thebackground track density (4 � 2tr. cm-2) suggesting nearly com-plete cut off of 220Rn into the pinhole chamber. This is in accordancewith the model prediction. Since the estimated uncertainty asso-ciated with the 220Rn transmission is very high, comparisons havenot been made for the case of 220Rn. However, the experimentalresult of 220Rn reveals that the 220Rn transmission into a pinholechamber can be cut off by selecting suitable dimension of thepinhole.

4. Effect of turbulence & its minimisation

The model described in section 2 is valid for transmissionthrough pinhole by molecular diffusion, which is possible only in astable environment. However, in most dwellings some amount ofair turbulence is always expected. Hence it is necessary to study theeffect of turbulence on transmission of 222Rn and 220Rn throughpinhole for optimising the design. The effect of turbulence for thecase of a combination of multiple pin-holes as compared to a singlepinhole of an equivalent area also needs to be investigated.

For this purpose, transmission through four different configu-rations of pinholes with varying dimensions and number of pin-holes, but with similar model-predicted transmission factors, wasstudied. The devices were exposed in the calibration chamber withthe fan inside the chamber running on throughout the exposureperiod. The turbulence velocity in this condition was estimated tobe 6.3 cm s�1 using the formula given byMishra et al. (2009), whichis representative of the condition for extreme turbulence indwellings. The experimental procedure was similar to thatdescribed in section 3. The results of the study for the four differentconfigurations are given in Table 4 for the case of 222Rn and inTable 5 for the case of 220Rn.

It may be observed that in case of 222Rn, the values of trans-mission factors for all combinations were almost same as thatpredicted by model. But in case of 220Rn, the values of transmissionfactors were 2e5 times higher than the model predicted values. Itwas also observed that for equivalent area of pinholes, the values oftransmission factor for the cases of multiple pin-holes were closerto the model-predicted value than that for single pinhole. This in-dicates that in presence of turbulence, thoron cut-off will be better

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Table 3Results of 220Rn transmission into the pin-hole chamber through a pin-hole of various dimensions in “Fan off” condition.

Hole dia (mm) Hole length (mm) No of sets Track density (tr. cm-2) 220Rn transmission percentage, FTn (%)obtained from experiment

Pin-hole chamber, Tp Reference chamber, Tr

0.5 5 3 5 � 1 137 � 5 0.8 � 21 5 3 5.3 � 1.5 134.7 � 3.5 1.0 � 22 5 3 5 � 1.7 157.3 � 4.5 0.7 � 23 5 3 5.3 � 0.7 156.7 � 6.4 0.9 � 12 2 3 4 � 1 203 � 5.6 0.0 � 1

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e9 5

in case of multiple holes of reasonably small radius thanwith singlepinhole of equivalent area.

5. Development of the pin-hole based 222Rn/220Rndiscriminating measurement device

An ideal device for 222Rn/220Rn measurement would be the onefor which the calibration factor will be independent of indoorenvironmental conditions such as air turbulence. High turbulencewill lead to a significant penetration of 220Rn into the radon chambercausing a lot of interference in 222Rn calibration factor. The present222Rn/220Rn discrimination design based on multiple pinholes ofreasonably small radius is thus an ideal choice. From Tables 4 and 5,it may be noted that the configurationwith 4 pin-holes, each having1mmdiameter and 2mm length, is best suited as it has 220Rn cut offof about 98% (Table 5) with 222Rn transmission of about 97%(Table 4). At the same time, turbulence penetration of 220Rn at aturbulence level of 6.3 cm s-1 (an extreme case of high turbulenceindoors)was found to be negligible and nearly same as that found inthe case of ‘fan off’ condition (nearly stable environment). Theschematic diagram and photograph of the device based on thisconfiguration is shown in Fig. 4.

The device has two identical cylindrical chambers of length of4.1 cm and radius 3.1 cm (same dimension as in the case of con-ventional twin cup dosimeter) which are separated by the pinholebased 222Rn/220Rn discriminating plate. The gas enters into the first“radon þ thoron” chamber through the glass fibre filter paperwhich filters out decay products and subsequently diffuses to thesecond “radon” chamber through four pin-holes which cut off220Rn. The LR-115 film kept in the ‘radon þ thoron’ chamber reg-isters the alpha tracks due to both 222Rn and 220Rn and their decayproducts. The LR-115 film kept in the ‘radon’ chamber registers thealpha tracks only due to 222Rn and its decay products.

