American International Journal of Research in Science, Technology, Engineering & Mathematics

Transcription

1 American International Journal of Research in Science, Technology, Engineering & Mathematics Available online at ISSN (Print): , ISSN (Online): , ISSN (CD-ROM): AIJRSTEM is a refereed, indexed, peer-reviewed, multidisciplinary and open access journal published by International Association of Scientific Innovation and Research (IASIR), USA (An Association Unifying the Sciences, Engineering, and Applied Research) Estimation of, Equilibrium Equivalent Concentration and its Seasonal Variation in Indoor Atmosphere by Using Solid State Nuclear Track Detector (SSNTD) Technique M. S. A. Khan Department of Physics, Gandhi Faiz-a-Aam (P.G.) College, Shahjahanpur - (U.P.), India. Affiliated to M. J. P. Rohilkhand University Bareilly- (U.P.), India * salim_labphysics@rediffmail.com Abstract: Estimation of equilibrium factor (F), equilibrium equivalent concentration (EEC), inhalation dose and its seasonal variation in indoor atmosphere of Hardoi District were carried out in sixty dwellings of 1 different villages. In the present study the average values of radon and thoron equilibrium factor was found to.26 and.6 respectively in summer season,.31 and.5 respectively in rainy season,.32 and respectively in autumn season,.37and.17 respectively in winter season. The average values radon and thoron equilibrium equivalent concentration (EEC) was found to 4.88 Bqm 3 and.96 Bqm 3 in summer season, 7.56 Bqm 3 and.96 Bqm 3 in rainy season, 9.66 Bqm 3 and 3.22 Bqm 3 in autumn season, 14.8 Bqm 3 and 4.25 Bqm 3 respectively in winter season. The total inhalation dose was found to vary from.2 msvy 1 to.69 msvy 1 with an average value of.61 msvy 1 in summer season,.33 msvy 1 to 1.93 msvy 1 with an average value of.6 msvy 1 in rainy season,.16 msvy 1 to 3.16 msvy 1 with an average value of 1.57 msvy 1 in autumn season, and 1. msvy 1 to 4.38 msvy 1 with an average value of 2.19 msvy 1 in winter season. The equilibrium factor was found close to the worldwide value (.4) for indoor condition. At all the places, the average inhalation dose was found below to the action level 3-1 msvy 1 as recommended by the International Commission on Radiation Protection (ICRP). Keywords: DRPS/DTPS,, SSNTD, EEC,,, Inhalation Dose. I. Introduction is a naturally occurring radioactive gas generated by the decay of uranium-bearing minerals in rocks and soil. It is the most important contributor to human exposure from natural radioactive sources. As it decays, radon gas produces radioactive decay products generally known as radon progeny (short lived decay products) that form solids which can be inhaled and lodge in lung tissue. These solids pose a potentially greater health risk than gas when they are airborne. Exposure to radon and its progeny in indoor atmosphere can result into significant inhalation risk to population particularly to those living in homes with much higher levels of radon. It is now well established fact that radon when inhaled in large quantity causes lung disorders and is the second major cause of lung cancer after smoking. The exposure of population to high concentrations of radon and its daughters for a long period leads to pathological effects like the respiratory functional changes and the occurrence of lung cancer [1]. During recent years, radon monitoring has become a global phenomenon due to its health hazard effects on population (Radiation workers and general public). It has been estimated that out of 2.2mSv of dose, which an individual receives annually from low- level exposure, 1.27 msv is due to radon isotopes and their short lived progeny ([2], [3], [4]). As the radon in the environment (indoor and outdoor), soil, ground water, oil and gas deposits contributes the largest fraction of the natural radiation dose to population, tracking its contraction is thus of fundamental interest from radiation protection, health and hygiene point of view (whether in mining developments, coal fields, thermal power plants, housing, building construction material etc.) It is well established that the absorbed radon dose in the lungs is mainly due to radon progeny especially polonium-214 (half life=.164 second) and polonium-218(half life=3.5minute) but not the radon gas. These short lived decay products are also radioactive and attached themselves to natural aerosol particles in the atmosphere. Both unattached decay products and decay products attached to the particles may be inhaled and may then stick to the walls of the lungs and other parts of the respiratory system. As these radon decay products undergo further decay, they emit alpha particles which irradiate the cells lining the walls of the respiratory system. Equilibrium factor between radon, thoron and its short lived progenies is very important for dose assessment from inhalation of radon and thoron. It must be determined in each radon monitoring. In reality, the concentrations of radon, thoron and its progeny vary significantly with time, place, mode of ventilation, humidity etc. Therefore, an assumed equilibrium AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 23

