Journal of Environmental Radioactivity 100 (2009) 94–98
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Short communication
Comparative study of natural radioactivity levels in soil samples from the Upper Siwaliks and Punjab, India using gamma-ray spectrometry Joga Singh a, *, Harmanjit Singh a, Surinder Singh a, B.S. Bajwa a, R.G. Sonkawade b a b
Department of Physics, Guru Nanak Dev University, Amritsar 143 005, Punjab, India Health Physics Lab, Inter University Accelerator Centre, New Delhi 110067, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2007 Received in revised form 18 August 2008 Accepted 19 September 2008 Available online 14 November 2008
Natural radioactive materials under certain conditions can reach hazardous radiological levels. So, it becomes necessary to study the natural radioactivity levels in soil to assess the dose for the population in order to know the health risks and to have a baseline for future changes in the environmental radioactivity due to human activities. The natural radionuclide (226Ra, 232Th, and 40K) contents in soil were determined for 26 locations around the Upper Siwaliks of Kala Amb, Nahan and Morni Hills, Northern India, using high-resolution gamma-ray spectrometric analysis. It was observed that the concentration of natural radionuclides viz., 226Ra, 232Th and 40K, in the soil varies from 28.3 0.5 to 81.0 1.7 Bq kg1, 61.2 1.3 to 140.3 2.6 Bq kg1 and 363.4 4.9 to 1002.2 11.2 Bq kg1 respectively. The total absorbed dose rate calculated from activity concentration of 226Ra, 232Th and 40K ranged from 71.1 to 162.0 nGy h1. The radium equivalent (Req) and the external hazard index (Hex), which resulted from the natural radionuclides in soil, were also calculated and found to vary from 149.4 to 351.8 Bq kg1and from 0.40 to 0.95 respectively. These values in Upper Siwaliks area were compared with that from the adjoining areas of Punjab. The radium equivalent activities in all the soil samples were lower than the limit (370 Bq kg1) set in the Organization for Economic Cooperation and Development (OECD) report and the dose equivalent was within the safe limit of 1 mSv y1. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Natural radioactivity Gamma-ray spectrometry HPGe detector Reeq activities Annual effective dose External health index
1. Introduction The knowledge of radionuclide distribution and radiation levels in the environment is important for assessing the effects of radiation exposure due to both terrestrial and cosmogenic sources. Terrestrial radiation is due to radioactive nuclides present in varying amounts in soils, building materials, water, rocks and atmosphere. Some of the radionuclides from these sources are transferred to man through food chain or inhalations, while the cosmogenic radiation originates from outer space as primary cosmic rays. Everyone on the planet is exposed to some background level of ionizing radiation. External exposures occur as a result of irradiation, and internal exposures occur because of inhalation and ingestion. The natural radioactivity in soil comes from 238U, 232Th and from natural 40K. Some other terrestrial radionuclides, including those of the 235U series, 87Rb, 138La, 147Sm and 176Lu exist in nature but at such low levels that their contributions to the dose in the humans are small. Artificial radionuclides can also be present such as 137Cs,
* Corresponding author. Tel.: þ91 183 2258806x3342; fax: þ91 183 258820. E-mail address:
[email protected] (J. Singh). 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.09.011
resulting from fallout from weapons testing. The radiological implication of these radionuclides is due to the gamma-ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters. Therefore, the assessment of gamma radiation dose from natural sources is of particular importance as natural radiation is the largest contributor to the external dose of the world population (UNSCEAR, 1988). Natural environmental radioactivity and the associated external exposure due to gamma radiation depends primarily on the geological and geographical conditions, and appear at different levels in the soils of each region in the world (UNSCEAR, 2000). The natural radioactivity of soil sample is usually determined from 226Ra, 232Th and 40K contents (OECD, 1979). Since 98.5% of the radiological effects of the uranium series are produced by radium and its daughter products, the contribution from the 238U and the other 226Ra, precursors are normally ignored (Zastawny et al., 1979). Nationwide surveys have been carried out to determine the radium equivalent activity of soil samples in many countries (Singh et al., 2003; Al-Jundi et al., 2003; Mireles et al., 2003; Ibrahim, 1999; Sroor et al., 2001; Ibrahiem et al., 1993). Such investigations can be useful for both the assessment of public dose rates and the performance of epidemiological studies, as well as to keep reference-data records, to ascertain possible changes in the
J. Singh et al. / Journal of Environmental Radioactivity 100 (2009) 94–98
environmental radioactivity due to nuclear, industrial, and other human activities. The present work investigates the activity concentration of radioactive elements such as 226Ra, 232Th, 40K and gamma ray absorbed dose in soil samples collected from different locations of Upper Siwaliks of Kala Amb, Nahan and Morni hills, Northern India using gamma-ray spectrometry. The latitude and longitude of Kala Amb, Nahan and Morni Hills are 30 300 0 N 77 120 0 E, 30.55 N 77.3 E and 30 410 60 N 77 4’ 60 E respectively. The work in this study area has been carried out for the first time. The activity measurements have also been carried out in Ropar district of Punjab (neighboring state) for comparison. The area surveyed in the present investigations with sampling locations is shown in Fig. 1.
