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Year : 2012  |  Volume : 35  |  Issue : 1  |  Page : 43-51  

Natural radioactivity and dose rates for soil samples around Tiruchirapalli, South India using γ-ray spectrometry

1 Environmental Impact Assessment Division, Hubert Enviro Care Systems (P) Limited, Ashok Nagar, Chennai, India
2 Department of Physics, Perunthaivar Kamarajar Institute of Engineering and Technology, Karaikal, India
3 P.G. and Research Department of Zoology, The New College, Chennai, India

Date of Web Publication6-May-2013

Correspondence Address:
Bojarajan Senthilkumar
Environmental Impact Assessment Division, Hubert Enviro Care Systems (P) Limited, Ashok Nagar, Chennai
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0464.111409

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The activity concentrations and the gamma-absorbed dose rates of the naturally occurring radionuclides 226 Ra, 232 Th, and 40 K were determined for 40 soil samples collected from Tiruchirapalli, South India, using g-ray spectrometry. The average activity concentrations of 226 Ra, 232 Th, and 40 K in the soil samples were found to be 29.9, 39.0, and 369.7 Bq kg−1 , respectively. The measured activity concentrations of both 226 Ra and 40 K in the soil were lower than the world average, whereas, the activity of 232 Th was higher than the world average. The concentrations of these radionuclides were also compared with the average activity of the Indian soil. The radiological hazard index was calculated and compared with the internationally approved values. The average external absorbed gamma dose rate was observed to be 79.9 nGy h−1 , with a corresponding average annual effective dose of 97.9 mSv y−1 , which was above the world average values. The values of Ra eq and H ex were found to be within the criterion limit, whereas, the radioactivity level index ( Ig) and total gamma dose rate were above the worldwide average values.

Keywords: Dose rate, natural radioactivity, soil, Tiruchirapalli, γ-ray radiation hazard indices

How to cite this article:
Senthilkumar B, Manikandan S, Musthafa MS. Natural radioactivity and dose rates for soil samples around Tiruchirapalli, South India using γ-ray spectrometry. Radiat Prot Environ 2012;35:43-51

How to cite this URL:
Senthilkumar B, Manikandan S, Musthafa MS. Natural radioactivity and dose rates for soil samples around Tiruchirapalli, South India using γ-ray spectrometry. Radiat Prot Environ [serial online] 2012 [cited 2021 Apr 13];35:43-51. Available from: https://www.rpe.org.in/text.asp?2012/35/1/43/111409

  Introduction Top

Natural radioactivity is widespread in the earth's environment; it exists in the soil, plants, water, and air. Exposure to environmental gamma radiation is mainly due to terrestrial and cosmic sources, and their presence is strongly influenced by the local geology. [1]

Natural radionuclides in the soil generate a significant component of the background radiation exposure to the population. [2] Radionuclides in soils, belonging to the 226 Ra and 232 Th series, as well as radioisotope of potassium ( 40 K), are the major contributors of outdoor terrestrial natural radiation. [3] As these radionuclides are not uniformly distributed, knowledge of their distribution in the soils and rocks plays an important role in radiation protection and measurement. [4] Also, the accumulation of these radionuclides in the soil may substantially contribute to the collective radiation dose received by the local population living within the particular environment. The specific levels of natural radionuclides are related to the types of rock from which the soils originate. Higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks. There have been many surveys to determine the background levels of radionuclides in soils, which in turn are related to the absorbed dose rates in air. The action level of the radiation dose for the public due to the gamma radiation, caused by building materials used in house building is 1 mSvy -1 . The action level of the radiation dose for the public due to the gamma radiation caused by landfill materials, materials used in landscaping, and materials used in road, street, and related buildings is 0.1 mSvy -1 . [1],[2],[3],[4],[5],[6] Therefore, measurements of natural radioactivity in the soil are of great interest to many researchers throughout the world and these have led to worldwide national surveys in the last two decades.

The main objective of this study is to identify and determine the natural radionuclide concentration in soil samples and to estimate the radiological effects and γ-ray absorbed dose of naturally occurring radionuclides, such as, 226 Ra, 232 Th, and 40 K, collected from different locations of the Tiruchirapalli area, South India. This study would be useful for establishing the baseline data on the background gamma radiation levels in different parts of Tiruchirapalli for assessment of radiation exposures to the population.