The present device has many salient features. Firstly, it elimi-nates the requirement of a cellophane membrane as discriminatorused in the conventional twin cup dosimeter. This reduces theuncertainty due to change in thoron cut-off with different batchesof membranes. Another important feature is that it has single facefor gas entry which rules out uncertainty arising due to unequalentry of 222Rn gas as in the case of conventional twin chamberdevice (discussed in detail in section 7). Besides, use of multiplepin-holes in the discriminator, and placement of the discriminatorafter the first chamber (and hence at some distance from theentrance) minimises the effect of turbulence on thoron cut-off.

Table 4Results of 222Rn transmission into the pinhole chamber through single vs. multiple pin-h

Net area of pinholes (mm2) Hole length (mm) Hole diameter (mm)

3.14 2 12

12.56 5 24

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Both chambers are internally coated with metallic powder(nickel) to have zero electric field inside the chamber. This ensuresthat the deposition of decay products formed from 222Rn and 220Rngases is uniform throughout the inner surfaces of the chamber(Hopke et al., 1993).

6. Determination of the calibration factors

Having developed the pin-hole based twin chamber device, it isnow required to calibrate the device and obtain the calibrationfactors for 222Rn and 220Rn measurements. The calibration factor isbasically converts the observed track densities to the activity con-centration of 222Rn and 220Rn in the air. If T is the backgroundcorrected track density observed on a SSNTD due to exposure in agiven mode to a concentration C of a given species for a time t, then

T ¼ k:C:t (9)

Where,

k is the calibration factor (tr. cm�2 d-1/(Bq m�3))t is the time in daysC is the concentration in Bq m�3

T is the track density in tracks cm�2

The calibration factors for 222Rn and 220Rn for the new twinchamber device may be obtained using following Eq. (10)(a,b) and11(a,b) respectively as follows:

kR;1 ¼ TR;1 � BtCR

(10a)

kR;2 ¼ TR;2 � BtCR

(10b)

kT ;1 ¼ TT ;1 � BtCT

(11a)

kT ;2 ¼ TT ;2 � BtCT

(11b)

Where, kR,1, kR,2, kT,1 and kT,2 are the calibration factors (tr. cm�2 d�1/(Bq m�3)) of 222Rn and 220Rn in the ‘radon þ thoron’ and ‘radon’chamber of the device respectively. TR,1, TR,2, TT,1 and TT,2 are the

oles of equivalent area in "fan on" condition.

No of holes Number of sets Radon transmission percentage (%)

Experiment Model (Eq. (7))

4 3 97.3 � 1.45 98.41 3 97.1 � 1.734 3 97.8 � 1.69 98.91 3 96.9 � 2.12

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Table 5Results of 220Rn transmission through single vs. multiple pin-holes of equivalent area in "fan on" condition.

Net area of pinholes (mm2) Hole length (mm) Hole diameter (mm) No of holes Number of set Thoron transmission percentage (%)

Experiment Predicted from model (Eq. (7))

3.14 2 1 4 3 1.8 � 0.7 12 1 3 4.8 � 1.7

12.56 5 2 4 3 2.2 � 0.6 1.64 1 3 6.1 � 1.4

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e96

total track densities (tr. cm�2) in LR-115 detectors for the‘radon þ thoron’ and ‘radon’ chamber for 222Rn and 220Rn exposurerespectively. B is the background track density in unexposed LR-115detector, measured as 4 � 2 tr. cm�2. CR and CT are the averageconcentrations (Bq m�3) of 222Rn and 220Rn at the entry facerespectively, and t is the exposure period (d).

Experiments were performed in a calibration chamber for thedetermination of calibration factors for both 222Rn and 220Rn.Calibration chambers of different types and sizes are being used theworld over for the standardisation of SSNTDs and associated pas-sive detectors (Azimi-Garakani, 1992). The chamber used in thepresent study was a stainless steel cubical chamber of volume0.5 m3 222Rn gas was introduced into the calibration chamber froma Pylon-make standard 222Rn source (Model RN -1025, Source ac-tivity of 110.6 kBq as on March 1996). A fan was placed inside thechamber for mixing and achieving spatial uniformity in the 222Rnconcentration. Gas samples were collected through outlets pro-vided on the chamber. RAD 7, a continuous 222Rn monitor was usedfor the measurement of 222Rn. The measurement devices werevertically hung to the arms of the central rod provided in thechamber. Initially, the fan inside the chamber was kept on for 5 minfor uniform mixing and later, it was switched off throughout theexposure period. The experimental set up used for the determi-nation of 222Rn calibration factor is shown in Fig. 3.

