220Rn passive measurement device with single entry face

Radiation Measurements 58 (2013) 52e60

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

A new pin-hole discriminated device with single entry face

222

Rn/220Rn passive measurement

B.K. Sahoo, B.K. Sapra*, S.D. Kanse, J.J. Gaware, Y.S. Mayya Radiological 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

a b s t r a c t

Article history: Received 28 March 2012 Received in revised form 31 July 2013 Accepted 8 August 2013

Solid State Nuclear Track Detector (SSNTD) based diffusion chambers have been widely used for residential 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 222Rn and 220Rn measurements. However, the estimation of the gas concentrations using this dosimeter was based on the assumption of the same entry rate of the gases into the two cups of the dosimeter, which may not be valid for dosimeters deployed in turbulent environmental conditions. To overcome this limitation, 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 diffusion into a pin-hole chamber and validating the same by carrying out experiments in a test chamber. The device has been calibrated at Bhabha Atomic Research Centre, Mumbai following the standard procedures to correlate the number of tracks registered in the LR-115 detector placed in the two chambers to the 222Rn and 220Rn concentration in the environment. Salient features of the device include (i) the pinholes act as 222Rn/220Rn discriminator and eliminate the requirement of membrane filter used in the earlier twin cup design (ii) the single entrance design for gas transmission and (iii) use of multiple pinholes of reasonably small radius minimises effect of turbulence on 222Rn/220Rn transmission factors so that the calibration factor is independent of indoor turbulence. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Radon Thoron Pin-hole Diffusion chamber SSNTD

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

* Corresponding author. Environmental & Bio-dosimetry Section, Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. Tel.: þ91 22 2559 2209; fax: þ91 22 2551 9209. E-mail addresses: [email protected], [email protected] (B.K. Sapra). 1350-4487/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2013.08.003

radon is the second most important cause of lung cancer, after smoking (WHO, 2009). Epidemiological studies have provided convincing evidence of an association between indoor radon exposure and lung cancer, even at relatively low radon levels commonly found in residential buildings (Darby et al., 2005; Krewski et al., 2005). Due to the increasing concern about the risk associated with indoor radon, projects for monitoring of indoor radon 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, LR115) have been widely used because of their cost effectiveness, portability and easy-to-use feature. Most importantly, unlike the

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60

case of active instruments, these detectors do not require power supply and provide time integrated radon concentration encompassing both diurnal and seasonal variations. Initially SSNTD in the bare mode was used for passive measurements of radon (222Rn) (Stranden et al., 1979; Ramachandran et al., 1986; Andriamanatena and Enge, 1995; Ramola et al., 1996); however, the response included the tracks from the decay products and 220Rn (thoron) as well. In order to remove interference from decay products in 222Rn gas measurements, a system with Solid State Nuclear Track Detector (SSNTD) enclosed in a diffusion chamber was developed (Nikezic and Baixeras, 1995; Nikezic et al., 1996; Nikezic and Stevanovic, 2007). In this system, the particulate decay products are filtered out using a suitable filter paper at the entry face, through which gases diffuse easily. However, these systems cannot distinguish 222Rn and 220Rn, which is a major issue in environments with elevated 220Rn concentrations. This necessitates development of a discrimination technique for 222Rn and 220Rn so as to accurately quantify individual gas concentrations using passive detector systems. For discriminating 222Rn and 220Rn measurements, twin chamber device with a diffusion barrier, which cuts off short lived 220Rn but allows 222Rn to pass through it, have been designed. While some 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-115 SSNTD based cylindrical twin cup dosimeterusing membrane for thoron cut-off, developed by Eappen and Mayya (2004), has been widely used for measurement of 222Rn and 220Rn in dwellings. The detector has two entrances (facing opposite to each other), both using glass fibre filters to cut off entries of decay products. In addition, one entrance uses a cellophane membrane to cut-off 220 Rn transmission so that only 222Rn enters into the so called ‘radon’ chamber. The other entrance allows both 222Rn and 220Rn into the so called ‘radon þ thoron’ chamber. A subtraction technique is used to remove the contribution of radon to the tracks in radon þ thoron chamber and obtain thoron concentration. However, it was observed that in some cases the track densities of SSNTD detector placed in ‘radon’ chamber exceeded the track densities of detector placed in ‘radon þ thoron’ chamber, resulting in a physically unacceptable negative 220Rn concentration. One possible reason for this is the different entry rates of 222Rn through two entrances of the dosimeter which may arise from turbulence or air flow in one direction (as in case of one entrance facing a fan and other being opposite to it). This ambiguity of different 222Rn entry in two chambers can be removed by developing a twin chamber device having a single entrance. The conventionally used dosimeter which uses membrane based 222Rn e 220Rn discrimination technique cannot be easily converted to a twin chamber with single entrance. However, it is possible to achieve the required design by replacing the membrane with a pin-hole based discriminating design. Though pinhole based diffusion barrier has been used in some detectors there has been no theoretical basis to decide the pin-hole dimensions for desired 220Rn cut off and 222Rn transmission into the diffusion chamber. In this paper, we discuss the development of a pin-hole based 222 Rn/220Rn discrimination technique, established by modelling 222 Rn and 220Rn diffusion into the pin-hole chamber and validating the model predictions with the experimental observations. Based on this, a new pin-hole based 222Rn/220Rn discriminating device has been designed and developed. This LR-115 track detector based device has a single face for gas entry and gives time integrated measurement of 222Rn and 220Rn in dwellings. The optimal configuration of pin-hole dimensions was decided with the help of model predictions as well experimental measurements in turbulent environmental conditions. In order to minimise the effect of