2. Geology of the area The Upper Siwaliks consist lithologically either of very coarse conglomerates, the boulder – conglomerates or massive beds of sand, grit and brown and red earthy clays. The former occur at the points of emergence of the large rivers – the Ravi, Tavi, Chenab, and Jhelum and of their chief tributaries, whereas the later occupy the intervening ground. The clays in the upper parts of the series are indistinguishable from the alluvial clays of the Punjab plains into which they pass by an apparently conformable passage upwards. Fossils are numerous in the Upper Siwaliks at some localities. This area appears to have been a Favourite haunt of a highly diversified elephant population, as is evident from the profusion and wide distribution of their skeletal remains. Insicors of Eliphas, Stegodon, Mastodon, their molars, skull plates, madibles maxillae, limb bones, etc., are commonly found in the sands and conglomerates. Other fossils are referable to Bubalus, Bos, Hippopotamus, Rhinoceros, Sus, Equus, Cervus, Apes, Gavialis and numerous Chelonian bones (Wadia, 1989).
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3. Experimental procedure 3.1. Measurement of natural radioactivity In order to measure the natural radioactivity in soil, surface soil samples were collected from different locations of Upper Siwaliks of Kala Amb, Nahan and Morni Hills and Ropar district of Punjab, Northern India. One sample from each location was collected by digging a hole at a depth of 0.5 m before the ground surface. After collection, samples were crushed into fine powder by using Mortar and Pestle. Fine quality of the sample was obtained using scientific sieve of 100 micron-mesh size. Before measurement, samples were dried in an oven at a temperature of 110 C for 24 h. Each sample was packed and sealed in an airtight PVC container and kept for about 4 week’s period to allow radioactive equilibrium among the daughter products of radon (222Ra) and its short-lived decay products. An average 0.25 kg of soil was used per sample. Using HPGe detector based on high-resolution gamma spectrometry system, the activity of soil samples was determined. The details of the technique are the same as reported elsewhere (Mehra et al., 2007). The efficiency calibration for the system was carried out using secondary standard source of uranium ore in geometry available for the sample counting. The secondary standard was calibrated with the primary standard (RGU-1) obtained from the International Atomic Energy Agency. Gamma transitions of 1461 keV for 40K, 186 keV and 609 keV for 226Ra, 338, 463, 911, 968 keV of 228Ac, 727 keV of 212Bi, 238 keV of 212Pb for 232Th were used for the laboratory measurement of activity concentration potassium, radium and thorium. The samples were counted for a period of 72,000 s, a typical spectra is shown in Fig. 2 and, based on the applied conditions, the achieved detection limits were 2 Bq kg1, 3 Bq kg1 and 2 Bq kg1 for 226Ra, 232 Th and 40K. The concentrations of radionuclides were calculated using the following equation:
ActivityðBqÞ ¼
CPS 100 100 SDcps 100 100 B:I: Eff B:I: Eff
where, CPS ¼ Net count rate per second B.I. ¼ Branching Intensity, and Eff ¼ Efficiency of the detector.
Fig. 1. Map showing the study area with sampling locations.
Fig. 2. The gamma-ray spectrum of sample showing various peaks of radionuclides.
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Table 1 Analytical results for the activity concentrations of 226Ra, 232Th and of 40K (Bq kg1) determined for each of the measured samples together with their total uncertainties radium equivalent, external hazard index, total absorbed dose and effective dose rate at various locations of Upper Siwaliks, Northern India. S. No.
