Natural radioactivity in soil samples of Yelagiri Hills, Tamil Nadu, India and the associated radiation hazards

Radiation Physics and Chemistry 81 (2012) 1789–1795

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Natural radioactivity in soil samples of Yelagiri Hills, Tamil Nadu, India and the associated radiation hazards R. Ravisankar a,n, A. Chandrasekaran b, P. Vijayagopal c, B. Venkatraman c, G. Senthilkumar d, P. Eswaran e, A. Rajalakshmi f a

Post Graduate and Research Department of Physics, Government Arts College, Thiruvanamalai 606603, Tamil Nadu, India Global Institute of Engineering & Technology, Vellore 632509, Tamilnadu, India c Radiation Safety Section, Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India d Department of Physics, University of College of Engineering, A Constituent College of Anna University, Chennai, Arani 632317, Tamil Nadu, India e Department of Physics, Vel Tech (Owned by RS Trust), Chennai, Avadi 600062, India f Department of Physics, SSN College of Engineering, Kalvakkam, Chennai 110, Tamil Nadu, India b

H I G H L I G H T S c c c

Soil radioactivity is used for base line data in future impact assessment. We report the results of radiation hazard parameters in soils of Yelagiri hills. The level of the natural radiation in the studied area does not exceed the norm.

a r t i c l e i n f o

abstract

Article history: Received 4 April 2012 Accepted 5 July 2012 Available online 20 July 2012

The natural radioactivity of soils at Yelagiri hills has been studied in this paper. The radioactivities of 25 samples have been measured with a NaI(Tl) detector. The radioactivity concentrations of 238U, 232Th and 40K ranged from r 2.17 to 53.23, 13.54 to 89.89 and from 625.09 to 2207.3 Bq kg  1, respectively. The measured activity concentrations for these radionuclides were compared with world average activity of soil. The average activity concentration of 232Th in the present study is 1.19 times higher than world median value while the activity of 238U and 40K is found to be lower. In order to evaluate the radiological hazard of the natural radioactivity, the radium equivalent activity Raeq, the absorbed dose rate DR, the annual effective dose rate and the external hazard index (Hex) have been calculated and compared with the internationally approved values. The study provides background radioactivity concentrations in Yelagiri hills. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Natural radioactivity Soil Gamma ray spectrometer Radium equivalent activity Absorbed dose rate External hazard index

1. Introduction Human beings have always been exposed to natural radionuclides arising from within and outside of the earth. The exposure to ionizing radiations from natural sources occurs because of the naturally occurring radioactive elements in the soil and rocks, cosmic rays entering the earth’s atmosphere from outer space and the internal exposure from radioactive elements through food, water and air. Natural radioactivity is wide spread in the earth’s environment and it exists in various geological formations in soils, rocks, water and air (Ibrahiem et al.,1993; AlyAbdo et al., 1999; Malanca et al.,1996; Myrick

n

Corresponding author. Tel.: þ91 9443520534; fax: þ91 9443520534. E-mail address: [email protected] (R. Ravisankar).

0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.07.003

et al.,1983; Maul and Ohara,1989. The natural radioactivity in soil comes from U and Th series and natural K. Artificial radionuclides can also be present from global fallout of 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 daughter products. 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). The measurement of natural radioactivity due to gamma rays is needed to implement precautionary measures whenever the dose is found to be above the recommended limits. The growing worldwide interest in natural radiation exposure has lead to extensive survey in many countries. External Gamma dose estimation due to the terrestrial sources is essential not only

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because it contributes considerably (0.46 mSv y  1) to collective dose but also because of the variations of the individual doses related to this pathway. The doses vary depending upon the concentrations of the natural radionuclides 238U, 232Th and their daughter products and 40K present in soils and rocks, which in turn depend upon the local geology of each region in the world (Radhakrishna et al.,1993; Quindos et al.,1994). To evaluate the terrestrial gamma dose rate for outdoor occupation, it is very important to estimate the natural radioactivity of soil samples and is usually determined from the 238U, 232Th and 40K contents