  Materials and Methods Top

Background of the study area

Tiruchirapalli is located in the central part of Tamil Nadu, South India (10° 49' N and 78° 42' E) on the banks of the River Cauvery [Figure 1]. It is the fourth largest city in the state of Tamil Nadu and spreads to an area of 5114 km 2 . The soil types of the study area are River alluvium, red sandy soil, black soil, and red soil. Geologically, the region mainly comprises of charnockites, gneisses, and granites of the early Proterozoic age. Thin strips of recent alluvium occur on either side of the rivers Coleroon and Cauvery, which flow in an easterly direction in the northern part of the study area. Limestone in Tiruchirapalli occurs in crystalline and non-crystalline (amorphous) varieties and the crystalline limestones of the Precambrian age are mainly distributed in most parts of Tiruchirapalli city. The average annual rainfall is 700 mm and the temperature is generally very high during summer, and it ranges from 29.1 to 38.2°C. As per the 2001 census, Tiruchirapalli has a population of 0.7 million, which contributes to 40% of the total population of Tiruchirapalli district. The density of the city population is 3601 persons per km 2 .
Figure 1: Map of Tiruchirapalli showing the sampling locations

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Sample collection and preparation

Soil samples were collected from 40 different locations in Tiruchirapalli that were close to the populated areas of the city [Figure 1]. At every sampling site, the soil samples were collected from the surface layer (0-20 cm depth) of the four corners and the center of a square area corresponding to 1 m 2 . The five samples of this soil mixture, weighing approximately 1.5 kg, were considered as representative of the sampling site. After removing the stones and organic materials, the samples were dried in an oven at about 100°C, for 24 hours, to remove the moisture content, and then pulverized, homogenized, and sieved through a 1 mm mesh. Then, a sample of 250 g was weighed and packed in a standard plastic container (12 cm height × 6.5 cm diameter). After proper tightening of the lid, the containers were sealed with Teflon tape and left for four weeks, in order to maintain radioactive equilibrium between 226 Ra and its daughters before they were taken for counting by γ- ray spectrometry.

Measurement of natural radioactivity

The concentration of natural radionuclides ( 226 Ra, 232 Th, and 40 K) in the soil samples was determined using a 5" × 4" Na (I) γ-ray spectrometric system coupled with a 1K multichannel analyzer (NETS-3MU, Electronic Enterprises Private Limited, India). The detector was housed inside a massive lead shield to reduce the background of the system. It was calibrated using a standard solution of 226 Ra in equilibrium with its daughters (obtained from NBS, USA), mixed with a simulated soil matrix, and counted in the same geometry as that of the soil samples. Three International Atomic Energy Agency (IAEA) standard reference materials (a standard soil of known radioactivity Fsoil- 6, Uranium ore sample-RGU1, and Thorium ore sample-RGTh1) were also used for checking the calibration of the system. The minimum detectable activity (MDA) for each radionuclide was determined from the background radiation spectrum, which was estimated to be 8.5 Bq kg -1 for 226 Ra, 1.0 Bq kg -1 for 232 Th, and 13.2 Bq kg -1 for 40 K.

Each sample, after attaining the equilibrium, was kept on top of the Na (I) detector, and counted for a period of 50000 seconds. The activity of 226 Ra was evaluated from the gamma line 1.76 MeV for 214 Bi of the 226 Ra peak, while the 2.61 MeV gamma line of the 208 Ti peak was used to determine 232 Th, and the 40 K activity was determined from the gamma peak at 1.46 MeV. The activity of each radionuclide in the sample was calculated using the total net counts under the selected photo peaks after subtracting the appropriate background counts, and applying the appropriate factors for photo-peak efficiency, gamma intensity of the radionuclide, and weight of the sample. The analysis of the gamma spectra obtained was performed using the software Microsoft Excel.