53

C(t) Fig. 1

C0 Fig. 1. Schematic of a pin-hole chamber system.

turbulence penetration of 222Rn and 220Rn, multiple pin-holes of reasonably small radius (0.5 mm) were used and an arrangement was made for deploying the device in a face-down condition. The device has been calibrated in a laboratory calibration facility at the Bhabha Atomic Research Centre, Mumbai to correlate the number of tracks registered in the LR-115 detector to the 222Rn and 220Rn concentration in the environment. 2. Model development for pin-hole based discriminator

222

Rn e

220

Rn

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

V

vC ðt Þ ¼ JA  lC ðt ÞV vt

(1)

where V is the volume of the chamber, J is the 222Rn/220Rn transmission flux through the pin-hole, A (¼p a2) is the area of the hole and 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 the concentration of the gas between the outside and inside air, by applying the Fick’s law of diffusion as follows:

J ¼ D

Co  CðtÞ d

(2)

Where, C0 is the 222Rn/220Rn gas concentration in the outside air at the 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

vCðtÞ ¼ lp C0  le CðtÞ vt

(3)

where we denote

lp ¼

AD Vd

and

(3a)

54

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60

20

Rn-220 transmission (%)

15

80

Rn-220 Rn-222

60

10 40 5

0

20

0

5

10

15

20

25

30

35

40

45

Rn-222 transmission (%)

100 2

D=0.1 cm /s n =1 2 A = 0.2 cm d =0.2 cm V =100 cc

0 50

222

Rn and

220

Rn transmission into pin-hole chamber.

le ¼ l þ lp

(3b)

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

CðtÞ ¼ C0




lp le

1  ele t



(4)

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


 Cs ¼ C0

lp le

(5)

Using Eq. (4) and Eq. (5), we may now define the following two quantities in order to assess the relative performance of the pinhole chamber for the transmission of 222Rn vis’a vis’ 220Rn. Table 1 Model predicted transmission time and percentage transmission of 222Rn and 220Rn into 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 s1. Hole diameter (mm)

1 2 1 2 3 2 3 4 3 4 4 3 4 3 4 5 3 4 6 8

Hole length (mm)

1 1 2 2 2 3 3 3 4 4 5 1 1 2 2 2 1 1 2 3

Chamber volume (cm3)

150 150 150 150 150 150 150 150 150 150 150 500 500 500 500 500 1000 1000 1000 1000

Transmission time, T95 (min)

Percentage transmission, F (%)

222

222

220

96 99 93 98 99 97 99 96 99 98 98 98 99 97 98 99 97 98 98 99

0.4 1.6 0.2 0.8 1.8 0.6 1.2 0.4 1.6 1.0 0.8 1 2 0.5 1 1.5 0.5 1 1 1

Rn

919 236 1769 468 210 696 105 919 236 376 468 348 197 687 391 252 687 391 348 294

220

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

Rn

Rn

T95 ¼

Rn

3Vd

lVd þ AD

(6)

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

F ¼

tim e (m in) Fig. 2. Transient response of

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

AD AD þ lVd

(7)