(OECD, 1979). In Uranium series, 98.5% of the radiological effects are produced by radium and its daughter products (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. Naturally occurring heaviest toxic element uranium is found in traces in almost all types of rocks, soils, sands and waters. Due to its property to dissolve in aqueous solution in hexavalent (U6 þ ) form and to precipitate as a discrete mineral in tetravalent (U4 þ )

Table 1 Activity concentration, Radium equivalent activity, absorbed dose rate, annual effective dose rate and external hazard in soils of Yelagiri hills. Activity concentration (Bq/kg)

Ra(eq) (Bq/kg)

Absorbed dose rate (nGy/h)

Annual effective dose rate (mSv/y)

External hazard (Hex)

189.50 76.12 78.94 218.31 141.34 144.33 140.14 218.17 263.01 112.68 243.81 205.24 141.99 188.85 138.91 219.45 140.68 131.87 161.07 245.53 166.38 155.14 119.40 152.85 220.82 168.58

109.00 41.63 44.77 111.68 73.51 80.08 72.64 110.58 130.10 63.23 123.41 106.78 79.81 108.31 75.39 121.63 76.51 72.44 80.75 132.40 81.99 76.72 56.29 79.70 106.19 88.62

0.130 0.049 0.053 0.134 0.088 0.096 0.087 0.132 0.156 0.075 0.148 0.128 0.095 0.129 0.090 0.145 0.091 0.086 0.096 0.158 0.098 0.092 0.067 0.095 0.127 0.106

0.554 0.218 0.229 0.612 0.400 0.416 0.396 0.610 0.730 0.326 0.681 0.579 0.411 0.551 0.397 0.633 0.403 0.380 0.449 0.703 0.462 0.432 0.328 0.432 0.608 0.478

Locations 238

232

40

24.47 21.1 13.54 83.77 36.24 28.45 38.95 89.48 114.75 22.81 100.42 71.31 28.05 26.06 41.45 43.34 29.52 13.78 61.69 46.79 57.6 50.24 35.95 44.47 89.89 48.56

2207.3 656.51 851.13 1137 913.95 1391.6 872.86 1025.2 936.11 1143.8 1128.8 1250.2 1455.5 2165.5 1137.8 2105.4 1219.1 1263 700.93 2052.8 630.41 625.09 261.11 983.08 557.88 1146.88

Th

BDL BDL BDL 18.93 25.55 6.24 23.35 18.45 33.39 BDL 21.2 15.76 BDL BDL BDL 10.1 13.13 23.76 23.79 34.93 39.89 39.55 49.72 20.45 53.23 19.16

K

2500 238-U

2000

232-Th 1500 40-K 1000

Locations Fig. 1. Locations Vs Activity Concentrations (Bq/kg).

Putthoor

Thayaloor

pallakanvoor

Nilavoor

Meatokanivoor

Kallivoor

pothvoor

Meatokalli

kottaivoor

Mangalam

Arthanavoor

Nali bend

Began bend

Adiyaman bend

Oori bend

Aayai bend

Paari bend

Kaari bend

Oviyar bend

Kabilar bend

Ilango bend

Kambar bend

Thiruvalluvar bend

0

Pavendar bend

500

Bharathiyar bend

Activity Concentrations (Bq/kg)

Pavendar bend Bharathiyar bend Thiruvalluvar bend Ilango bend Kambar bend Kabilar bend Oviyar bend Paari bend Kaari bend Oori bend Aayai bend Adiyaman bend Nali bend Began bend Arthanavoor kottaivoor Mangalam Meatokalli pothvoor Kallivoor Nilavoor Meatokanivoor Pallakanvoor Thayaloor Putthoor Average

U

R. Ravisankar et al. / Radiation Physics and Chemistry 81 (2012) 1789–1795

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form, uranium forms deposits in the earth’s crust where the geological conditions become favorable. The objective of this study is focused on determining the activity concentrations of 238U, 232Th and 40K in soil samples collected from different locations of Yelagri Hills, Tamil Nadu using gamma ray spectrometry.

