GEO-HAZARDS: Recent Research Editors: R.C. Ramola and G.S. Gusain Copyright © 2015, Narosa Publishing House, New Delhi, India
STUDY OF SEASONAL VARIATION OF INDOOR RADON CONCENTRATION AND ITS CORRELATION WITH THORON CONCENTRATION IN SHAHJAHANPUR DISTRICT OF CENTRAL UTTAR PRADESH Anil Kumar*, R. B. S. Rawat*, Brij Nandan Singh*, Ram Singh** *
**
Department of Physics, S.S. (P.G.) College, Shahjahanpur 24200l (U.P.), India Department of Physics, Govt. Girls Degree College, Aliganj Lucknow 2260 16 (U.P.), India. E-mail:
[email protected]
Measurement of indoor radon & thoron concentration were carried out in the houses of Shahjahanpur district of Central Uttar Pradesh, India using Solid State Nuclear Track Detector (SSNTD) based dosimeter developed at the Bhabha Atomic Research Centre (BARC), Mumbai. Three detector films are used for measurements, two in the cup modes for radon & thoron concentration and one in bare mode for their progeny. More than 100 houses of different types of construction in 25 locations were taken to cover whole area of the district. The objective was finding the seasonal variation in concentration of radon and thoron. So keeping this in view the measurements were made in residential houses from December 2010 to November 2011 to cover all four seasons for a calendar year. Measured concentration of radon in different seasons in these dwellings varied from 07 Bq/M 3 to 75 Bq/M3 with an average of 22.74 Bq/m3, while that of thoron varied from 06 Bq/M 3 to 42 Bq/M3 with an average of 16.56 Bq/m3. It is observed that the concentration of radon is maximum in winter and minimum during summer. The maximum concentration in winter is essentially influenced 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 long time and radon accumulated inside the room. The variation in indoor radon and thoron concentration for different seasons will be discussed in the paper. A weak positive correlation was observed between radon and thoron. KEYWORDS: Indoor radon, thoron, seasonal variation, Central Uttar Pradesh & correlation. INTRODUCTION Studies on natural radioactivity and natural environmental radiations are of great importance as they are of interest in health physics, radiation physics and all the allied branches of natural science. The
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measurement of the concentration of radon and its progeny in indoor atmosphere has been the interest of many research scientists all over the world. In our daily life we come across both natural as well as man –made sources of radiation. The natural sources are cosmic rays and the natural radioactivity present in the soil and rocks. Whereas the man made sources of radiation are nuclear installations, nuclear explosions, radionuclide’s and radiation sources used in industry, medicine and agriculture, air travel and various types of electronic devices.Radon is present in trace amounts almost everywhere on the earth, being distributed in the soil, the groundwater and in the lower atmosphere. Radioactive radon can migrate from soils and rocks and accumulate in surrounding enclosed areas such as homes and underground mines. It is important that sources of radon as well as radon infiltration mechanisms be understood before making attempts to control the indoor radon. The radon in the indoor is balanced by the rate of entry from source and the rate of removal, primarily by ventilation. Thus to understand the occurrence of high indoor radon concentration, one must understood the source material, transport mechanics and rate of removal. It has been estimated that the radon, largely in homes, constitutes about 50% of the dose equivalent received by general population from all sources of radiation, both naturally occurring and manmade. The concentration of radon in the atmosphere varies depending on the place, time, and height above the ground and meteorological conditions. METHODOLOGY Twin cup radon dosimeter is used in present study for the measurement of indoor radon and thorn concentration. The exposure of the detector inside the cup is termed as cup mode and the one exposed open is termed as the bare mode. One of the cups(Fig.1) has its entry covered with a glass fiber filter paper permeates both radon and thoron gases into the cup and is called the filter cup. The other cup is covered with a semi permeable membrane (Ward et al., 1977) sandwiched between two glass fiber filter papers and is called membrane cup. The SSNTD film (LR-115, Type II) inside the membrane cup registers tracks contributed by radon only, while that