2 factor F can not reflect the actual conditions. This problem can not solved through active measurements based on air filtering, since they only give short term measurements. Therefore, long term measurements of the radon, and its progeny concentration or the equilibrium factor F is needed to accurately assess the health hazards contribution from radon, and its progeny. Large scale measurements have been undertaken in many countries to identify dwellings which have high radon level [5]. The degree of ventilation in a dwellings and the plating out of a radon progeny onto surfaces decide the extent of equilibrium between them. Since the daughter products are mainly responsible to the inhaled dose, the measurement of equilibrium factor F radon and its progeny is desirable. It is therefore, necessary to estimate the equilibrium factor F for different types of dwelling construction. The equilibrium factor F is defined as the ratio of total potential alpha energy for the actual daughter concentration to the total potential alpha energy of the daughters which would be in equilibrium with the radon or thoron concentration. The concept of the equilibrium factor gives rise to an alternate unit for the radon concentration in terms of an equilibrium equivalent value with its progeny nuclides. The EEC of radon is equal to that quantity of radon concentration which is in secular equilibrium with its progeny nuclides giving equivalent PAEC for the progeny nuclides actually present in the atmosphere. Health effects associated with inhalation of radon and its progeny is related to the energy released by the alpha particle decay, generally known as potential alpha particle energy concentration. The objective of this work was to estimate the radon and thoron equilibrium factor, equilibrium equivalent concentration and inhalation dose in the indoor atmosphere of the study area. II. Study Area The measurements annual exposure, equilibrium equivalent concentration and inhalation dose due to indoor radon, thoron and their progeny were made in houses of Hardoi District of Uttar Pradesh, India. It is located between and north latitudes and between and 8 46 east longitudes. It has an average elevation of 134meter from seas level. The total geographical area of the district is 5,947 sq. km. Most of the houses in this area are made of mud paste and poorly ventilated. Buildings are constructed of concrete, cement, bricks and blocks. The selection of dwellings for installing dosimeters was done taking into account the degree of ventilation, type of floor, number of windows and doors as they all responsible for variation in indoor radon concentration. Most of the houses in the study area are made of mud paste and poorly ventilated. Buildings are constructed of concrete, cement, bricks and blocks. The material used for constructing the mud houses are sand, lime bricks, mortar and clay. The selected sites are shown in map. Fig.1: Map of Hardoi District and selected sites AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 231

3 III. Experimental Technique In order to measure the equilibrium factor F one has to know that the concentration of parent nuclei and progenies. For the measurement of parent radon concentration in the indoor atmosphere, pin hole based twin cup dosimeter (Fig.2) was used in the present study with single face for entry of radon and thoron gases from environment. The design of this dosimeter has two compartments separated by a central pin-holes disk made up of HDPE material acting as 22 Rn discriminator. The first compartment is known as radon+thoron chamber and the second compartment is known as radon chamber. Filter paper was used to cover the entry point of the compartment blocking the entry of the progeny. The air containing radon and thoron from the first compartment diffuses to the second compartment through pin-holes, acting as a diffusion barrier. This diffusion barrier cut-off the entry of thoron into the second chamber. Hence only radon gas enters into the second compartment. In order to register the tracks produced by alpha particle, LR-115 detector film was placed in first and second chamber. The tracks registered in first and second chamber are corresponding to the radon+thoron and radon concentration in the indoor atmosphere respectively. For