The inner lid was placed in and closed tightly with the outer cap. The container was sealed hermetically and externally with adhesive tape and transported to the laboratory. The sample is kept aside for about a month to ensure equilibrium between 226Ra and its daughters and 228Ra and its daughters before being taken for gamma spectrometric analysis.

2. Geology of study area

3.2. Estimation of natural radioactivity levels by gamma ray spectrometry

Yelagiri is a hill station/village in Vellore district of Tamil Nadu, India, situated off the Vaniyambadi–Tirupattur road. Located at an altitude of 1410 m above Mean Sea Level and spread across 30 km2, the Yelagiri village (also spelled Elagiri at times) is surrounded by orchards, rose-gardens, and green valleys. Yelagiri comprises of 14 hamlets and a number of temples spread over a couple of hills. The highest point in Yelgiri is the Swamimalai Hill, standing at 4338 ft; Swamimalai is a popular destination for trekkers. The hill provides a good number of trekking trails through thick reserved forests. Mangalam, a small village, is at the base of this hill. There are other trekking options that include smaller peaks like Javadi Hills, Palamathi Hills etc.

A 300  300 NaI(Tl) scintillation detector has been used for spectral measurements to enable one to cover the energy spectrum of the naturally occurring radionuclides up to 2.6 MeV (208Tl, a daughter product of 232Th). The detector is shielded by 15 cm thick lead on all sides including top to reduce background due to cosmic ray component by almost 98%. The inner sides of the lead shielding are lined by 2 mm thick cadmium and 1 mm Table 2 Comparison of natural radioactivity levels in soil and absorbed dose at different locations of Yelagiri Hills (India) with those in other countries as given in UNSCEAR (2000). Country

Concentration in soils (Bq/Kg)

3. Materials and methods

238

Egypt United states Bangladesh China Hong Kong SAR India Japan Korea Iran Denmark Belgium Luxembourg Switzerland Bulgaria Poland Romania Greece Portugal Spain Present study Median

3.1. Sample collection

17 40 34 32 59 29 33 – 28 17 26 35 40 45 26 32 25 44 32 19.16 35

40

Th

18 35 – 41 95 64 28 – 22 19 27 50 25 30 21 38 21 51 33 48.56 30

K

320 370 350 440 530 400 310 670 640 460 380 620 370 400 410 490 360 840 470 1146.88 400

32 47 – 62 87 56 53 79 71 52 43 49 45 70 45 59 56 84 76 88.62 57

300 250 200 150 100 50

Fig. 2. Locations Vs Radium equivalent activity (Bq/kg).

Putthoor

Thayaloor

pallakanvoor

Meatokanivoor

Nilavoor

Kallivoor

pothvoor

Meatokalli

kottaivoor

Locations

Mangalam

Arthanavoor

Nali bend

Began bend

Aayai bend

Adiyaman bend

Oori bend

Kaari bend

Paari bend

Oviyar bend

Kabilar bend

Kambar bend

Ilango bend

Thiruvalluvar bend

Bharathiyar bend

0 Pavendar bend

Radium equivalent activity (Bq/kg)

The soil profiles were sampled using an automatic core driller. Cores extruded and sectioned at 10 cm diameter and 25 cm depth were used to take soil samples (Ravisankar et al., 2007). The soil samples were collected at 25 sites with the only constraint that no sampling site should be taken to a field boundary, a road, a tree, a building or other obstruction. For each soil sample collected, an area of 1 m  1 m was marked and carefully cleared of debris to a few centimeters depth. Surface soils were then taken from different places randomly within the marked and cleared area, and mixed together thoroughly, in order to obtain a representative sample of that area. The collected samples after coning, quartering and sieving were used for different analyses. These samples were sun dried for 10 days and kept in an oven at 105 1C, for about 2 h. After homogenization, samples were sieved through 1 mm mesh-sized sieve to remove stone, pebbles and other macro-impurities. The homogenized sample was packed in a standard 250 ml airtight PVC plastic container.