The devices were exposed to different 222Rn concentrationsvarying between 2 kBq m�3 and 34 kBq m�3. The exposure periods

Fig. 4. Schematic diagram of the new pin-hole based 222Rne220Rn measure

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were varied from 14 h to 168 h depending upon the radon con-centration level such that a reasonable number of tracks will beformed in the detectors. After the completion of each exposure, theSSNTD films (LR-115) were taken out of the device and etched in 2.5(N) NaOH solutions using standard etching procedure as discussedearlier (Sec. 3). After etching, these films were thoroughly washedwith distilled water and kept for drying. The clear polyester basewas then peeled off and the films were subjected to spark countingto get the total track density. The actual track density was thencalculated by subtracting the background tracks in unexposed LR-115 detector. The 222Rn calibration factor in the two chambers ofthe device was determined using Eq. (10 (a, b)). The uncertaintyassociated with each 222Rn calibration factor was derived byapplying error propagation formula (Bevington and Robinson,2003) to Eq. (10 (a,b)) in a similar way as done for Eq. (8 a). Theoverall uncertainty was then estimated using uncertainties inmeasured 222Rn concentration, total and background trackdensities.

For estimating the 220Rn calibration factor of the device, theabove mentioned procedure was not suitable since 220Rn beingshort lived would decay out rapidly in its large volume (0.5 m3).This would lead to non-uniformity of the gas concentration,resulting in a 220Rn profile inside the chamber (Yamasaki, 1995).Hence, an alternative chamber of volume 250 cm3 was designed.This was a cylindrical chamber, made up of High Density PolyEthylene (HDPE) material. One side of the chamber was kept open

ment device and its photograph showing the deployment orientation.

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Fig. 5. Schematic of the experimental set up used for determination of 220Rn cali-bration factor of the 222Rne220Rn measurement device.

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e9 7

such that the entry side of the measurement device made anair-tight coupling with this opening. Inside this cylindrical cham-ber, an arrangement was made to release 220Rn gas at thebottom generated from standard 220Rn source (Pylon, Canada,Model TH -1025, Source activity of 117.1 as on 8 November 1996).The release was directed towards the wall of the chamber so as tohave reasonable mixing. Sampling provision was made close to theentry face of the device to measure the representative 220Rn con-centration by RAD7, a continuous radonmonitor. The calibration setup used for the 220Rn calibration is shown in Fig. 5

The 220Rn exposure experiment was carried out for a period of1 h. This period is sufficient enough to get a reasonable trackdensity in detectors as the 220Rn concentrations were in the rangeof 310e446 kBq m�3. After the completion of exposure, the LR-115detectors were etched and tracks were counted by spark counter ina manner as discussed earlier (Sec. 3.1). The actual track densitywas then calculated by correcting for the background tracks and the220Rn calibration factors in both chambers of the device weredetermined using Eq. (11(a,b)). The uncertainty associated witheach 220Rn calibration factor was derived by applying error prop-agation formula (Bevington and Robinson, 2003) to Eq. (11 (a,b)) ina similar way as done for Eq. (8 a). The overall uncertainty was thenestimated using uncertainties in measured 220Rn concentration,total and background track densities.

The calibration factors obtained for 222Rn and 220Rn in bothchambers are tabulated in Table 6 and Table 7 respectively. The222Rn calibration factor in ‘radon þ thoron’ chamber was found tobe 0.0172 tr.cm�2 d�1/(Bq m�3) and in the ‘radon’ chamber it wasfound to be 0.0170 tr.cm�2 d�1/(Bq m�3). Similarly the 220Rn cali-bration factor in ‘radon þ thoron’ chamber is 0.010 tr.cm�2 d�1/(Bq m�3) and in ‘radon’ chamber, the value is 0.00052 tr.cm-2 d�1/(Bq m�3) indicating more than 98% of 220Rn cut-off takes place. The220Rn calibration factor is less due to its short diffusion length in airand nearly 44% of initial 220Rn decays out before entering to the 1stchamber of the device (Eappen et al., 2008). It may be noted that

Table 6222Rn calibration factors of the present 222Rne220Rn measurement device obtained from

Sr. no CR (kBq m�3) Exposureperiod t (h)