Using Eqs. (6) and (7), the transmission time (T95) and transmission factor (F) of 222Rn and 220Rn into the pinhole chamber are estimated for various pinhole radii and lengths. The results are listed in Table 1. It may be noted that Eqs. (6) and (7) will also be valid for multiple pin-holes but in such cases, A will be the sum of areas of individual holes. Hence, the predicted values of the transmission factor (F) and transmission time (T95) of 222Rn and 220Rn into a pinhole chamber will remain same for a given total area of the pinholes irrespective of single or multiple pin-holes (all other parameters remaining the same). For example, T95 and F for a single hole of diameter 1 mm will be the same as that for the case of 4 holes of 0.5 mm diameter each. For designing 222Rn/220Rn discriminating device, a suitable combination of area and length of pinhole 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 comparison with experiments discussed in the following section. 3. Experimental validation For the purpose of experimental validation, few units of cylindrical twin chamber device with two identical chambers were fabricated (see schematic diagram of Fig. 4). Gas enters into the 1st chamber through a filter paper and then into the 2nd chamber through discriminating pin-hole disc which separates the two chambers. The 1st chamber is the reference chamber and the 2nd one is the pin-hole chamber. Each chamber has a length of 4.1 cm and radius of 3.1 cm (same dimensions as in the twin cup dosimeter developed by Eappen and Mayya (2004)). Two LR-115 detector films of size 3 cm  3 cm were fixed at end of the two chambers so as to register the tracks due to alphas emitted from 222Rn, 220Rn gases and their decay products formed in the chamber volume. The ratio of track densities obtained in the detector belonging to pinhole chamber and to that in reference chamber is a measure of transmission factor of 222Rn or 220Rn. Experiment was carried out for pin-hole diameters of 0.5 mm, 1 mm, 2 mm and 3 mm and pinhole lengths of 2 mm and 5 mm. The experimental procedure comprise of controlled exposure of LR-115 detector loaded twin chambers in a calibration chamber, etching of LR-115 films for developing the track, and counting the developed tracks using spark counter. The exposure of twin chamber device to 222Rn/220Rn was carried out in a cubical stainless steel calibration chamber of volume of 0.5 m3 in ‘fan off’ condition as the model is based on diffusion theory. The schematic diagram of the experimental arrangement is shown in Fig. 3. The chamber has arrangement for deploying the

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60

Fan Radon-Thoron measurement device Calibration chamber

Radon source Moisture absorber

RAD 7

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

twin chamber devices to be exposed. Separate experiments were carried out in 222Rn and 220Rn environment to determine their respective transmission factors. Radon (222Rn)/thoron (220Rn) was introduced into the calibration chamber from a standard source. The source used for 222Rn was Model RN -1025 from Pylon Electronics Inc., Ottawa, Canada with activity of 110.6 kBq as on March 1996; whereas source used for 220Rn was Model TH-1025 from Pylon Electronics Inc., Ottawa, Canada with activity of 117.1 kBq as on 8 November 1996. Initially, the fan inside the chamber was kept on for 5 min for uniform mixing and later on it was switched off throughout the exposure period. The exposure period was decided to be at least one day for 222Rn and 30 min for 220Rn on the basis of model predicted value of T95 (Eq. (6)). After the completion of exposure, the LR-115 films were removed from the twin chamber device and were chemically etched using 2.5N NaOH solutions at a temperature of 60  C for 90 min without stirring (Eappen and Mayya, 2004). The counting of etched tracks in LR-115 film was carried out using an automated spark counting technique. Each film was pre-sparked at 900 V prior to counting and then counted at the operating sparking voltage of 500 V. The background tracks in LR-115 detector were also measured by following the same etching and counting procedure using the unexposed LR-115 detector. The experimental transmission percentage (Fexp) was then calculated using the following formula

Fexp ¼

Tp  B  100 Tr  B

(8a)

Where Tp and Tr are the total track densities in LR-115 detectors placed in pinhole chamber and reference chamber respectively, B is the average background track density in unexposed LR-115 detectors which was found to be 4  2 tr. cm2 for control samples from the same batch of LR-115 films. The uncertainty (sF ) associated with each experimentally determined transmission factor was derived by applying error propagation formula (Bevington and Robinson, 2003) to Eq. (8 a). The overall uncertainty was estimated using the expression:

sF ¼

100 ðTr  BÞ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2 ðTr  BÞ2 s2p þ Tp  B s2r þ Tp  Tr s2B 2 (8b)