232

U

Absorbed dose rate (nGy/h)

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equilibrium with their respective parent radionuclides. All the soil samples were subjected to gamma ray spectral analysis with a counting time of 20,000 s.

4. Results and discussions 4.1. Activity concentration The results for the activity concentrations of natural radionuclides U, 232Th and 40K in soil samples of different locations of Yelagri Hills, Tamil Nadu are reported in Table 1. The activity concentrations of 238U, 232Th and 40K ranges from r2.17 to 53.23 Bq kg  1, 13.54 to 114.75 Bq kg  1 and 261.11 to 2207.3 Bq kg  1, respectively. Except for 4 locations the obtained results for 238U have lower values of activity concentrations, when compared with worldwide average value (35 Bq kg  1 for 238U ) of this radionuclide in the soil (UNSCEAR, 2000). Similarly except 9 locations, the obtained results for 232Th have higher values of activity concentrations, when compared with worldwide average value (30 Bq kg  1 for 232Th, respectively) of this radionuclide in the soil (UNSCEAR, 2000). On one location the obtained results for 40K have values higher than the worldwide median values (400 Bq kg  1). The lowest value, 238

140 120 100 80 60 40 20

Locations

0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 Pavendar bend Bharathiyar bend Thiruvalluvar bend Ilango bend Kambar bend Kabilar bend Oviyar bend Paari bend Kaari bend Oori bend Aayai bend Adiyaman bend Nali bend Began bend Arthanavoor kottaivoor Mangalam Meatokalli pothvoor Kallivoor Nilavoor Meatokanivoor pallakanvoor Thayaloor Putthoor

Annual effective dose rate (mSv/y)

Fig. 3. Locations Vs Absorbed dose rate (nGy/h).

Locations

Fig. 4. Locations Vs Annual effective dose rate (mSv/y).

Putthoor

Thayaloor

pallakanvoor

Nilavoor

Meatokanivoor

Kallivoor

pothvoor

Meatokalli

kottaivoor

Mangalam

Arthanavoor

Nali bend

Began bend

Adiyaman bend

Oori bend

Aayai bend

Kaari bend

Paari bend

Oviyar bend

Kabilar bend

Ilango bend

Kambar bend

Thiruvalluvar bend

Pavendar bend

0 Bharathiyar bend

Absorbed dose rate (nGy/h)

thick copper to cut off lead x-rays and cadmium x-rays respectively. This graded lining shield further reduces the background especially in the low energy region. Standard sources of the primordial radionuclides obtained from IAEA in the same geometry and having the same density, as that of the prepared soil samples, were used to determine the efficiency of the detector for various energies in the prescribed geometry. The soil samples were placed on the top of 300  300 NaI (Tl) crystal and using the gamma ray spectrometer and multichannel analyzer, count spectra were obtained for each of the soil sample. The activity content of the three primordial nuclides viz., 40K, 232Th and 238U are deduced from the count spectra. The region under the peaks corresponding to 1.46 MeV (40K), 1.764 MeV (214Bi) and 2.614 MeV (208Tl) energies are considered to arrive at the radioactivity levels of 40K, 238U and 232Th respectively. The minimum detectable activity (MDA) of each of the three primordial radionuclides is determined from the background radiation spectrum obtained for the same counting time as was done for the soil samples and is estimated as 2.15 Bq kg  1 for 232Th, 2.22 Bq kg  1 for 238U and 8.83 Bq kg  1 for 40K. The sealed containers were left for at least 4 weeks (47 half life’s of 222Rn) before counting by gamma ray spectrometry in order to ensure that the daughter products of 226Ra up to 210Pb and 228Th up to 208Pb achieve

