ASSESSEMENT OF INDOOR RADON, THORON AND THEIR PROGENY IN DWELLINGS OF BAREILLY CITY OF NORTHERN INDIA USING TRACK ETCH DETECTORS

ASSESSEMENT OF INDOOR RADON, THORON AND THEIR PROGENY IN DWELLINGS OF BAREILLY CITY OF NORTHERN INDIA USING TRACK ETCH DETECTORS DEEPAK VERMA *, M. SHAKIR KHAN Department of Applied Physics, Z. H. College of Engineering & Technology, Aligarh Muslim University, Aligarh-202002, Uttar Pradesh, India *Corresponding author e-mail: dpkapd@gmail.com Received June 11, 2013 The levels of radon, thoron and their progeny in the dwellings of Northern India have been determined by using twin chamber dosimeter cups. The concentrations of radon and thoron were found to vary from 16.62 155.12 Bq/m 3 and 4.16 24.93 Bq/m 3. Their progeny concentrations were found to vary from 1.80 16.77 mwl and 0.112 0.674 mwl. The values of life time fatality risk and annual effective dose were found to vary from 0.24x10-4 1.21 x10-4 and 0.31 2.78 msv/y, respectively. The variation in the above parameters strongly influenced by the type of dwelling construction and their ventilation conditions. Key words: Radon, Dosimeter cup, LR-115 detector, Action level. 1. INTRODUCTION The exposure of population to natural sources of radiation has become an important issue in terms of radiological protection. Mostly natural radiation comes from radon ( 222 Rn), thoron ( 220 Rn) and their solid short-lived daughter products. There is a concern about possible links between emissions of radon, and certain types of malignant diseases have led to surveys to measure radon concentrations inside the dwellings. Radon and thoron originates from uranium ( 238 U) and thorium ( 232 Th) respectively, which is present in the earth s crust in varying concentrations. The radon potential for a given region is likely to be the result of a combination of properties of the underlying rocks and soils, such as the distribution of uranium and radium, porosity, permeability, and moisture content, as well as meteorological and seasonal variations. The diffusion length of radon, i.e., the average distance an atom can move through dry soil before decaying, is about 1.6 m, while it is only 2 cm for thoron because of relatively short half-life, i.e., 55.6 s. This property of radon makes it a greater health hazardous gas [1]. Rom. Journ. Phys., Vol. 59, Nos. 1 2, P. 172 182, Bucharest, 2014

2 Study of radon, thoron and their progeny in some dwellings 173 The air pressure inside a dwelling is usually lower than the air pressure in soil beneath it because of the effects of wind and temperature. The radon gas, arising from soil and rocks, seeps through the foundations, basements, piping of buildings, which can accumulate in the air dwellings that are poorly ventilated. The radon has less probability of decay in the lungs, because of its half life of 3.82 days. Therefore, it may be exhaled in the process of respiration before undergoing a radioactive decay. The short lived decay product of radon such as 218 Po, 214 Pb, 214 Bi and 214 Po are relatively short lived radioactive heavy metals. These daughters, when drawn into the respiratory tract usually remain stuck in the mucus lining of the tract and may be lodged in the lungs [2]. In the process of decaying they deposit large amount of energy in the surrounding tissues results in severe type of biological damage, which may ultimately cause lung cancer. In comparison to radon, there is no much information known about the levels and risks associated with thoron present in indoors. As half-lives of thoron progenies (i.e. 212 Pb 10.6 h and 212 Bi 60.6 min) are comparable with biological elimination rate. The inhalation radiation dose into lung is relatively smaller than that from radon progeny. Radiation exposure due to inhalation of thoron progeny is estimated to be 10± 20% as compared with short-lived radon progeny [3]. The measurements of radon, thoron and their daughters in the dwellings are important to determine whether or not the particular living place needs to have the action against possible higher levels of radon and its daughters. With this objective, we have measured radon, thoron and their progeny levels inside the dwellings of Bareilly city of Northern India (figure 1). 2. GEOLOGY OF STUDY AREA Bareilly is situated on the bank of river Ramganga and there are other seven rivers passing through this district. The lower Himalayan range is about 40 km away from the study area. According to 2011census, the population of Bareilly was 8, 98,167. Study area is located at 28.10 N latitude and 78.23 E longitude having an elevation of 166 m. The city lies entirely in the Ganga plains, which provide fertile alluvial soil that is suitable for agriculture. The city has a semi-arid climate with high variation between summer and winter temperature. Summers are long, from early April to October, with the monsoon season in between. Winter starts in October and peaks in January and is notorious for its heavy fog. The average annual rainfall in this area is about 1200 1500 mm (47 59 inch), most of which is during the monsoon period in July and August. Geographically, it forms the outer gateway to enter in the Uttarakhand State. Bareilly is a centre for manufacturing of furniture and for trade in cotton, cereal and sugar.