  Results and Discussion Top

Activity concentration of radionuclides in soil

The radionuclide activity concentration in the soil samples taken from 40 locations are presented in [Table 1]. Specific activities of 226 Ra, 232 Th, and 40 K are reported in Bq kg -1 dry weight. The mean activity concentrations of 226 Ra, 232 Th, and 40 K in the surface soil samples from Tiruchirapalli are 29.9, 39.0, and 369.7 Bq kg -1 , respectively. The higher activity concentration of 226 Ra and 232 Th in the soil found in some areas may be correlated with the presence of radioactive rich granites, sandstones, and quartzites. However, a detailed geochemical investigation is required to reach some conclusion. The results obtained in this study are comparable to the worldwide average concentration of these radionuclides in soils reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), [7] which are 35, 30, and 400 Bq kg -1 for 226 Ra, 232 Th, and 40 K, respectively. These values are also comparable to the average values in the soils of India [Table 2]. These results are also substantiated by the statistical data of the natural radionuclide concentrations in the soil samples. It can be observed that the skewness and kurtosis coefficients of all radionuclides are not closer to the null value indicating the non-existence of a normal distribution [Table 3] and [Figure 2]. The positive values of the kurtosis coefficient for the radionuclides 232 Th and 226 Ra indicate that the distribution is higher and narrower than normal. The correlation between 226 Ra and 232 Th in the soil samples is found to be reasonably good (R = 0.74) indicating symmetrical distribution in the soil samples [Figure 3].
Figure 2: Frequency distributions of the activities of 226Ra, 232Th, and 40K (Bq kg-1 dry weight) and relative frequency of distribution of the gamma dose rate at 1 m above ground level (nGy h-1)

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Figure 3: Correlation between 226Ra and 232Th in the soil samples of Tiruchirapalli, South India

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Table 1: Activity concentrations of 226Ra, 232Th, 40K in the soil samples from Tiruchirapalli, South India

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Table 2: Comparison of activity concentration of 226Ra, 232Th, and 40K in surface soil samples in the vicinity of MAPS, Kalpakkam with other parts of India

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Table 3: Statistical data for radioactivity concentrations of 226Ra, 232Th, 40K, Raeq, and absorbed dose rate for surface soil samples from Tiruchirapalli, South India

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Absorbed dose rate from the soil

The absorbed dose rate, D (nGy h -1 ), at a height of 1 m above the ground surface, in air, due to the concentration of 226 Ra, 232 Th, and 40 K in the soil samples are presented in [Table 4]. The dose was calculated using the absorbed dose rate activity conversion factors, depending on the radionuclides analyzed in the soil. The conversion factor described by UNSCEAR [7] was adopted and the gamma absorbed dose rates were calculated using the equation followed by Kannan et al., [15] as given below.
Table 4: The dose rate, effective dose rate, Radium equivalent activity (Raeq), external hazard index (Hex) radioactivity level index (Iγ ), and total gamma-radiation dose rate of the soil samples from Tiruchirapalli, South India

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The total absorbed external gamma dose rate varies from 39.5 to 124.9 nGy h−1 , with an average of 79.9 nGy h−1 . This is higher than the reported global average of 55 nGy h -1 , but falls within the reported world range of 28-120 nGy h -1 . [16] The differences are considered to be due to the geomorphology and land use patterns, which vary from one place to another and from one locality to another, in the same zone. The estimation of the mean dose rate may be important for determining the radiation detriment to the population as a whole. A common feature in any environmental radiation measurement is the considerable variation in soil radioactivity with location depending on the soil physicochemical parameters. Therefore, the estimation of contribution by each of the radionuclides 238 Ra, 232 Th, and 40 K, to the total external radiation dose rate was estimated to be 17.2, 63.4, and 19.3%, respectively, and is important in estimating the contribution of these radionuclides to the local population. The absorbed gamma dose rate due to the thorium series in these soil samples has been found to be nearly 3.7 times greater than that due to 226 Ra, and this may be due to the higher concentration of the 232 Th concentration in the soils.

To estimate the annual effective dose rates, the conversion coefficient from the absorbed dose in the air to the effective dose (0.7 Sv Gy -1 ) and the outdoor occupancy factor (0.2) proposed by UNSCEAR [7] were used. The effective dose rate was calculated from the formula adopted by Yang et al. [17]

In estimating the effective dose in any environment, the two factors of importance are the conversion coefficient from Gy h -1 to Sv h -1 and the occupancy factor. The former gives the equivalent human dose in Sv y -1 from the absorbed dose rate in air (Gy h -1 ), while the latter gives the fraction of the time an individual is exposed to outdoor radiation. The first factor has been recommended by the UNSCEAR [7] as 0.7 Sv Gy -1 and the second factor as 0.2, which suggests that from the absorbed dose in air to the effective dose received by adults and considering that people in India, on an average, spend ~20% of their time outdoors, the annual effective doses are calculated. [5],[15] This factor suits the pattern of life in the studied area, yielding the outdoor effective dose as given in [Table 4]. The indoor dose rates have not been evaluated because the essential data on the average buildup of radon gas in the indoor atmosphere are not available. The corresponding outdoor annual effective dose range from 48.3 to 153.1 μSv y -1 , with an average value of 97.9 μSv y -1 , was calculated, which is slightly above the world average of 80 μSv y -1 . [7] Thus, our results are relatively higher than the average worldwide values reported for the normal background regions.

