Uranium, Radium and Radon Exhalation Rate in the Soil Samples

Uranium, Radium and Radon Exhalation rate in the soil samples from some areas of Punjab and Haryana states, India Harmanjit Singh, Joga Singh, B.S.Bajwa, Surinder Singh*, Department of Physics, Guru Nanak Dev University, Amritsar-143005, India Abstract Uranium, radium concentration and radon exhalation rate have been determined in soil samples from some areas of Punjab and Tusham (Haryana), using the LR-115 plastic track detectors. ‘Can Technique’ using these detectors, has been used for the estimation of radium concentration & radon exhalation rate and uranium estimation has been carried out in the samples using the fission track technique. Uranium content in these samples has been found to be varying from 1.03 to 9.96 ppm in the Punjab region and varies from 3 to 13.4 ppm for Tusham region. Radium concentration observed for soil samples has been found to be varying from 0.82 to 8.6 BqKg-1 for Punjab area and from 2 to 9.31 BqKg-1 for Tusham. The Radon exhalation rate in these samples has been found to be varying from 1.2 to 8.83 mBqKg-1h-1 for Punjab area and from 3.11 to 11.54 mBqKg-1h-1 for Tusham. A positive correlations with (R2 =0.61) and (R2=0.60) have been found between radon exhalation rate and uranium concentration in the samples from both for the Punjab and Tusham regions respectively. _____________________________________________________________ *Corresponding author: Ph +91-183-2257007. Fax +91-183-2258820 E-mail- [email protected] Introduction The radioactive element Uranium being parent element of the well- known radioactive decay series i.e. Uranium series, is present in earth’s crust in varying amounts at almost all the places on earth. Radon is produced in the soil due to the presence of 238U and is transported to atmosphere by turbulent diffusion (Tanner et al, 1980). Once formed by the decay of the parent 226Ra, the 222Rn atoms are free to diffuse through the interstices between mineral or soil particles where they become a minor constituent of soil. The processes effective in transporting 222Rn from soil to the surface are related directly to the size and configuration of the space occupied by the soil gas. Radon concentrations in soil pores at depth are dependent directly upon the radium content of the soil, emanating power for radium and soil moisture content. Exhalation designates the escape of radon from a material to the atmosphere. In the soil, radon molecules can escape from grains of soil by diffusion or recoil into the soil pores (Porstendorfer et al, 1991). This process is called emanation. The number of radon atoms released per unit surface area per unit time from a material is termed as exhalation rate. Normally the dominant contributor to indoor radon is the emanation from soil and fractured bedrock close to the surface. If uranium rich material lies close to the surface of the earth, there can be high radon emanation rate, resulting in high radon exposure hazards. Radon (222Rn) a radioactive inert gas is responsible for about half of the radiation dose received by general population (UNSCEAR, 1994). On the basis of epidemiological studies it has been established that the enhanced levels of indoor radon in dwellings can cause health hazards and may cause serious deceases like lung cancer in human beings (Bichichio et al., 1998;Field et al., 2000). To estimates the radon risk it is necessary to check the correlation between uranium concentration and radon emanation potential of the soil samples. A number of studies of exhalation rates of radon/thoron from soil and building materials are available (Wilkening et al., 1972; Folkerts, 1983; Singh and Prasad, 1997). Thus to check the radon risk it is necessary to check the correlation between the uranium concentration and radon emanation potential of the source material. In the present study we have applied solid- state nuclear track detectors (LR-115) for the microanalysis of uranium, radium and radon exhalation rate measurements in the soil samples taken from wide range of areas of Punjab and Tusham ring complex, Haryana regions and even their correlations have been studied Experimental Technique (I) Radium Concentration and radon exhalation rate The ‘can technique’ using nuclear track detectors provides a simple method to measure the actual radon emanation in the laboratory. Thus to measure the radon exhalation rate from the soil samples, “Can Technique” has been used (Abu-Jarad et.al. 1988 and Khan et al., 1992). In this technique, about 250 gm of the soil sample was placed in an emanation chamber (1 lt glass bottle), which was then closed for a period of three weeks in order to get equilibrium between radium and radon. In each can one LR-115 plastic detector was attached below cork