R. Ravisankar et al. / Radiation Physics and Chemistry 81 (2012) 1789–1795

of 261.11 Bq kg  1 of 40K, was found in the soil sample from the Pallakanvoor village of Yelagri Hills and the high value of 2207.3 Bq kg  1 in the soil sample from the Pavender bend of Yelagri. The average activity concentration of 232Th and 40K in the present study is 1.38 and 2.86 times higher than world median value while the activity of 238U is found to be lower. Fig. 1 shows the locations and activity concentration of natural radionuclides. The average values of activity do not provide an exact indication of radiation hazard associated with the materials.

was calculated from the formula: Effective dose rate (mSv/year)

HR ¼ DR nGy=h  8766 h=year  0:2ðoutdoor occupancy factorÞ 0:7 Sv=Gyðconversion factorÞ  106
HR ¼ DR nGy=h  1:23  103 Table 3 Statistical data of radioactivity concentrations of soil samples of Yelagiri Hills of Tamil Nadu.

4.2. Radium equivalent activity (Raeq)

232

To assess the hazard of materials that contain K, U and Th, we have computed the radium-equivalent activity Raeq (Beretka and Matthew, 1985): Raeq ¼ AU þ1:43ATh þ 0:07AK

1793

ð1Þ

where AU, ATh and AK are the activity concentrations of 238U, 232Th and 40 K, respectively (in Bq kg  1). As can be seen from Table 1, the Raeq values for the soil samples varied from 76.12 to 263.01 Bq kg  1 with the average of 168.88 Bq kg  1. All the values of the Raeq obtained for the soil sample is 168.88 Bq kg  1 which is less than the recommended value of 370 Bq kg  1 (Beretka and Matthew, 1985) and as such does not pose any radiological hazard. Fig. 2 shows the locations and Radium equivalent activity (Raeq).

Th

232

Th,

ð3Þ

238

40

K (Bq kg 1) in

U and

238

40

U

K

Range

BDL–53.23

13.54–114.75

261.11–2207.3

Arithmetic mean Standard deviation Skewness Kurtosis Frequency-distribution

19.66 16.28 0.48  0.58 Log-normal

48.56 28.20 0.90  0.11 Log-normal

1146.90 520.38 0.79 0.14 Log-normal

4.3. Estimation of absorbed gamma dose rate and the annual effective dose rate To provide a characteristic of the external terrestrial gamma radiation, we have calculated the absorbed dose rates in air at 1 m above the ground were calculated using the formula DR ¼ 0:43Q U þ 0:666Q Th þ 0:042Q K

ð2Þ 238

U, where QU, QTh and QK are the activity concentration of the 232 Th and 40K ( in Bq kg  1) respectively. To estimate the annual effective dose, one has to take into account the conversion factor from absorbed dose in air to effective dose and the outdoor occupancy factor. In the recent UNSCEAR (2000) reports, a value of 0.7 Sv Gy  1 was used for the conversion factor from absorbed dose in air to effective dose received by adults, and 0.2 for the outdoor occupancy factor, implying that 20% of time is spent outdoors on average around the world. The effective dose rate

Fig. 6. The frequency distribution of the activity of

0.7 0.6 0.5 0.4 0.3 0.2

Fig. 5. Locations Vs External hazard (Hex).

Thayaloor

pallakanvoor

Nilavoor

Meatokanivoor

Kallivoor

pothvoor

Meatokalli

kottaivoor

Locations

Mangalam

Arthanavoor

Nali bend

Began bend

Adiyaman bend

Oori bend

Aayai bend

Kaari bend

Paari bend

Oviyar bend

Kabilar bend

Kambar bend

Ilango bend

Thiruvalluvar bend

0

Pavendar bend

0.1 Bharathiyar bend

External hazard (Hex)

0.8

238

U.

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R. Ravisankar et al. / Radiation Physics and Chemistry 81 (2012) 1789–1795

where DR (nGy/h) is given by the Eq. (2). The estimated results for DR and the corresponding HR are given in Table 1. The estimated DR and HR values for all the studied samples ranged from 41.63 to

132.40 nGy h  1 and 0.049 to 0.158 mSv/y respectively. From the data in Table 1, the estimated mean value of DR in the studied samples is 88.62 nGy h  1 which is slightly higher than world average (populated-weighted) indoor absorbed gamma dose rate of 84 nGy h  1. However, the estimated mean value of effective dose rate is less than the permissible limit. Table 2 lists the comparison of natural radioactivity levels in soil and absorbed dose at Yelagiri hills (India) with those in other countries. Figs. 3 and 4 shows the locations and absorbed dose rate and annual effective dose rate respectively.