in the filter cup records due to radon and thoron. The third SSNTD film exposed in the bare mode registers alpha tracks contributed by the concentrations of both gases and their alpha emitting progeny. The dosimeter is kept at a height of 2 m from the ground and care was taken to keep the bare card at least 10 cm away from any surface. This ensures the errors due to tracks from deposited activity from nearby surfaces are avoided, since the ranges of alpha particles from radon/thoron progeny fall within 10 cm distance. After the exposure period of 90 days, the SSNTD films were retrieved and chemically etched in 2.5N NaOH solution at 60oC for 70 min with mild agitation throughout (Srivastava et al., 1995; Ramola et al., 1996, 1997, 2005; Miles, 1997; Ramachandran, 1998). The tracks recorded in all the three SSNTD films were counted using a spark counter. The measured track densities for indoor radon was converted into concentration (Bq/m3) using the following calibration factor 0.023 tracks cm-2 d-1=1 Bq/m3 (K.P. Eappen, Y.S. Mayya , 2004)
Figure 1 Twin cup radon dosimeter
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STUDY AREA Shahjahanpur district is one of the districts of Uttar Pradesh state of India. It is a part of Bareilly division. Geographically, it is situated at 27.35 N latitude and 79.37 E longitudes. Adjoining districts of Shahjahanpur are Lakhimpur Kheri, Farukhabad, Hardoi Badaun, Bareilly and Pilibhit District. Shahjahanpur is a district having three main rivers, the Ganga, the Ramganga and the Garrah and a large fraction of its total population lived near the banks of above said rivers. The ground of Shahjahanpur is almost plane but at some places, there are ups and downs. The soil of Shahjahanpur is of mainly thee types- bhud, domat and matiyaar. The dwellings in the study area are mainly made of bricks using cement and concrete. 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.
Figure 2. Map of Shahjanhapur Area
RESULTS AND DISCUSSION The aim of present work was to measure the concentration of radon and thoron in indoor atmosphere in Shahjahanpur district. The houses chosen for installing dosimeters are new as well as old one. In rural area like Mirjapur and Kalan, some dosimeters are installed in mud houses. The results of systematic study are obtained by considering the room as a space in which the radon and thoron levels are directly related to the dynamic and static parameters. The minimum radon concentration (7 Bq/m3) was recorded in Shahjahanpur city in summer while the highest concentration (75 Bq/m3) is recorded in Mirjapur in winter with average value 27.71 Bq/m3. The least concentration in summer was due to increase in temperature which results virtual mixing and rising of aerosol and dust particles to higher altitude, so there will be the reduction in aerosol and dust particles near the earth surface and hence the radon concentration decreases. The maximum concentration in winter is essentially influenced by the intense temperature inversion, which generally occurs in winter when the wind velocity is low. The
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maximum concentration in winter is also the result of decreased ventilation because in winter season the houses are closed for long time and radon accumulated inside the room. The minimum thoron concentration (6 Bq/m3) was recorded in Khudaganj, Kant, Dadraul and Kalan and Piprola in summer while the highest concentration (42 Bq/m3) is recorded in Mirjapur in winter. The average value of thoron concentration in study area was found 16.62 Bq/m3 for the complete year. The observed values of radon concentrations in mud houses were found comparably higher than that in other houses. Since the ground floor of mud houses allow more radon to diffuse inside the house because of high porosity of the material used. In mud houses, local soil and stone are used for construction purposes. The emanation of radon from building materials (local soil and stone) is higher than that for normal building material and thus contributes additional radon inside the house. The concentration of radon and thoron in study area were observed below the recommended action level set by various organizations. In a national radon survey done by BARC, Mumbai and published by Head, Library and information services Division in September 2003, the minimum and maximum concentration of radon and thoron in India was reported 4.6 Bq/m3and 147.3 Bq/m3, 3.5 Bq/m3 to 42.8 Bq/m3 respectively. The correlation coefficient between radon and thoron is 0.64.