head at a certain distance (about 15 cm) from the surface of the material (soil sample). The sensitive part of the detector was faced to the emanating radon from the soil sample so that it could record the alpha particles resulting from the decay of radon in the whole volume of the can. Such an assembly is left for a time period of 90days. After fixed time, the detectors were taken out, etched (2.5 N NaOH at 60 °C for 90 minute) and counted for alpha particle tracks under standard conditions. The radon exhalation rate (in terms of mass) is calculated by using equation (Khan et al., 1992) CV/M -1 -1 EM (mBqkg h ) = ——————— T+1/ (e-T-1) Where EM=Radon Exhalation Rate; C=Integrated Radon Exposure V=Volume of the can; M=Mass of the soil sample (250gm) T=Exposure Time; =Decay Constant In the present work we also calculated Radium concentration in soil samples using ‘can technique’. The effective radium content of the soil samples was calculated by using the relation (Somogyi, 1990): -1 CRa (Bqkg ) = ———[h. A/M] KTe where M = Mass of the soil sample (250 g) A = Area of cross section of emanation chamber (7.55×10-3 m2) h = Distance between the detector and the top of the soil sample (0.153m) K = Sensitivity factor (0.0245 tracks cm-2 day-1 per (Bq m-3)) by Azam et al., 1995 =Background corrected track density (tracks cm-2) The effective exposure time was given by the equation -T Te =T- 1/ (1-e ) where = Decay Constant for 222Rn (ii) Estimation of Uranium content Fission track technique enables us to obtain the activity of radium from uranium concentration and therefore, the potential radon. For the analysis of the uranium concentration of the soil samples, the fission track registration technique (Jojo et al., 1994;Azam and Prasad, 1989) has been used. In this technique a homogeneous mixture of accurately weighed powdered soil sample and methylcellulose was taken in the ratio 1:2 by weight. With about 150 mg of this mixture, using a hydraulic pellet-making machine made a thin pellet of about 1mm thickness and 1.3 cm diameter. Pellets of standard sample of known uranium concentration were also prepared in a similar manner. Each of these pellets was sandwiched between a pair of Lexan plastic detectors. The induced fission tracks were recorded in the Lexan detectors after irradiating these pellets with a thermal neutron dose of 2×1015 (nvt) from CIRUS reactor at BARC, Trombay. After irradiation the detectors were etched (6.25 N NaOH solution at 700C for 25 min) and scanned manually using Carl Zeiss binocular optical microscope at a magnification of 400×. Uranium concentrations were calculated by using the equation (Azam and Prasad, 1989;Jojo et al., 1994) Ux=Us (Tx/Ts)(Is/Ix) where the subscripts x and s represent the sample and standard respectively; T is the fission track density and I, the isotropic abundance 235 238 ratio of U and U. The correction factor (Rs/Rx) has been taken as unity. Is/Ix has been taken as unity assuming that the isotropic abundance ratio is the same for the sample and standards (Scentfle et al., 1957) Results and Discussion The uranium, radium and radon exhalation rate observed in the wide range of soil samples collected from different villages along the Amritsar to Budhlada track in the Punjab state (going towards Tusham region, Haryana) in Amritsar, Ferozepur, Faridkot & Bathinda Districts of Punjab are reported in Table 1. Uranium concentration in the collected soil samples from these areas has been found to be varying from 1.03 ppm in village Doburji to 9.8 ppm in the village Budhlada. The radium concentration in these samples has been found to be varying from 0.84 Bq kg-1 to 7.06 Bq kg-1. The radon exhalation rate in these collected samples has also been measured and has been found to be varying from 1.03 mBqkg-1h-1 in village Balachak to 8.83 mBqkg-1h-1 in village Budhlada. Similarly uranium, radium and radon exhalation rate observed in thirteen soil samples collected from different villages around the Tusham ring complex, Bhiwani District of Haryana are reported in Table 2. Uranium concentration in the collected soil samples varies from 3 ppm in Bhiwani city to 13.4 ppm in Tusham city. The radium concentration in these samples has been found to be varying from 2 Bq kg-1 in village Dang kalan to 9.31 Bq kg-1 in village Dulhadi. The radon exhalation rate in these collected samples has also been measured and has been found to be varying from 3.11 mBqkg-1h-1 in village Dang kalan to 11.54 mBqkg-1h-1 in village Dulhadi. It can be observed that wherever the uranium content in the samples is more the obvious offshoot the radium and exhalation rate has been also found to high in those samples as well, indicating as expected a good correlation between them.

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