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

55

chamber respectively, sB is the uncertainty associated with average background track density in unexposed LR-115 detectors from the same batch of LR-115 films (taken as 2 tr.cm2) The results of transmission factors for 222Rn and 220Rn are presented in Table 2 and Table 3 respectively. The results for 222Rn transmission factors obtained from experiments were compared with values obtained from Model (Eq. (7)) wherein the diffusion coefficient of 222Rn in air was taken as 0.1 cm2 s1 (Rogers and Neilson, 1991). A fairly good matching between the experimentally estimated and theoretically predicted transfer factors for 222Rn is evident from Table 2. In case of 220Rn, the observed tracks densities in pinhole chamber were found to be very close to the background track density (4  2tr. cm-2) suggesting nearly complete cut off of 220Rn into the pinhole chamber. This is in accordance with the model prediction. Since the estimated uncertainty associated with the 220Rn transmission is very high, comparisons have not been made for the case of 220Rn. However, the experimental result of 220Rn reveals that the 220Rn transmission into a pinhole chamber can be cut off by selecting suitable dimension of the pinhole. 4. Effect of turbulence & its minimisation The model described in section 2 is valid for transmission through pinhole by molecular diffusion, which is possible only in a stable environment. However, in most dwellings some amount of air turbulence is always expected. Hence it is necessary to study the effect of turbulence on transmission of 222Rn and 220Rn through pinhole for optimising the design. The effect of turbulence for the case of a combination of multiple pin-holes as compared to a single pinhole of an equivalent area also needs to be investigated. For this purpose, transmission through four different configurations of pinholes with varying dimensions and number of pinholes, but with similar model-predicted transmission factors, was studied. The devices were exposed in the calibration chamber with the fan inside the chamber running on throughout the exposure period. The turbulence velocity in this condition was estimated to be 6.3 cm s1 using the formula given by Mishra et al. (2009), which is representative of the condition for extreme turbulence in dwellings. The experimental procedure was similar to that described in section 3. The results of the study for the four different configurations are given in Table 4 for the case of 222Rn and in Table 5 for the case of 220Rn. It may be observed that in case of 222Rn, the values of transmission factors for all combinations were almost same as that predicted by model. But in case of 220Rn, the values of transmission factors were 2e5 times higher than the model predicted values. It was also observed that for equivalent area of pinholes, the values of transmission factor for the cases of multiple pin-holes were closer to the model-predicted value than that for single pinhole. This indicates that in presence of turbulence, thoron cut-off will be better

Table 2 22 Results of 2Rn transmission into the pin-hole chamber through a pin-hole of various dimensions in “Fan off” condition.

Hole dia (mm)

0.5 1 2 3 2

Hole length (mm)

No of sets

5 5 5 5 2

3 3 3 3 3

222 Rn transmission percentage FRn (%)

Track density (tr. cm-2) Pin-hole chamber, Tp 156.7 258.3 329 340 295

    

8.5 5.1 6.2 23.9 8.7

Reference chamber, Tr 283.7 329.7 341.7 353.3 305

    

4.5 4.7 4.2 29 8.9

Experiment 55 78 96 96 97

    

3 2 2 10 4

Model (Eq. (7)) 60 86 96 98 98

56 Table 3 Results of

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60

220

Rn 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) Pin-hole chamber, Tp

0.5 1 2 3 2

5 5 5 5 2

3 3 3 3 3

5 5.3 5 5.3 4

    

1 1.5 1.7 0.7 1

in case of multiple holes of reasonably small radius than with single pinhole of equivalent area. 5. Development of the pin-hole based discriminating measurement device