the measurement of progeny concentration in the study area, direct radon progeny concentration sensors (DRPS/DTPS) have been used. Direct radon/thoron progeny sensor is a passive, deposition based technique for estimating the time-integrated equilibrium equivalent radon and thoron concentration (EEC Rn & EEC Tn ) in the indoor atmosphere. These sensors are used to accommodate the passive nuclear track detector (LR- 115) mounted with absorbers of appropriate thickness. For radon progeny (Rn 222 ), the absorber is a combination of aluminized mylar and cellulose nitrate of effective thickness 37µm to detect mainly alpha particles of energy 7.67 MeV emitted from Po 214 [6]. For thoron progeny (Rn 22 ), the absorber is 5 µm aluminized mylar which selectivity detects only 8.78 Mev α-particles emitted from Po 212 (thoron progeny). DRPS and DTPS give the direct measure of progeny activity concentration in air. The absorber thickness ensues that lower energy alpha emission do not pass through the absorber. The basic principal of operation of this sensor is that the LR-115 detector detects the alpha particles emitted from the deposited progeny atoms ([7], [8]). The dosimeter containing detector film (LR-115) and the sensors was hanged overhead on the ceiling at a height of minimum1.5 meter from the floor and at least 1 cm from any surface and it was exposed adjacent to each other for a period of 9 days. The selection of dwellings for installing dosimeters was done taking into account the degree of ventilation, type of floor, number of windows and doors as they all responsible for variation in indoor radon concentration. After exposure period the dosimeters are retrieved and all the detector films are etched with 2.5N NaOH solutions for 9 min. ([9], [1], [11], [12], [13], [14]) at a bath temperature of about 6 C. The tracks produced by alpha particles in the film were counted by using a spark counter. The track densities were used to calculate the radon and progenies concentrations and hence these were related to find the equilibrium factor of radon. The exposure cycle has been extended in a time integrated four quarterly cycles to cover all the four seasons of a calendar year (summer, rainy, autumn and winter). The radon and thoron concentration in pin-hole and the filter compartment of the dosimeter can be calculated by using the following formula: CRn (Bq/m 3 ) = T1 / krn t and CRn (Bq/m 3 ) = (T2 t CRn krn ) / ktn t Where C Rn and C Th are the radon and thoron concentration in Bq/m 3, T 1 and T 2 represents is the track density of radon and thoron respectively, t is the exposure time (9 days). k Rn (=.17 ±.2 tracks cm 2 per.d.bq m 3 ) and k Tn (=.1 ±.1 tracks cm 2 per.d.bq m 3 ) are the calibration factors of radon and thoron in radon ± thoron chamber. The tracks recorded in the exposed detector (LR-115) film is related to Equilibrium Equivalent progeny (EEC) using the sensitivity factor. The number of tracks per unit area per unit time (T) can be correlated to equilibrium equivalent progeny concentration (EEC) in air using the sensitivity factor(s) [8].The following formulae is used to calculate the EEC Tn [15] and EEC Rn [6]. EECRn (Bq/m 3 ) = Tracks (Total DTPS) / kt t & EECTn (Bq/m 3 ) =Tracks (only DTPS) / kr t Where k T and k R are the calibration factors for DTPS and DRPS respectively. The values of sensitivity factors DTPS and DRPS in natural environment have been calculated by Mishra et al [16] to be equal to.94 tracks cm 2 d 1 /EEC Tn (Bqm 3 ) for DTPS and.94 tracks cm 2 d 1 / EEC Rn (Bqm 3 ) for DRPS.The same sensitivity factors have been used to calculate the values of EEC Tn and EEC Rn in the present study. The equilibrium factor F for radon and thoron can be calculated as: FRn = EECRn (Bqm 3 )/CRn (Bqm 3 ) and FTn = EECTn (Bqm 3 ) /CTn (Bqm 3 ) Once equilibrium factor is determined, the inhalation dose of radon/ thoron and their progeny can be calculated by using the conversion factors for radon concentration (C Rn ), thoron concentration (C Tn ), EEC Rn and EEC Tn are.17 nsvh 1 Bqm 3,.11 nsvh 1 Bqm 3, 9 nsvh 1 Bqm 3 and 4 nsvh 1 Bqm 3 respectively as recommended by United Nations Scientific committee on the Effect of Atomic Radiation [17]. These equations are given by Rn.I.D. (msvy 1 ) = [(CRn.17) + (EECRn 9)] & Tn.I.D. (msvy 1 ) = [(CTn.11) + (EECTn 4)] AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 232