Sample location
226
Ra (Bq/Kg)
232
Th (Bq/Kg)
40 K (Bq/Kg)
Raeq (Bq/Kg)
Absorbed dose rate (nGy h1)
Annual effective dose (mSv) Indoor
Outdoor
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Chandigarh Kharag-mangoli Ramgarh Billa Alipur Batour Raipur Rani Pyarewala Laha Raomajra Kala Amb Moonginand Devi ka Bagh Ambwala Nahan Do Sadka Laadu Banethi Neemwala Kanderan Badyal Morni Tikari Bhoori Mandana Bearwala
40.4 0.7 42.1 0.7 30.6 0.5 60.6 0.9 38.8 0.8 36.7 0.6 36.8 1.0 70.4 1.2 81.0 1.7 37.8 0.6 42.7 0.7 41.0 0.7 74.8 1.2 28.3 0.5 39.6 0.7 43.4 0.7 41.3 0.7 39.4 0.7 51.0 0.3 45.2 0.9 40.7 0.8 40.4 0.7 42.0 0.7 41.5 0.8 47.8 0.8 37.9 0.6
70.3 1.4 83.7 1.6 61.2 1.3 127.9 1.6 71.0 1.5 62.7 1.3 65.0 1.4 106.3 2.0 127.4 2.1 74.3 1.6 70.9 1.6 91.6 1.7 140.3 2.6 65.2 1.4 78.8 0.8 93.7 1.7 79.4 1.7 75.1 1.5 99.0 2.0 91. 1.7 85.7 1.8 73.8 1.6 79.3 1.6 64.3 1.3 89.8 1.8 79.4 1.7
504.9 6.5 479.3 6.2 406.4 5.4 756.7 9.0 500.7 6.4 529.6 6.7 391.8 5.2 506.4 6.5 1002.2 11.2 704.1 8.5 363.4 4.9 866.8 10.0 990.7 11.1 730.4 8.7 680.0 8.3 775.9 9.2 633.9 7.8 552.8 7.0 952.5 10.7 611.5 7.6 761.7 9.0 483.5 6.3 423.1 5.6 389.1 5.2 633.5 7.8 580.9 7.3
179.7 198.8 149.4 301.8 178.9 167.1 160.0 261.4 340.4 198.2 172.1 238.8 351.8 177.8 204.7 237.1 203.8 189.3 265.9 222.9 221.9 183.2 187.9 163.4 224.9 196.3
85.5 94.0 71.1 143.1 85.1 79.9 75.6 122.2 162.0 95.6 80.8 115.5 167.4 86.7 98.3 113.9 97.5 90.3 128.2 106.1 106.9 86.9 88.6 77.0 107.1 93.8
0.42 0.46 0.35 0.70 0.42 0.39 0.37 0.60 0.79 0.47 0.40 0.57 0.82 0.43 0.48 0.56 0.48 0.44 0.63 0.52 0.52 0.43 0.43 0.38 0.53 0.46
0.10 0.12 0.09 0.18 0.10 0.10 0.09 0.15 0.20 0.12 0.10 0.14 0.21 0.11 0.12 0.14 0.12 0.11 0.16 0.13 0.13 0.11 0.11 0.09 0.13 0.12
0.49 0.54 0.40 0.81 0.48 0.45 0.43 0.71 0.92 0.54 0.46 0.64 0.95 0.48 0.55 0.64 0.55 0.51 0.72 0.60 0.60 0.49 0.51 0.44 0.61 0.53
28.3–81.0 42.9
61.2–140.3 85.5
363.4–1002.2 623.5
149.4–351.8 214.5
71.1–162.0 102.3
0.35–0.82 0.50
0.09–0.21 0.13
0.40–0.95 0.58
Range Average
External hazard index (Hex)
The annual effective doses were determined as follows:
3.2. Radium equivalent activity It was calculated through the following relation (Yu et al., 1992):
IndoorðnSvÞ ¼ ðAbsorbed DoseÞnGyh1 8760 h 0:8 0:7 SvGy1
Raeq ¼ CRa þ 1:43CTh þ 0:07CK where CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K in Bq kg1, respectively. While defining Raeq activity, it has been assumed that 370 Bq kg1 226Ra or 259 Bq kg1 232Th or 4810 Bq kg1 40K produce the same gamma dose rate.