222

Rn/220Rn

An ideal device for 222Rn/220Rn measurement would be the one for which the calibration factor will be independent of indoor environmental conditions such as air turbulence. High turbulence will lead to a significant penetration of 220Rn into the radon chamber causing a lot of interference in 222Rn calibration factor. The present 222 Rn/220Rn discrimination design based on multiple pinholes of reasonably small radius is thus an ideal choice. From Tables 4 and 5, it may be noted that the configuration with 4 pin-holes, each having 1 mm diameter and 2 mm length, is best suited as it has 220Rn cut off of about 98% (Table 5) with 222Rn transmission of about 97% (Table 4). At the same time, turbulence penetration of 220Rn at a turbulence level of 6.3 cm s-1 (an extreme case of high turbulence indoors) was found to be negligible and nearly same as that found in the case of ‘fan off’ condition (nearly stable environment). The schematic diagram and photograph of the device based on this configuration is shown in Fig. 4. The device has two identical cylindrical chambers of length of 4.1 cm and radius 3.1 cm (same dimension as in the case of conventional twin cup dosimeter) which are separated by the pinhole based 222Rn/220Rn discriminating plate. The gas enters into the first “radon þ thoron” chamber through the glass fibre filter paper which filters out decay products and subsequently diffuses to the second “radon” chamber through four pin-holes which cut off 220 Rn. The LR-115 film kept in the ‘radon þ thoron’ chamber registers the alpha tracks due to both 222Rn and 220Rn and their decay products. The LR-115 film kept in the ‘radon’ chamber registers the alpha tracks only due to 222Rn and its decay products. The present device has many salient features. Firstly, it eliminates the requirement of a cellophane membrane as discriminator used in the conventional twin cup dosimeter. This reduces the uncertainty due to change in thoron cut-off with different batches of membranes. Another important feature is that it has single face for gas entry which rules out uncertainty arising due to unequal entry of 222Rn gas as in the case of conventional twin chamber device (discussed in detail in section 7). Besides, use of multiple pin-holes in the discriminator, and placement of the discriminator after the first chamber (and hence at some distance from the entrance) minimises the effect of turbulence on thoron cut-off. Table 4 Results of

222

Reference chamber, Tr 137 134.7 157.3 156.7 203

    

5 3.5 4.5 6.4 5.6

220 Rn transmission percentage, FTn (%) obtained from experiment

0.8 1.0 0.7 0.9 0.0

    

2 2 2 1 1

Both chambers are internally coated with metallic powder (nickel) to have zero electric field inside the chamber. This ensures that the deposition of decay products formed from 222Rn and 220Rn gases 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 is now required to calibrate the device and obtain the calibration factors for 222Rn and 220Rn measurements. The calibration factor is basically converts the observed track densities to the activity concentration of 222Rn and 220Rn in the air. If T is the background corrected track density observed on a SSNTD due to exposure in a given 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. cm2 d-1/(Bq m3)) t is the time in days C is the concentration in Bq m3 T is the track density in tracks cm2 The calibration factors for 222Rn and 220Rn for the new twin chamber device may be obtained using following Eq. (10)(a,b) and 11(a,b) respectively as follows:

kR;1 ¼

TR;1  B tCR

(10a)

kR;2 ¼

TR;2  B tCR

(10b)

kT;1 ¼

TT;1  B tCT

(11a)

kT;2 ¼

TT;2  B tCT

(11b)

Where, kR,1, kR,2, kT,1 and kT,2 are the calibration factors (tr. cm2 d1/ (Bq m3)) 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

Rn transmission into the pinhole chamber 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 sets

3.14

2

12.56

5

1 2 2 4

4 1 4 1

3 3 3 3

Radon transmission percentage (%) Experiment 97.3 97.1 97.8 96.9

   

1.45 1.73 1.69 2.12

Model (Eq. (7)) 98.4 98.9

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60 Table 5 Results of

220

57

Rn 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

3.14

2

12.56

5

1 2 2 4

4 1 4 1

total track densities (tr. cm2) in LR-115 detectors for the ‘radon þ thoron’ and ‘radon’ chamber for 222Rn and 220Rn exposure respectively. B is the background track density in unexposed LR-115 detector, measured as 4  2 tr. cm2. CR and CT are the average concentrations (Bq m3) of 222Rn and 220Rn at the entry face respectively, and t is the exposure period (d). Experiments were performed in a calibration chamber for the determination of calibration factors for both 222Rn and 220Rn. Calibration chambers of different types and sizes are being used the world over for the standardisation of SSNTDs and associated passive detectors (Azimi-Garakani, 1992). The chamber used in the present study was a stainless steel cubical chamber of volume 0.5 m3 222Rn gas was introduced into the calibration chamber from a Pylon-make standard 222Rn source (Model RN -1025, Source activity of 110.6 kBq as on March 1996). A fan was placed inside the chamber for mixing and achieving spatial uniformity in the 222Rn concentration. Gas samples were collected through outlets provided on the chamber. RAD 7, a continuous 222Rn monitor was used for the measurement of 222Rn. The measurement devices were vertically hung to the arms of the central rod provided in the chamber. Initially, the fan inside the chamber was kept on for 5 min for uniform mixing and later, it was switched off throughout the exposure period. The experimental set up used for the determination of 222Rn calibration factor is shown in Fig. 3. The devices were exposed to different 222Rn concentrations varying between 2 kBq m3 and 34 kBq m3. The exposure periods