4 Fig. 2: A view of Pin-hole based radon- thoron dosimeter IV. Result and Discussion The observed values of indoor 222 Rn & 22 Rn concentration, equilibrium equivalent concentration, inhalation dose and equilibrium factor in the dwellings in four different seasons of a calendar year from the study area are given in the table 1.From the observation it was found that in summer season the radon concentration varies from 12Bq/m 3 to 3Bqm 3 with an average value of 18.8Bq/m 3 where as thoron concentration was found to vary1bq/m 3 to 2Bq/m 3 with an average value of 16Bq/m 3. In rainy season the radon concentration was found to vary from 15.5Bq/m 3 to 35Bq/m 3 with an average value of 24.4Bq/m 3 where as throng concentration was found to vary from12bq/m 3 to 24Bq/m 3 with an average value of 19.5Bq/m 3 respectively. In autumn season the radon concentration was found to vary from 7Bq/m 3 to 37Bq/m 3 with an average value of 3.2Bq/m 3 where as thoron concentration was found to vary from1bq/m 3 to 33Bq/m 3 with an average value of 21.5Bq/m 3. In winter season the radon concentration w3as found to vary from 3.5Bq/m 3 to 49Bq/m 3 with an average value of 4Bq/m 3 where as thoron concentration was found to vary from 2Bq/m 3 to 4Bq/m 3 with an average value of 25Bq/m 3. During summer season the equilibrium factor due to radon was found to vary from to 3 with an average value of.26 where as equilibrium factor due to thoron was found to vary from.3 to.19 with an average value of.6.in rainy season the equilibrium factor due to radon was found to vary from.18 to Table 1. Observed values of equilibrium factor (F), equilibrium equivalent concentration (EEC) and inhalation dose in different season from the study area Summer Rainy Autumn Winter Season Concentration (Bqm 3 ) Concentration (Bqm 3 ) Equilibrium Equivalent Concentration EEC Rn (Bqm 3 ) Equilibrium Equivalent Concentration EEC Tn (Bqm 3 ) (F Rn ) (F Tn ) Inhalation Dose AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 233

5 M. S. A. Khan, American International Journal of Research in Science, Technology, Engineering & Mathematics, 15(3), June-August, (msvy 1 ) Inhalation Dose (msvy 1 ) Total Inhalation Dose (msvy 1 ) with an average value of.31 where as equilibrium factor due to thoron was found to vary from.4 to with an average value of.5. In autumn season the equilibrium factor due to radon was found to vary from.14 to 35 with an average value of.32 where as equilibrium factor due to thoron was found to vary from.3 to.25 with an average value of.in winter season the equilibrium factor due to radon was found to vary from.26 to 41 with an average value of.37 where as equilibrium factor due to thoron was found to vary from.8 to.27 with an average value of.17.the total inhalation dose in summer season was found to vary from.2 msvy 1 to.69 msvy 1 with an average value of.61 msvy 1.In rainy season it was found to vary from.33 msvy 1 to 1.93 msvy 1 with an average value of.6 msvy 1.In autumn season the total inhalation dose was found to vary from.16 msvy 1 to 3.16 msvy 1 with an average value of 1.57 msvy 1 and in winter season it was found to vary from 1. msvy 1 to 4.38 msvy 1 with an average value of 2.19 msvy 1. It is observed that the radon, thoron concentration and the total inhalation dose are higher in winter and lower in summer. The corresponding values of equilibrium factors are also higher in winter and lower in summer. It is observed that the equilibrium factor due to radon and thoron are different in different seasons. This variation in equilibrium factor with season is due to ventilation of the houses during the different seasons. The maximum value of the equilibrium factor in winter is essentially by the intense temperature inversion, which generally occurs in winter season when the wind velocity is low. The maximum concentration in winter is also the result of decreased ventilation because in this season the houses are closed for a long time and therefore radon, thoron and their progenies accumulated inside the room. So that radon and thoron equilibrium factor increases. The lower value of equilibrium factor in summer is due to the fact that in summer the rooms are kept well ventilated. The variations of equilibrium factor F and equilibrium equivalent concentration (EEC) in different seasons are shown in the graph 1, 2, 3, 4&5 respectively. The equilibrium factor, equilibrium equivalent concentration (EEC) and total annual effective dose in the study area were observed below the recommended limits Summer Fig Rainy.4.5 Fig. 4 AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 234

6 EECRn and EECTn (Bq/m3) M. S. A. Khan, American International Journal of Research in Science, Technology, Engineering & Mathematics, 15(3), June-August, Autumn.25.3 Fig. 5 Winter Fig Summer Rainy Autumn Winter Fig. 7 V. Conclusions Measured values of equilibrium factor, equilibrium equivalent concentration (EEC) and inhalation dose from the study area are given in the table 1.The seasonal variation of equilibrium factor F, equilibrium equivalent concentration (EEC) are shown in the figure 1,2,3,4 &5. The observed result shows quite higher equilibrium factor and equilibrium equivalent concentration (EEC) in winter season as compared to the other season. The maximum value of the equilibrium factor in winter is essentially by the intense temperature inversion, which generally occurs in winter season when the wind velocity is low. The maximum concentration in winter is also the result of AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 235