Figure 3.Thoron concentration in Shahjanhapur
Figure.4 Radon,, thoron concentration in rainy season in Shahjanhapur
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Figure 5. Radon, thoron concentration in summer season in Shahjanhapur
Fig.6: Radon, thoron concentration in winter season in Shahjanhapur
Figure 7.Radon, thoron concentration in autumn season in Shahjanhapur CONCLUSIONS Based on the result, it was concluded that radon concentration vary with, temperature, humidity and atmospheric pressure As humidity decreases the temperature increases which results in the maximum vertical mixing and rising of dust particles and vice versa. The significant variation of radon concentration was observed for different seasons. It is found that the average radon concentration is maximum during winter season and minimum during summer season. The radon concentration gradually decreases towards summer and monsoon, the factors that may affect is high temperature and low pressure in summer. During the monsoon with south west winds having strong wind velocity and
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heavy precipitation, a decrease in radon concentration was found. Decrease of radon in monsoon is due to other factors also i.e. soil is saturated with water during monsoon. Seasonal variation of radon concentration depends on several parameters that include type of houses, radon source, living habits of inhabitants, ventilation system of the houses, outdoor climate and metrological parameters like moisture, pressure, wind speed and temperature differences .This seasonal variation may be attributed to the fact that during hot summer season people lives self ventilated, houses keep open which result the minimum radon concentration. On the other hand they normally keep the windows closed in winter season that results in poor ventilation and hence increased radon levels. Geological impacts on the values had minor effect as the entire selected area was of almost same soil and rocks. On analyzing the results, the observed values of radon & thoron concentrations in mud houses were found comparably higher than that in other houses. The probable cause could be the exhalation of radon gas from soil. In the area close to industrial area radon concentration is found little higher than the areas in localities. It may be due to the drainage of the harmful chemicals including radioactive elements. Recently thermal power plant was established in Rosa, Shahjahanpur. It will be quite interesting to note change in concentration of radioactive elements in the atmosphere of Shahjahanpur. Although the results obtained from site Roza indicates no specific significant change in concentration in comparison to other sites, but the recorded concentration will play an important role in all comparative studies proposed in forth coming time and in estimating total radiation dose for habitants of Shahjahanpur. ACKNOWLEDGEMENTS The authors wish to acknowledge to Prof. R.C. Ramola, Department of Physics, H.N.B. University Garhwal, Tehri for his generous help and encouragement throughout tenure of my research work. I am highly obliged to Dr. K.P. Eappen, Scientist, Bhabha Atomic Research Centre, Trombey, Mumbai, for helpful suggestions and discussion about radon measurement techniques and to generous cooperation of the people of the Shahjahanpur district. REFERENCES Eappen K.P., Mayya Y.S., Radiation Measurements 38 (2004) 5 – 17). Ward, W.J., Fleischer, R.L. and Mogro-Campero, A. (1977) Barrier technique for separate measurement of radon isotopes. Rev. Sci. Instrum. 48, 1440-1441 Miles, J.C.H. (1997) Calibration and standardization of etched track detectors,In Radon measurements by etched track detectors, applications in radiation protection, Ramachandran, T.V. (1998) Proc. 11th National Symposium on Solid State Nuclear Track detectors Ramola, R.C. Rawat, R.B.S., Kandari, M.S., Ramachandran, T.V., & Choubey, V.M., 1998. A study of seasonal variation of radon levels in different types of houses. Jour. Environ. Radioactivity. 39 (1), 1-7. Ramola, R.C., Negi, M.S. & Choubey, V.M. 2005. Radon, thoron and their progeny concentrations in dwelling of Kumaun Himalaya- survey and outcomes. J. Envir. Radioactivity, 79 (1), 85-92. Ramola, R.C., Rawat, R.B.S., Kandari, M.S., Ramachandran, T.V., Eappen, K.P. and Subba Ramu, M.C. (1996) Calibration of LR-115 Plastic track detector for environmental radon measurements. Indoor Built Environ. 5, 364-366. Ramola, R.C., Singh M., Singh S., and Virk H.S., (1987). Ind. J. Pure Appl. Phy. 25, 127-129. Srivastava, D.S., Singh, P., Rana, N.P.S., Naqvi, A.H., Azam, A., Ramachandran, T.V., Subba Ramu M.C., Muraleedharan T.V. and Shaikh A.N.,Methods and Measurements of Indoor Levels of Rn222 and its Daughters, BARC Rep. No. 1390, 1988.
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