OutdoorðnSvÞ ¼ ðAbsorbed DoseÞnGyh1 8760 h 0:2 0:7 SvGy1 3.5. External hazard index (Hex)
3.3. Calculation of air-absorbed dose rate The external terrestrial g-radiation absorbed dose rate in air at a height of about 1 m above the ground was calculated by using the conversion factor of 0.0414 nGy h1/Bq kg1 for 40K, 0.461 nGy h1/ Bq kg1 for 226Ra, and 0.623 nGy h1/Bq kg1 for 232Th (UNSCEAR, 1993) assuming that 137Cs, 90Sr and the 235U decay series can be neglected as they contribute very little to the total dose from environmental background (Kocher and Sjoreen, 1985; Jacob et al., 1986; Leung et al., 1990).
D nGyh1 ¼ 0:461CRa þ 0:623CTh þ 0:0414CK where, CRa, CTh and CK are the activity concentrations (Bq kg radium, thorium and potassium in the samples.
The external hazard index Hex can be calculated by the following equation (Beretka and Mathew, 1985):
Hex ¼ CRa =370 þ CTh =259 þ CK =4810 1 where CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K in Bq kg1, respectively. The value of this index must be less than the unity in order to keep the radiation hazard to be insignificant. The maximum value of Hex equal to unity corresponds to the upper limit of Raeq (370 Bq kg1). 4. Results and discussion
1
) of
3.4. Calculation of annual effective dose Annual estimated average effective dose equivalent received by a member was calculated using a conversion factor of 0.7 Sv Gy1, which was used to convert the absorbed rate to human effective dose equivalent with an outdoor occupancy of 20% and 80% for indoors (UNSCEAR, 1993).
The results for the activity concentrations of natural radionuclides 226Ra, 232Th and 40K together with their average values and range in soil samples of 26 locations around the Upper Siwaliks of Kala Amb, Nahan and Morni Hills and 11 locations in Ropar district of Punjab, Northern India were reported in Table 1 and Table 2 respectively. The world average concentrations are 35, 30 and 400 Bq kg1 for 226Ra, 232Th and 40K respectively. In general, the average activity concentrations of 226Ra, 232Th and 40K in soil of these areas were higher than the world figures reported in UNSCEAR (2000). Comparatively high values of 226Ra, 232Th and 40K
J. Singh et al. / Journal of Environmental Radioactivity 100 (2009) 94–98
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Table 2 Analytical results for the activity concentrations of 226Ra, 232Th and of 40K (Bq kg1) determined for each of the measured samples together with their total uncertainties radium equivalent, external hazard index, total absorbed dose and effective dose rate at various locations of Ropar district of Punjab, Northern India. S.No.
Sample location
226
Ra (Bq/Kg)
232
Th (Bq/Kg)
40 K (Bq/Kg)
Raeq (Bq/Kg)
Absorbed dose rate (nGy h1)
Indoor
Outdoor
1 2 3 4 5 6 7 8 9 10 11
Roulmajra Khatana Noohon Dakala Ratanpur Daburji Gonumajra Chandpur Lohgarh Fide Ghanoulia Haripur
41.0 0.8 37.5 0.6 37.9 0.6 41.7 0.7 38.2 0.7 41.6 0.7 39.2 0.8 35.9 0.6 34.0 0.7 33.3 0.6 34.6 0.7
81.4 1.7 76.1 1.5 72.8 1.5 81.0 1.7 79.5 1.5 81.0 1.7 78.7 1.5 68.7 1.4 70.4 1.7 65.6 1.4 68.3 1.6
634.5 7.8 562.8 7.1 512.7 6.6 538.5 6.8 578.8 7.2 530.6 6.8 605.4 7.5 422.9 5.6 557.0 9.3 623.4 7.7 527.5 9.1
206.3 189.7 181.5 199.0 196.5 198.3 198.4 166.7 177.5 175.1 174.4
98.7 90.6 86.4 94.6 93.5 94.2 94.9 79.0 85.0 84.4 84.0
0.48 0.44 0.42 0.46 0.46 0.46 0.47 0.39 0.42 0.41 0.41
0.12 0.11 0.11 0.12 0.12 0.12 0.12 0.10 0.10 0.10 0.10
0.56 0.51 0.49 0.54 0.54 0.54 0.54 0.45 0.48 0.47 0.47
33.3–41.7 37.7
65.6–81.4 74.9
422.9–634.5 554.0
166.7–206.3 187.6
79.0–98.7 89.6
0.39–0.48 0.44
0.10–0.12 0.11
0.45–0.56 0.51
Range Average
in soil samples from Upper Siwaliks area may be due to the abundance of high activity fossil bones at some localities (Bajwa et al., 1993; Bajwa and Virk, 1996). Very high values of uranium concentration ranging from 1475 to 2062 Bq kg1 were reported by the authors. No regular trend in the variation of the terrestrial radioactivity was observed from the study area. Residential houses and dwellings in Upper Siwaliks and near vicinity (Punjab) were mostly built with bricks where soil was the source material. There was not any statistical information about cancers, average lifetime of people, typical health diseases, etc. in the study area. To assess the radiological risk of soils used as a building material, it was useful to calculate the radium equivalent activity and external hazard index (Orgun et al., 2005; Beretka and Mathew, 1985). The values of radium equivalent activity (Raeq) around Upper Siwaliks and Punjab were less than the acceptable