No. ofsets

TR,1 (tr.cm�2)

1 1.93 � 0.4 168 2 213 � 92 10.2 � 1.1 50 2 346 � 83 15.8 � 1.2 67.2 2 926 � 84 17.3 � 1.2 54 2 596 � 115 27.4 � 1.8 28 2 605 � 316 32 � 2.3 14 2 323 � 137 34.2 � 2.3 45 2 1101 � 16Average � SD

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the typical value of 222Rn and 220Rn concentrations in dwellings arein the range from 10 to 40 Bq m�3 (UNSCEAR, 2000). Since typicalbackground tracks observed in LR-115 detector is about 4 tr.cm�2, itis preferred to deploy the measurement device in the field for aperiod of at least 3e4 months depending upon the concentrationlevel so as to get track densities 3 times above the background inthe exposed LR-115 film. It may be remarked that, the backgroundtrack density in other film detectors such as CR-39 is more ascompared to LR-115 detector. Hence the use of LR-115 detector inthis measurement device provides another advantage for detectinglow level of 222Rn and 220Rn concentration in indoor environment.

It may be noted that the calibration factors obtained are basedon a given protocol. They may change depending upon the exper-imental conditions such as operational characteristic of the sparkcounter or etching conditions. It would be appropriate to establisha-priori relative factors for changed measurement conditionsthrough inter-comparison exercises before using these calibrationfactors.

7. Performance evaluation of the present device againstconventional twin cup dosimeter

The new device is developed aiming to remove the ambiguity ofobtaining negative 220Rn concentration due to non-uniform entryof 222Rn into the two compartments which normally happens inpresence of turbulence. To evaluate its performance in such con-ditions, a comparison study was made by exposing the new deviceand the conventional twin cup dosimeter to 222Rn in a test chamberunder “fan on” condition, a case of high turbulence. The two deviceswere deployed side by side at five different positions in thechamber. As per the deployment protocol, all new devices weredeployed vertically with entry face down, while the conventionaldevices were deployed in horizontal manner. Since conventionaldevice had two entrances viz. ‘membrane face’ and ‘filter face’, theirrespective positions with respect to position of the fan were noteddown. Three out of five such devices had ‘membrane face’ (entranceto “radon” chamber) directed towards the fan, while the remainingtwo devices had ‘membrane face’ directed opposite to the fan. Thefanwas kept on throughout the exposure period. The experimentalprocedure for the exposure and processing of the exposed LR-115detectors was as described in Section 3. The remarks about theentrance position of devices with respect to position of fan, totaltrack densities obtained for the detectors belonging to ‘radon’ and‘radon þ thoron’ chambers and their differences are tabulated inTable 8. The following observations can be made from the table.

(i) The difference in track densities between ‘radon þ thoron’and ‘radon’ chambers of the conventional twin chamber device isnegative in some cases (3) and positive in some other cases (2)which may arise due to non-uniform entry of 222Rn into therespective chambers of the device through its two opposite faces.Negative results occur when the membrane face of the device was

experiments.

TR,2 (tr.cm�2) kR,1 (tr.cm�2 d�1/(Bq m�3)) kR,2 (tr.cm�2 d�1/(Bq m�3))

211 � 8 0.0155 � 0.0033 0.0153 � 0.0032337 � 4 0.0161 � 0.0018 0.0157 � 0.0017924 � 4 0.0208 � 0.0016 0.0208 � 0.0016594 � 7 0.0152 � 0.0011 0.0152 � 0.0011604 � 10 0.0188 � 0.0016 0.0188 � 0.0013311 � 14 0.0171 � 0.0014 0.0164 � 0.0014

1081 � 11 0.0171 � 0.0012 0.0168 � 0.00110.0172 � 0.002 0.0170 � 0.002

iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003

Table 7220Rn calibration factors of the present 222Rne220Rn measurement device obtained from experiments.

