Comparison of indoor radon level with radon exhalation rate from soil in Garhwal Himalaya
Introduction
The radon problem presents a global concern of radiation hygiene to the world population. Radon is present in trace amounts almost everywhere on the earth, being distributed in soil, groundwater and the lower atmosphere. The distribution of radon in ground depends on a large number of factors, such as the initial distribution of uranium in the soil, soil temperature, microcraks, granulation, porosity, humidity and surface winds. Radon can migrate from soils and rocks and accumulates in surrounding enclosed areas, such as homes and underground mines. The activity concentrations of radon, thoron and their progeny are largely influenced within the house by factors such as topography, type of houses, building materials, temperature, ventilation, wind speed and even the life style of the people living in the house (Ramola et al., 1987, Ramola et al., 2000, Subba Ramu et al., 1988, Martz et al., 1991). The major source of radon in the house is the soil beneath the house but the household water, building materials and cooking gas also influence the indoor radon concentration (Ramola et al., 2005, Prasad et al., 2008a). The measurements of radon, thoron and their progeny are important because the radiation dose to human population due to inhalation of radon and its progeny contribute more than 50% of the dose from all sources of radiation both naturally occurring and man made (UNSCEAR, 2000).
The fraction of radon atoms generated in the soil grains and reaching the pore volume of the soil is known as emanation coefficient or exhalation rate. This coefficient depends basically on the soil grain size distribution, on the porosity and on the water content. Radon exhalation from the earth crust and building materials form the main source of radon in indoor and outdoor environments (Mayya, 2004). The assessment of radiological risk related to inhalation of radon and radon progeny is based mainly on the integrated measurement of radon (George, 1984). The radon exhalation study is helpful to analyze the behavior of radon in the atmosphere.
Section snippets
Geology of study area
Garhwal Himalaya has its unique geological setting. The higher Himalayas and the inner lesser Himalayas are the two main lithotectonic zones found in the Garhwal Himalayas. On the basis of lithology grade of metamorphism and orientation of structural elements, the rocks of the lesser Himalayan provinces are divided into different formations. The junction between the lesser Himalayas and outer Himalayas is marked by the MBT (Main boundary thrust) while the Higher Himalayas and Lesser Himalayas
Twin cup dosimeter technique
In present study, radon and thoron measurements were carried out in the dwellings of Ukhimath region of Garhwal Himalayas using nuclear track detector in a twin cup radon dosimeter. The details methodology is given elsewhere (Ramola et al., 2005). The twin cup dosimeter was kept in the house at a height of 1.5 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 nearly surfaces are
Result and discussion
The measured concentrations of indoor radon and thoron are shown in Table 1. The radon concentration was found to vary from 13 ± 2 Bq/m3 to 178 ± 8 Bq/m3 with an average of 61 ± 5 Bq/m3, while the thoron concentration varies from 9 ± 2 Bq/m3 to 94 ± 6 Bq/m3 with an average of 33 ± 3 Bq/m3. The relatively high concentrations of radon and thoron in some house are because of poor ventilation of houses in winter season, when most of the doors and windows remain closed due to low temperature in the region (Ramola
Conclusions
The concentrations of radon and thoron and resulting dose in the study area were found to be lower than the ICRP recommended value of 200 Bq/m3 (ICRP, 1993) and thus are within the safe limit. Radon exhalation rate from the soil was found positively correlated with the indoor radon concentration and radium content in the soil. It was observed that the soil and rocks of the study area have a significant contribution to the indoor radon concentration in the area.
Acknowledgements
The authors are grateful to Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, for providing financial support in the form of a project. Authors (GSG and GP) are also thankful to Council of Scientific and Industrial Research, New Delhi for providing Senior Research Fellowship.
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