4.4. Radiation hazard indices Finally, in order to evaluate the hazard of the natural g radiation, the external hazard index (Hex) was calculated from the formula Hex ¼

AU ATh AK þ þ o1 370 Bq=kg 259 Bq=kg 4810 Bq=kg

ð4Þ

where AU, ATh, and AK are the activity concentrations of 238U, 232Th and 40K, respectively. The calculated external hazard values are between 0.218 and 0.730 (Table 1). The mean value of the external hazard index (0.478) is less than the recommended value. Fig. 5 shows locations and external hazard indices. Fig. 7. The frequency distribution of the activity of

232

Th.

4.5. Statistical analysis

Fig. 8. The frequency distribution of the activity of

40

K.

The statistical values corresponding to the activities measured for 232Th, 238U and 40K in soil samples collected at different locations are presented in Table 3. Table 3 lists the mean value, standard deviation, range, skewness, kurtosis coefficient and the type of theoretical frequency distribution that best fits each empirical distribution. Figs. 6–8 shows corresponding frequency distribution of the activities detected for the cited radionuclides. It can be observed from Table 3 that the positive value of skewness obtained for 232Th, 238U and 40K activity concentrations shows that their distribution are asymmetric with right tail being longer than the left as can be seen in Figs. 6–8. From the negative values obtained for the kurtosis coefficient of 232Th and 238U indicates that the distributions are lower and their distributions are asymmetric, while the positive value of kurtosis coefficient of 40 K indicate that the distribution are higher and narrower than normal. As a result of this analysis, the log normal distribution for 232 Th, 238U and 40K is obtained. Correlation graph between the 232 Th and 238U activities have been drawn (Fig. 9). Absence of a very good correlation (R2 ¼0.24) between 232Th and 238U is possibly due to differential transportation of radionuclides in soil.

Fig. 9. Correlation between

238

U and

232

Th.

R. Ravisankar et al. / Radiation Physics and Chemistry 81 (2012) 1789–1795

5. Conclusion This study is focused only on the concentration of 232Th, 238U and 40K in soil samples and the resulting radiation dose from these radionuclides. In addition to that, the estimation of the radium equivalent activity, absorbed gamma dose rate, the annual effective dose rate and external hazard index with the investigated soils have been made. In regard to the results of the above radiation hazard parameters do not exceed international recommended values. The data presented in this study will serve as a base line survey for primordial radionuclides concentration in the study area and also gives a base line for proper assessment of radiation exposure to the dwellers. Further investigation is still needed to measure the soils from deep layers. References Al-Jundi, J., Al-Bataina, B.A., Abu-Rukan, Y., Shehadeh, H.H., 2003. Natural radioactivity concentration in soil samples along the Amman Aquaba highway, Jordan. Radiat Meas. 36, 555–560. AlyAbdo, A.A., Hassan, M.H., Huwait, M.R.A., 1999. Radioactivity assessment of fabricated phosphogypsum mixtures. In: Proceedings of the Fourth Radiation Physics Conference. 15–19 November, Alexandria, Egypt, pp. 632–640. Beretka, J., Matthew, P.J., 1985. Natural radioactivity of Australian building materials, industrial wastes and by products. Health Phys. 48, 87–95. Ibrahim, N., 1999. Natural activities of 238U, 232Th and 40K in building materials. J. Environ. Radioact. 43, 255–258. Ibrahiem, N.M., El Ghani, A.H., Abd Shawky, S.M, Ashraf, E.M., Farouk, M.A., 1993. Measurement of radioactivity levels in soil in the Nile delta and middle Egypt. Health Phys. 64, 620–627.

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