160
Annual effective dose (mSv)
safe limit of 370 Bq kg1 (OECD, 1979). The calculated values of Hex for the soil samples ranged from 0.40 to 0.95 around Upper Siwaliks with average value of 0.58 (Table 1) and from 0.45 to 0.56 in Ropar with average value of 0.51 (Table 2). Since all these values are lower than unity, therefore, according to the Radiation Protection 112 (European Commission, 1999) report, soil from these regions is safe and can be used as a construction material without posing any significant radiological threat to the population. The calculated total absorbed dose and annual effective dose rates of samples were also shown in Tables 1 and 2. The International Commission on Radiological Protection (ICRP) has recommended the annual effective dose equivalent limit of 1 mSv y1 for the individual members of the public and 20 mSv y1 for the radiation workers (ICRP, 1993). These dose limits have been established on the prudent approach by assuming that there is no threshold dose
Upper Siwaliks Punjab Linear (Upper Siwaliks) Linear (Punjab)
140
100
232Th
(Bq Kg-1)
120
80
60
40 300
400
500
600
700 40K(Bq
Fig. 3. Variation of
40
K and
232
External hazard index (Hex)
800
900
1000
kg-1)
Th activity concentrations in the soils of Upper Siwaliks and Punjab, Northern India.
1100
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J. Singh et al. / Journal of Environmental Radioactivity 100 (2009) 94–98
below which there would be no effect. This means that any additional dose would cause a proportional increase in chance of a health effect. The world wide average annual effective dose is approximately 0.5 mSv and the results for individual countries being generally within the 0.3–0.6 mSv range (UNSCEAR, 2000). No regular trend in the variation of the annual effective dose and absorbed dose rate was observed from the soil samples. Our results for average annual effective dose were within the range of world wide average value. A detailed analysis of the results indicated that there was a good correlation (R2 ¼ 0.53, N ¼ 26) between the activity concentrations of 232Th and 40K in the soil samples around the Upper Siwaliks and a very poor correlation (R2 ¼ 0.09, N ¼ 11) in the soil samples of Ropar area (Fig. 3). For 95% Confidence level, the confidence limits lie in the range 85.5 41.3 for 232Th and 623.5 369.6 for 40K around the Upper Siwaliks and 74.9 11.6 for 232Th and 554.0 116.0 for 40K in Ropar district of Punjab. The respective correlation coefficients around Upper Siwaliks were slightly higher in value when compared with those for the soils from Rajasthan (Nageswara Rao et al., 1996) but were in good agreement with those reported for Spanish soils (Quindos et al., 1994).
5. Conclusions 1. The average values of activity concentrations of 226Ra, 232Th and 40K around Upper Siwaliks were found to be higher than the corresponding values from the adjoining areas of Ropar district of Punjab. This may be due to the abundance of high activity fossil bones at some localities of Upper Siwaliks. 2. The results obtained showed that the indoor and outdoor effective doses due to natural radioactivity of soil samples were lower than the average national and world recommended value of 1.0 mSv y1. 3. Our results revealed that there was a good correlation (R2 ¼ 0.53) between the activity concentrations of 232Th and 40 K in the soil samples around the Upper Siwaliks and a very poor correlation (R2 ¼ 0.09) in the soil samples of Ropar area. Acknowledgments The authors are thankful to the residents of the study area for their cooperation during the fieldwork, lab staff of G.N.D. University, Amritsar especially Mr. Santokh Singh, and officials of Health Physics Lab. of the Inter University Accelerator Center, New Delhi for their support in providing the necessary equipments for gamma-ray spectrometry.
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