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

222

3 3 3 3

1.8 4.8 2.2 6.1

   

0.7 1.7 0.6 1.4

Predicted from model (Eq. (7)) 1 1.6

were varied from 14 h to 168 h depending upon the radon concentration level such that a reasonable number of tracks will be formed in the detectors. After the completion of each exposure, the SSNTD films (LR-115) were taken out of the device and etched in 2.5 (N) NaOH solutions using standard etching procedure as discussed earlier (Sec. 3). After etching, these films were thoroughly washed with distilled water and kept for drying. The clear polyester base was then peeled off and the films were subjected to spark counting to get the total track density. The actual track density was then calculated by subtracting the background tracks in unexposed LR115 detector. The 222Rn calibration factor in the two chambers of the device was determined using Eq. (10 (a, b)). The uncertainty associated with each 222Rn calibration factor was derived by applying error propagation formula (Bevington and Robinson, 2003) to Eq. (10 (a,b)) in a similar way as done for Eq. (8 a). The overall uncertainty was then estimated using uncertainties in measured 222Rn concentration, total and background track densities. For estimating the 220Rn calibration factor of the device, the above mentioned procedure was not suitable since 220Rn being short 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 Poly Ethylene (HDPE) material. One side of the chamber was kept open

Rne220Rn measurement device and its photograph showing the deployment orientation.

58

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60

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

220

Rn cali-

such that the entry side of the measurement device made an air-tight coupling with this opening. Inside this cylindrical chamber, an arrangement was made to release 220Rn gas at the bottom 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 to have reasonable mixing. Sampling provision was made close to the entry face of the device to measure the representative 220Rn concentration by RAD7, a continuous radon monitor. The calibration set up used for the 220Rn calibration is shown in Fig. 5 The 220Rn exposure experiment was carried out for a period of 1 h. This period is sufficient enough to get a reasonable track density in detectors as the 220Rn concentrations were in the range of 310e446 kBq m3. After the completion of exposure, the LR-115 detectors were etched and tracks were counted by spark counter in a manner as discussed earlier (Sec. 3.1). The actual track density was then calculated by correcting for the background tracks and the 220 Rn calibration factors in both chambers of the device were determined using Eq. (11(a,b)). The uncertainty associated with each 220Rn calibration factor was derived by applying error propagation formula (Bevington and Robinson, 2003) to Eq. (11 (a,b)) in a similar way as done for Eq. (8 a). The overall uncertainty was then estimated using uncertainties in measured 220Rn concentration, total and background track densities. The calibration factors obtained for 222Rn and 220Rn in both chambers are tabulated in Table 6 and Table 7 respectively. The 222 Rn calibration factor in ‘radon þ thoron’ chamber was found to be 0.0172 tr.cm2 d1/(Bq m3) and in the ‘radon’ chamber it was found to be 0.0170 tr.cm2 d1/(Bq m3). Similarly the 220Rn calibration factor in ‘radon þ thoron’ chamber is 0.010 tr.cm2 d1/ (Bq m3) and in ‘radon’ chamber, the value is 0.00052 tr.cm-2 d1/ (Bq m3) indicating more than 98% of 220Rn cut-off takes place. The 220 Rn calibration factor is less due to its short diffusion length in air and nearly 44% of initial 220Rn decays out before entering to the 1st chamber of the device (Eappen et al., 2008). It may be noted that

Table 6 222 Rn calibration factors of the present Sr. no

3

CR (kBq m

1 1.93 2 10.2 3 15.8 4 17.3 5 27.4 6 32 7 34.2 Average  SD

      

0.4 1.1 1.2 1.2 1.8 2.3 2.3

)