174 Deepak Verma, M. Shakir Khan 3 Fig. 1 Location of study area. 3. MATERIALS & METHODS In this experiment, twin chamber dosimeter cups loaded with solid state nuclear track detectors (SSNTDs) have been used in order to assess the levels of radon, thoron and their daughters inside the dwellings of Bareilly city of Uttar Pradesh, India. These plastic dosimeters were developed and calibrated at Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai [4]. The SSNTDs used were 12 mm thick LR-115 type-ii strippable cellulose nitrate films, supported on a transparent 100 mm thick polyester (cellulose acetate) sheet, made by Kodak Pathe, France. Each cup has two chambers namely filter mode and membrane mode of equal volumes of a height 4.5 cm and a radius of 3.1 cm with the inner volume of 124 cm 3 as shown in figure 2. To achieve the maximum track registration for dosimeter cup, their dimensions are based on ratio of the effective volume of the cup to its total volume. With that dosimeter cup we can measure the concentration of radon as well as thoron and their progeny, all simultaneously at a given location. The mouth of first chamber i.e. filter mode being covered with glass fibre filter paper and the second one i.e. membrane mode with a semi-permeable membrane (polyethylene terephthalate, having permeability

4 Study of radon, thoron and their progeny in some dwellings 175 Fig. 2 Schematic diagram and actual photograph (a & b) of twin cup dosimeter. constant of 10-8 10-7 cm -2 s -1 for radon gas) [19] sandwiched between two glass fibre filter papers. These cups also have provision for exposing the detector in bare mode on the outer surface of the cup. The detectors were affixed inside bottom of each chamber. The detector fixed in membrane mode records the tracks of alpha particles emitted from radon ( 222 Rn) only, as the membrane allow more than 95% of radon gas to diffuse through it and due to short half life and diffusion properties suppress thoron gas to less than 1%, thus providing radon/thoron discrimination [4]. The radon gas inside the cup reached to a steady state condition within a time period of 4 to 5 hr. In filter mode, the glass fibre filter paper allow diffusing both radon and thoron gases, therefore detector in this mode records the tracks of alpha particles emitted from both radon and thoron and their alpha emitting progeny, namely 218 Po, 214 Po, 216 Po and 212 Po. In bare mode the track registered on detector may affect by some parameters like attachment of aerosol, deposition and recoil of radon, thoron and their progeny from aerosol and surfaces, and decay. It is assumed that the detectors in bare mode respond to the airborne alpha emitters and

176 Deepak Verma, M. Shakir Khan 5 not to the activity deposited on it [20]. These cups were exposed in 20 dwellings at a height of about 2.5 m from the ground, considering least disturbance to the occupants. The dosimeters hanged in such a way that the active surface of detector used in bare mode exposure is kept at a minimum distance of 10 cm away from any surface in order to avoid the tracks due to attenuated alpha particles reaching from these surfaces. The exposure was completed in each dwelling within a period of about three months. After exposure, the detectors were removed from the dosimeters and etched in 2.5 N NaOH solution at 60 ºC for 1.5 h in a constant temperature water bath to reveal the tracks. The tracks recorded on LR-115 detectors were counted using an optical microscope at a magnification of 100X. Background track density of an unexposed detector has also been counted. The corrected track density was then obtained by subtracting the number of tracks on an unexposed detector from the exposed detector. The corrected tracks were converted to radon and thoron gas concentrations using the following equations [4] C =ρ K d (1) 3 R(Bq/m ) m/ m. ( ) C =ρ dc K K d (2) 3 T(Bq/m ) f. R rf / tf. Where C R and C T are the concentration of radon and thoron (in Bq/m 3 ) respectively, ρ m is the track density of detector in membrane mode (in tracks/cm 2 ), K m is the calibration factor of radon in membrane mode (in tracks.cm -2 d -1 /Bq.m -3 ), d is the time of exposure (in days), ρ f is the track density of detector in filter mode (in tracks/cm 2 ), K rf is the calibration factor of radon in filter mode (in tracks.cm -2 d -1 / Bq.m -3 ) and K tf is the calibration factor for thoron in filter mode (in tracks.cm -2 d -1 / Bq.m -3 ). The numeric values of these factors [4] are given below: K rm = 0.019 ± 0.003 tracks.cm -2 d -1 /Bq.m -3 K rf = 0.020 ± 0.004 tracks.cm -2 d -1 /Bq.m -3 K tf = 0.016 ± 0.005 tracks.cm -2 d -1 /Bq.m -3 The radon and thoron progeny levels or PAEC values (in mwl) were calculated by using the equations [5, 6] ( ) ( ) PAEC mwl = ( C F ) / 3.7 (3) R R PAEC mwl = ( C F ) / 3.7 (4) Where F R and F T are equilibrium factors for radon and thoron having the value 0.4 and 0.1, respectively [7]. The values of life time fatality risk were obtained by using a factor of 3 x 10-4 WLM -1 [8]. A conversion factor of 3.9 msv/wlm and 3.4 msv/wlm for radon [5] and thoron [9] progeny, respectively was used to calculate the annual effective dose. T T