Radiation hazard indices

The distribution of 226 Ra, 232 Th, and 40 K in soil are not always uniform. The Ra eq activity is an index that has been introduced to represent the specific activities of 226 Ra, 232 Th, and 40 K by a single quantity, which takes into account the radiation hazards associated with them, and the maximum value of Ra eq must be less than 370 Bq kg -1 . [18] This index can be calculated according to the equation given by Beretka and Mathew: [18]

Where, CRa , CTh , and CK are the specific activities of 226 Ra, 232 Th, and 40 K in Bq kg -1 , respectively. The values of Ra eq for the soil samples are given in [Table 4]. The radium equivalent activity (R eq ) of all soil samples range from 84.3 Bq kg -1 to 280.4 Bq kg -1 , with an average value of 178.2 Bq kg -1 , and this value is much lower than the acceptable limit. [18] All the Ra eq values of samples are below the internationally accepted value, as mentioned earlier. Accordingly, any radium equivalent activity concentration that exceeds 370 Bq kg -1 may pose radiation hazards. [19]

The external hazard index (H ex ) is another radiation hazard index defined by Beretka and Mathew [18] to evaluate the indoor radiation dose rate due to external exposure to γ-radiation from the natural radionuclides in the construction building materials of dwellings. This index value must be less than unity to keep the radiation hazard insignificant, that is, the radiation exposure due to the radioactivity from the construction materials must be limited to 1.5 mSv y -1 based on the following criterion [18] :

Where, CRa , CTh , and CK are the specific activities of 226 Ra, 232 T h, and 40 K in Bq kg -1 , respectively. The calculated results of H ex for the soil samples range from 0.23 to 0.76, with an average of 0.48 [Table 4]. These values are far below the criterion limit (H ex = ≤1) as per the European Commission on Radiation Protection reports [19] , and the terrestrial soil around Tiruchirapalli is not contributing to higher exposure for the inhabitants and can be used as a construction material without posing any significant radiological threat to the population.

The radioactivity level index, Iγ, can be used to estimate the gamma-radiation hazard levels, typically for those associated with natural radionuclides. The representative level of radiation hazard index can be estimated according to the equation adopted by Sroor, et al.: [20]

The calculated values for the Iγ range, between 0.6 and 2.0, with an average value of 1.3, were higher than the international values (Iγ ≥1). In the present study, the calculated values for most sampling sites were higher than the international values and in the remaining sites the values were close to the upper limit.

The total gamma-radiation dose rate is modified to include the contributions from natural radionuclides and cosmic radiation, according to the following equation: [21]

Where C is the activity concentration of natural radionuclides and the number 34 is a factor included to take care of contribution from cosmic radiation. The results of the total gamma-radiation dose rates of natural radionuclides and cosmic radiation are presented in [Table 4]. As it can be seen from the table, the results of total gamma-radiation dose rates range between 73.5 and 158.9 nGy h -1 , with an overall average value of 113.9 nGy h -1 , which is above the reported values/ranges.

  Conclusion Top

In the present study, γ-ray spectrometry has been used to determine the soil radioactivity concentrations of 226 Ra, 232 Th, and 40 K in 40 soil samples collected from Tiruchirapalli, South India. The results show that the average concentrations of 226 Ra, 232 Th, and 40 K in the studied soil samples are comparable to the worldwide average concentration of these radionuclides in the soils and are also comparable to the average values of the soils of India [Table 2]. The total absorbed external gamma dose rates and the annual effective dose have been found to be relatively higher than the reported average global primordial radiation levels. The values of Ra (eq) and H ex are below the internationally accepted values. The calculated values for the Iγ and total gamma radiation dose are higher than the international values. The values obtained for natural radioactivity and γ-absorbed dose rates due to the activity concentration of 226 Ra, 232 Th, and 40 K in the soil and in air shows that Tiruchirapalli can be regarded as an area with normal natural background radiation.

  Acknowledgment Top

The authors wish to acknowledge the assistance of Mr. S. Jeevanandham, Department of Geology, National College, Tiruchirapalli, for assisting in sample collection and processing of samples for Gamma ray measurements. Acknowledgments are also extended to Mr. R. Elumalai, Institute for Ocean Management, Anna University, Chennai for his Technical Assistance during the measurement of Natural Radionuclides in soil samples and Mr. Rajkumar, IOM, Anna University for preparation of the map of the study area.

  References Top

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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3], [Table 4]


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