Sr. no CT (kBq m�3) Exposure period, t (h) TT,1 (tr.cm�2) TT,2 (tr.cm�2) kT,1 (tr.cm�2 d�1/(Bq m�3)) kT,2 (tr.cm�2 d�1/(Bq m�3))

1 310 � 7 1 117 � 5 7 � 2 0.009 � 0.0005 0.00023 � 0.000222 319 � 8 1 137 � 4 10 � 3 0.010 � 0.0004 0.00045 � 0.000273 340 � 8 1 123 � 3 12 � 2 0.008 � 0.0003 0.00056 � 0.000204 350 � 8 1 137 � 6 11 � 2 0.009 � 0.0005 0.00048 � 0.000195 353 � 8 1 185 � 4 17 � 3 0.012 � 0.0004 0.00088 � 0.000256 360 � 9 1 147 � 5 13 � 3 0.010 � 0.0004 0.00060 � 0.00024)7 360 � 9 1 151 � 6 10 � 2 0.010 � 0.0005 0.00040 � 0.000198 368 � 9 1 177 � 7 15 � 2 0.011 � 0.0005 0.00072 � 0.000199 376 � 9 1 169 � 7 9 � 2 0.011 � 0.0005 0.00032 � 0.0001810 446 � 10 1 178 � 5 15 � 3 0.009 � 0.0004 0.00059 � 0.00019Average � SD 0.010 � 0.001 0.00052 � 0.00019

Table 8Performance comparison between present device and the conventional twin cup dosimeter for 222Rn exposure in “fan on” condition.

Positionno.

Entrance position of devices with respect to position of the fan Observed track densities (tr.cm�2)

Present device Twin cup dosimeter Twin cup dosimeter Present device

‘Radon þ thoron’chamber

‘Radon’chamber

Difference ‘Radon þ thoron’chamber

‘Radon’chamber

Difference

1 Facing downwards Membrane face towards fan 624 684 �60 648 635 132 Facing downwards Membrane face towards fan 625 671 �46 654 638 163 Facing downwards Membrane face towards fan 629 678 �49 651 624 274 Facing downwards Membrane face opposite to fan 678 623 55 640 627 135 Facing downwards Membrane face opposite to fan 682 634 48 644 633 11Average 648 658 �10 647 631 16SD 30 28 57 6 6 6

B.K. Sahoo et al. / Radiation Measurements xxx (2013) 1e98

directed towards the fan while positive result occurs in the reversesituations. In case of negative results, themean concentration in the‘radon’ chamber may be higher than that in ‘radon þ thoron’chamber due to higher transmission rate of 222Rn into the formerone because of a pressure driven entry brought about by the fan.When the filter face of the device (entrance to ‘radon þ thoron’chamber) is opposite to membrane face (and hence opposite todirection of flow due to fan), the pressure driven entry into thischamber is least probable and the mean concentration in‘radon þ thoron’ chamber will be less as compared to ‘radon’chamber. The above explanation may hold good in the reversesituation where in the mean concentration in the ‘radon þ thoron’chamber will be higher than that in ‘radon’ chamber leading topositive results.

(ii) The track densities for ‘radon þ thoron’ and ‘radon’ cham-bers of the new pinhole based devices (5) are nearly equal, indi-cating a nearly uniform entry of 222Rn into the two chambers of thedevice. The small difference between the track densities may beattributed to small difference (w3%) in mean 222Rn concentrationbetween the two chambers which occurs due to decay of smallfraction of 222Rn while diffusing from ‘radon þ thoron’ to ‘radon’chamber through pin-holes. It may be noticed that the track densitydifference between the two chambers, though marginally small,remains positive, indicating that possibility of getting negative220Rn concentration is ruled out in the case of new device.

8. Conclusion

A new pinhole based 222Rn/220Rn measurement device has beendeveloped to replace the conventional membrane based twin cupdosimeter being used in India and it has several significant im-provements over the latter. The major improvements are: (i) thepin-holes based 222Rn/220Rn discrimination design eliminates therequirement of cellophane membrane filter (ii) the single entrancedesign for gas transmission rules out the possibility of encountering

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negative 220Rn concentration, and (iii) use of multiple pin-holes ofreasonably small radius and internal positioning of discriminatorminimises the effect of turbulence on 222Rn/220Rn transmissionfactors to a great extent and makes calibration factors of the deviceindependent of indoor turbulent conditions. With these salientfeatures, it is hoped that the new pinhole based twin chamberdevice will go long way as an ideal measurement device for map-ping 222Rn and 220Rn concentration in indoor environments.

Acknowledgements

Authors would like to thank Shri DAR Babu, Head, RadiologicalPhysics and Advisory Division, Bhabha Atomic Research Centre,Mumbai for his constant encouragement towards this work. Thehelp provided by Shri Manoranjan Dash, Atomic Energy RegulatoryBoard, Mumbai during experiments is kindly acknowledged.

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iminated 222Rn/220Rn passive measurement device with single entrys.2013.08.003