6 Study of radon, thoron and their progeny in some dwellings 177 4. RESULTS & DISCUSSION The measured values of radon, thoron and their progeny concentration, life time fatality risk and annual effective dose in different dwellings of study area are tabulated in table 1. These measurements were carried out in winter season (December to February, 2011). The indoor radon and thoron levels in this area are found to vary from 16.62 155.12 Bq/m 3 and 4.16 24.93 Bq/m 3 with an average value of 87.23 Bq/m 3 and 13.63 Bq/m 3, respectively. The corresponding standard deviations are 47.60 and 7.83. The average value of radon concentration (87.23 Bq/m 3 ) is higher than the average value of 40 Bq/m 3, reported for the dwellings worldwide [10]. This may be due to the difference in the concentration of radioactive elements, viz. uranium and radium in the soil and building materials of the study area. However, most of the dwellings have the radon concentration below the level of concern i.e. 150 Bq/m 3 while none of them have a value higher than the action level 200 600 Bq/m 3, recommended by ICRP [5]. The radium content in the soil of Bareilly city lies within the range of 11 40 Bq/kg, which is much below than the Organisation for Economic Co-operation and Development (OECD) recommended value of 370 Bq/kg. As radon is the daughter product of radium therefore low level of radium in soil resulting low level of indoor radon and it is very well reflected in our present study. Table 2 shows the values of radon concentration from different parts of India. We have also compared our indoor radon results with the other countries (table 3). In present study the radon/thoron (C R /C T ) ratio was found to vary from 3.0 to 9.33 with an average of 6.53. This ratio find good agreement with the ratio (5.1 to 9.0) reported for the dwellings of Brahmaputra valley of Assam [12]. However some other parts of India like Shilong, Karimganj, Aizwal and Agartala have the ratio between 2.0 to 5.0 (approx.) [13]. The annual effective dose received by the residents of studied areas varies from 0.31 to 2.78 msv/y with an average of 1.56 msv/y and a standard deviation of 0.85. In all the dwellings surveyed, the maximum annual effective dose (2.78 msv/y) is found less than the lower limit of the action level (3 msv/y) recommended by International Commission on Radiological Protection (ICRP). The lifetime fatality risk of the residents of the studied area varies from 0.24 x10-4 to 1.21 x10-4 with an average of 2.15 x10-4 and a standard deviation of 0.66. It is clear from table 1 that the values of radon concentration are higher than thoron. It may be due to the difference in half life of radon and thoron, which affects the exhalation rate from the wall and the concentration distribution inside the dwellings. The other reason may be that the dosimeter was generally placed at a height of about 2.5 meter from the ground; any effect of floor/subsurface soil on thoron concentration could not be detected. Hence in that condition the main sources of indoor thoron are only the walls of dwellings [11]. The difference in the values of radon concentration may be due to the wide variation in ventilation conditions, type of construction and other factors such as temperature, humidity etc. In the study area, most of the dwellings having brick walls, concrete roofs and cemented floors.