222

the typical value of 222Rn and 220Rn concentrations in dwellings are in the range from 10 to 40 Bq m3 (UNSCEAR, 2000). Since typical background tracks observed in LR-115 detector is about 4 tr.cm2, it is preferred to deploy the measurement device in the field for a period of at least 3e4 months depending upon the concentration level so as to get track densities 3 times above the background in the exposed LR-115 film. It may be remarked that, the background track density in other film detectors such as CR-39 is more as compared to LR-115 detector. Hence the use of LR-115 detector in this measurement device provides another advantage for detecting low level of 222Rn and 220Rn concentration in indoor environment. It may be noted that the calibration factors obtained are based on a given protocol. They may change depending upon the experimental conditions such as operational characteristic of the spark counter or etching conditions. It would be appropriate to establish a-priori relative factors for changed measurement conditions through inter-comparison exercises before using these calibration factors. 7. Performance evaluation of the present device against conventional twin cup dosimeter The new device is developed aiming to remove the ambiguity of obtaining negative 220Rn concentration due to non-uniform entry of 222Rn into the two compartments which normally happens in presence of turbulence. To evaluate its performance in such conditions, a comparison study was made by exposing the new device and the conventional twin cup dosimeter to 222Rn in a test chamber under “fan on” condition, a case of high turbulence. The two devices were deployed side by side at five different positions in the chamber. As per the deployment protocol, all new devices were deployed vertically with entry face down, while the conventional devices were deployed in horizontal manner. Since conventional device had two entrances viz. ‘membrane face’ and ‘filter face’, their respective positions with respect to position of the fan were noted down. Three out of five such devices had ‘membrane face’ (entrance to “radon” chamber) directed towards the fan, while the remaining two devices had ‘membrane face’ directed opposite to the fan. The fan was kept on throughout the exposure period. The experimental procedure for the exposure and processing of the exposed LR-115 detectors was as described in Section 3. The remarks about the entrance position of devices with respect to position of fan, total track densities obtained for the detectors belonging to ‘radon’ and ‘radon þ thoron’ chambers and their differences are tabulated in Table 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 is negative in some cases (3) and positive in some other cases (2) which may arise due to non-uniform entry of 222Rn into the respective chambers of the device through its two opposite faces. Negative results occur when the membrane face of the device was

Rne220Rn measurement device obtained from experiments.

Exposure period t (h)

No. of sets

TR,1 (tr.cm2)

168 50 67.2 54 28 14 45

2 2 2 2 2 2 2

213 346 926 596 605 323 1101

      

9 8 8 11 31 13 16

TR,2 (tr.cm2) 211 337 924 594 604 311 1081

      

8 4 4 7 10 14 11

kR,1 (tr.cm2 d1/(Bq m3)) 0.0155 0.0161 0.0208 0.0152 0.0188 0.0171 0.0171 0.0172

       

0.0033 0.0018 0.0016 0.0011 0.0016 0.0014 0.0012 0.002

kR,2 (tr.cm2 d1/(Bq m3)) 0.0153 0.0157 0.0208 0.0152 0.0188 0.0164 0.0168 0.0170

       

0.0032 0.0017 0.0016 0.0011 0.0013 0.0014 0.0011 0.002

B.K. Sahoo et al. / Radiation Measurements 58 (2013) 52e60 Table 7 220 Rn calibration factors of the present Sr. no

3

CT (kBq m

1 310 2 319 3 340 4 350 5 353 6 360 7 360 8 368 9 376 10 446 Average  SD

         

)

7 8 8 8 8 9 9 9 9 10

222

Rne220Rn measurement device obtained from experiments.

Exposure period, t (h)

TT,1 (tr.cm2)

1 1 1 1 1 1 1 1 1 1

117 137 123 137 185 147 151 177 169 178

         

TT,2 (tr.cm2)

5 4 3 6 4 5 6 7 7 5

7 10 12 11 17 13 10 15 9 15

         

1 2 3 4 5 Average SD

kT,1 (tr.cm2 d1/(Bq m3))

2 3 2 2 3 3 2 2 2 3

Table 8 Performance comparison between present device and the conventional twin cup dosimeter for Position no.

59

0.009 0.010 0.008 0.009 0.012 0.010 0.010 0.011 0.011 0.009 0.010

222

          

Observed track densities (tr.cm2)

Present device

Twin cup dosimeter

Facing Facing Facing Facing Facing

downwards downwards downwards downwards downwards

Membrane Membrane Membrane Membrane Membrane

face face face face face

towards fan towards fan towards fan opposite to fan opposite to fan

0.00023 0.00045 0.00056 0.00048 0.00088 0.00060 0.00040 0.00072 0.00032 0.00059 0.00052

          

0.00022 0.00027 0.00020 0.00019 0.00025 0.00024) 0.00019 0.00019 0.00018 0.00019 0.00019

Rn exposure in “fan on” condition.