7 decreased ventilation because in this season the houses are closed for a long time and therefore radon, thoron and their progenies accumulated inside the room. So that the equilibrium factor and equilibrium equivalent concentration (EEC) increases. The lower value of equilibrium factor in summer is due to the fact that in summer the rooms are kept well ventilated. The dose level i.e. inhalation dose at all places in the dwellings of study area were observed below the action level 3-1 msvy 1 as recommended by ICRP (1993).Thus present study concludes that the dwellings in the study area are safe without posing significant radiological threat to the human being. Although the results obtained from the study area do not show major concern but the recorded values will play an important role in all comparative studies proposed in forth coming time and in estimating total radiation dose for habitants of Hardoi District. VI. Acknowledgement The author is thankful to the residents living in the study area for their cooperation during the fieldwork. The author is also grateful to Dr. Mohammad Tariq, Department of Physics M.B.P. Govt. (P.G.) College Ashiana, Lucknow, Dr. R.B.S. Rawat and Dr. Anil Kumar Singh Department of Physics S.S. (P.G.) College Shahjahanpur for providing necessary help to carry out this work. References [1]. BEIR: Report of the Committee on the Biological Effects of Ionizing, Natl. Res council. Natl. Acad. Press., Washington D.C., [2]. IAEA, Bulletin of International Atomic Energy Agency. IAEA/PI/A 114 E, , [3]. K.K. Narayanan, D. Krishnan and M. C. Subbaramu ISRP (K)-BR-3, ISRP, Kalpakkam Chapter (1991). [4]. ICRP (International Commission on Radiological Protection): ICRP Publication No. 85, Annals of the ICRP, Pergamon Press, New York. 2. [5]. ICRP (International Commission on Radiological Protection). Protection against radon-222 at home and at work, ICRP Publication 65, Annals of the ICRP 23(2), Pergamon Press, Oxford, [6]. R. Mishra., Y.S Mayya and H.S. Kushwaha, Measurement of radon and thoron progeny deposition velocities on surface and their comparison with theoretical models. Aerosol Sci.4, 1-15, 299. [7]. S.Y.Y. Leung, D. Nikezic and Yu, K. N. Passive monitoring of the equilibrium factor inside a radon exposure chamber using barelr-115 SSNTD. Nucl. Instrum. Methods A564, , 26. [8]. R. Mishra and Y.S Mayya, Study of a deposition based direct thoron progeny sensor (DTPS) technique for estimating equilibrium equivalent thoron concentration (EETC) in indoor environment. Radiat Meas. 43:148-16, 28. [9]. V. M. Choubey, S.K. Bartaya and R.C. Ramola, in Himalayan Spring A Geographical Controle, Environment Geol., vol.39, pp , 2. [1]. R.C.Ramola, R.B.S.Rawat, M.S. Mandarin, T.V. Ramachandran, K.P. Eappen and M.C. Subba Ramu, Calibration of LR-115 Plastic track detector for environmental radon measurements. Indoor Built Environ. 5, , [11]. R.C. Ramola, R.B.S. Rawat, M.S. Kandari, T.V. Ramachandran and V.M. Choubey, Measurement of indoor radon levels around Uttar Kashi and Pauri Garhwal areas using nuclear track detectors techniques. Indian Jour. Environmental Protection, 17 (7), , [12]. R.C.Ramola, M.S. Negi and V. M. Choubey., thoron and their progeny concentrations in dwelling of Kumaun Himalayasurvey and outcomes. J. Envir. Radioactivity, 79 (1), 85-92, 25. [13]. J.C.H. Miles, Calibration and standardization of etched track detectors, In radon measurements by etched track detectors, applications in radiation protection, earth Sciences and the Environment (Eds, Durrani, S.A., and Iliac, R), World Scientific, Singapore, , [14]. T.V. Ramachandran, Proc. 11 th National Symposium on Solid State Nuclear Track Detectors, Amritsar, pp. 5-68, [15]. UNSCEAR: United Nations Scientific committee on the effects of Atomic Radiations, Report to the General Assembly, United Nations, New York, 2. [16]. R. Mishra and H.S. Kushwaha, Measurement of radon and thoron progeny deposition velocities on surface and their comparison with theoretical models. Aerosol Sci.4, 1-15, 21. [17]. Y.S. Mayya, K.P. Eappen, and K.S.V. Nambi, Methodology for mixed field inhalation dosimetry in monazite areas using a twincup dosimeter with three track detectors. Radiat. Prot. Dosim. 77, , AIJRSTEM ; 216, AIJRSTEM All Rights Reserved Page 236