178 Deepak Verma, M. Shakir Khan 7 S. No. Types of floor Table 1 Values of radon, thoron and their progeny (PAEC) concentration, life time fatality risk and effective dose Radon conc. (C R ) (Bq/m 3 ) Thoron conc. (C T ) (Bq/m 3 ) Ratio (C R /C T ) Radon progeny conc. (PAEC) (mwl)) Thoron progeny conc. (PAEC) (mwl) Life time fatality risk (x10-4 ) Effective dose (msv/y) 1 cement 55.40 9.70 5.71 5.98 0.440 0.79 1.02 2 granite 105.26 13.16 8.00 11.38 0.356 1.45 1.88 3 cement 27.70 4.85 5.71 2.99 0.122 0.33 0.50 4 cement 38.78 4.16 9.32 4.19 0.112 0.53 0.69 5 granite 127.42 24.93 5.11 13.77 0.674 1.78 2.31 6 granite 132.56 24.59 5.39 14.37 0.664 1.86 2.40 7 cement 77.56 8.31 9.33 8.38 0.224 1.11 1.38 8 granite 144.04 23.90 6.02 15.14 0.646 1.95 2.52 9 cement 16.62 5.54 3.00 1.80 0.150 0.24 0.31 10 cement 22.16 5.19 4.27 2.39 0.140 0.31 0.40 11 granite 116.34 12.47 9.33 12.58 0.337 1.60 2.07 12 granite 149.58 23.55 6.35 16.17 0.636 2.08 2.68 13 granite 121.88 18.70 6.52 13.18 0.505 1.69 2.19 14 cement 60.94 9.35 6.52 6.59 0.253 0.84 1.09 15 cement 33.24 4.50 7.39 3.59 0.122 0.46 0.59 16 granite 155.12 23.20 6.69 16.77 0.627 2.15 2.78 17 granite 110.80 12.81 8.65 11.97 0.346 1.52 1.97 18 cement 44.32 10.38 4.27 4.79 0.280 0.62 0.81 19 granite 138.50 24.24 5.71 14.97 0.655 1.93 2.50 20 cement 66.48 9.07 7.33 7.19 0.245 0.92 1.19 Min. 16.62 4.16 3.00 1.80 0.112 0.24 0.31 Max. 155.12 24.93 9.33 16.77 0.674 1.21 2.78 Av. 87.23 13.63 6.53 9.41 0.377 2.15 1.56 S. D. 47.60 7.83 5.12 0.211 0.66 0.85 Table 2 Comparison of indoor radon levels with different parts of India S. Indoor Radon Area No. of References No. level (Bq/ m 3 ) measurements 1 12-104 Faizabad (U.P.) 20 [11] 2 12-190 Jodhpur ( Rajasthan) 20 [14] 3 15-218 Amritsar (Punjab) 32 [15] 4 13-143 All India - [16] 5 66-104 Northern Haryana 80 [17] 6 36-140 Hamirpur and Una (Himachal 22 [18] Pradesh) 7 18-62 Jaipur(Rajasthan) 10 [20] 8 17-155 Bareilly (U.P.) 20 Present study

8 Study of radon, thoron and their progeny in some dwellings 179 However, there were some differences i.e., a) some houses have plastered and painted walls, while others have walls without plaster and paint, b) some houses have floor made of granite, while others have simple cemented floor. The higher values of radon (>100 Bq/m 3 ) found in those types of dwellings who have the granite flooring. Table 3 Comparison of indoor radon levels with different countries S. No. Country Average Indoor radon level (Bq/m 3 ) References 1 India (Bareilly) 87 Present study 2 Czech Republic 140 [9] 3 Spain 86 [9] 4 Finland 123 [23] 5 Austria 97 [24] 6 Belgium 48 [25] 7 France 68 [26] 8 Ireland 89 [27] 9 Italy 70 [28] 10 Luxembourg 115 [29] 11 Romania 112 [30] 12 Sweden 108 [31] 13 Turkey (Giresun) 130 [32] 14 USA 46 [33], [34] As we know granite is a rich source of radium ( 226 Ra) which further decay to radon ( 222 Rn) by alpha emission, so it may be a reason for higher concentration of radon in these dwellings. The use of Air Condition (A.C.) in above said dwellings makes poor ventilation, which is also an important reason of higher radon levels. However, it is clear from table 1 that lower concentration of radon (< 80 Bq/m 3 ) found in the houses having the cemented floor. Also the proper ventilation in these dwellings provides circulation of air from inside to outside and vice versa which results the low concentration of radon. Figure 3 shows the frequency distribution of the measured radon concentration among the dwellings of study area. The number of houses (%) having the radon concentration ranges from 0 75, 75 150 and >150 Bq/ m 3 are 45%, 50% and 5%, respectively. The maximum value of radon progeny (16.77 mwl) found in the present study is below the recommended maximum level of 21.50 mwl in India [16, 19].

180 Deepak Verma, M. Shakir Khan 9 Fig. 3 Numbers of dwellings (in %) having different concentration of radon. 5. CONCLUSIONS In the above study we have calculated the values of radon, thoron and their progeny levels in the indoor environment of some dwellings of Bareilly city in Northern India. Life time fatality risk and effective dose have also been calculated for the occupants of these dwellings. The conclusions of the present study are as follows: (1) The overall average value of radon in the present study (87.23 Bq/m 3 ) is found higher than the world average value of indoor radon level (40 Bq/m 3 ). Nevertheless, the present values are lower than the action level (200 400 Bq/m 3 ) recommended by ICRP. It is also clear from table 2 that the radon levels in the dwellings of Bareilly city lies (closely) within the range (13 143 Bq/m 3 ) of indoor radon levels in India measured by other researchers. (2) The maximum value of radon progeny (16.77 mwl) have been found below the action limit of 21.50 mwl reported by Ramachandran et al. for Indian dwellings. (3) The values of effective dose in some dwellings of study area found higher than the worldwide average radiation dose of 2.4 msv/y, but less than the lower limit of action level 3 msv/y recommended by ICRP. (4) The types of walls, floor and the ventilation conditions of the dwellings play an important role in order to decide the levels of indoor radon/thoron concentration and effective dose.

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