Entrance position of devices with respect to position of the fan Twin cup dosimeter

0.0005 0.0004 0.0003 0.0005 0.0004 0.0004 0.0005 0.0005 0.0005 0.0004 0.001

kT,2 (tr.cm2 d1/(Bq m3))

Present device

‘Radon þ thoron’ chamber

‘Radon’ chamber

Difference

‘Radon þ thoron’ chamber

‘Radon’ chamber

Difference

624 625 629 678 682 648 30

684 671 678 623 634 658 28

60 46 49 55 48 10 57

648 654 651 640 644 647 6

635 638 624 627 633 631 6

13 16 27 13 11 16 6

directed towards the fan while positive result occurs in the reverse situations. In case of negative results, the mean concentration in the ‘radon’ chamber may be higher than that in ‘radon þ thoron’ chamber due to higher transmission rate of 222Rn into the former one 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 to direction of flow due to fan), the pressure driven entry into this chamber 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 reverse situation where in the mean concentration in the ‘radon þ thoron’ chamber will be higher than that in ‘radon’ chamber leading to positive results. (ii) The track densities for ‘radon þ thoron’ and ‘radon’ chambers of the new pinhole based devices (5) are nearly equal, indicating a nearly uniform entry of 222Rn into the two chambers of the device. The small difference between the track densities may be attributed to small difference (w3%) in mean 222Rn concentration between the two chambers which occurs due to decay of small fraction of 222Rn while diffusing from ‘radon þ thoron’ to ‘radon’ chamber through pin-holes. It may be noticed that the track density difference between the two chambers, though marginally small, remains positive, indicating that possibility of getting negative 220 Rn concentration is ruled out in the case of new device.

8. Conclusion A new pinhole based 222Rn/220Rn measurement device has been developed to replace the conventional membrane based twin cup dosimeter being used in India and it has several significant improvements over the latter. The major improvements are: (i) the pin-holes based 222Rn/220Rn discrimination design eliminates the requirement of cellophane membrane filter (ii) the single entrance design for gas transmission rules out the possibility of encountering

negative 220Rn concentration, and (iii) use of multiple pin-holes of reasonably small radius and internal positioning of discriminator minimises the effect of turbulence on 222Rn/220Rn transmission factors to a great extent and makes calibration factors of the device independent of indoor turbulent conditions. With these salient features, it is hoped that the new pinhole based twin chamber device will go long way as an ideal measurement device for mapping 222Rn and 220Rn concentration in indoor environments. Acknowledgements Authors would like to thank Shri DAR Babu, Head, Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai for his constant encouragement towards this work. The help provided by Shri Manoranjan Dash, Atomic Energy Regulatory Board, Mumbai during experiments is kindly acknowledged. References Andriamanatena, R., Enge, W., 1995. Practical and theoretical calibration of Rn-sensitivities for plastic nuclear track detectors. Radiat. Meas. 25 (1e4), 625e 626. Azimi-Garakani, D., 1992. A comparison of different radon chambers. Nucl. Instrum. Methods Phys. Res. B 71, 99e102. Bevington, P.R., Robinson, D.K., 2003. Data Reduction and Error Analysis for the Physical Sciences. McGraw-Hill, New York. Darby, S., Hill, D., Auvinen, A., Barros-Dios, J.M., Baysson, H., Bochicchio, F., Deo, H., Falk, R., Forastiere, F., Hakama, M., Heid, I., Kreienbrock, L., Kreuzer, M., Lagarde, F., Mäkeläinen, I., Muirhead, C., Oberaigner, W., Pershagen, G., RuanoRavina, A., Ruosteenoja, E., Schaffrath Rosario, A., Tirmarche, M., Tomásek, L., Whitley, E., Wichmann, H.E., Doll, R., 2005. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. BMJ 330 (7485), 223e226. Doi, M., Kobayashi, S., 1994. The passive radonethoron discriminative dosimeter for practical use. Hoken Butsuri. 29, 155e156. Dudney, C.S., Hawthorne, A.R., Wallace, R.G., Reed, R.P., 1990. Radon-222, 222Rn progeny, and 220Rn progeny levels in 70 houses. Health Phys. 58, 297e311. Eappen, K.P., Mayya, Y.S., 2004. Calibration factors for LR-115 (Type-II) based radon thoron discriminating dosimeter. Radiat. Meas. 38, 5e17. Eappen, K.P., Sahoo, B.K., Ramachandran, T.V., Mayya, Y.S., 2008. Calibration factor for thoron estimation in cup dosimeter. Radiat. Meas. 43, S418eS421.

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