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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 1  |  Page : 37-41  

Natural radionuclide concentration in Hassan district of South India


Department of Physics, Mangalore University, Mangalagangotri, Karnataka, India

Date of Submission31-Jan-2018
Date of Decision19-Feb-2018
Date of Acceptance13-Mar-2018
Date of Web Publication31-May-2018

Correspondence Address:
Mr. B G Jagadeesha
Department of Physics, Mangalore University, Mangalore, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_13_18

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  Abstract 

Soil samples were collected from Hassan district of South India. The analysis was carried out using NaI (Tl) gamma ray spectrometer. The 232Th activity was found to vary from 20.4 ± 1.3 Bq/kg to 136.3 ± 3.3 Bq/kg with a mean value of 56.9 ± 2.0 Bq/kg, and 226Ra activity was found to vary from 22.5 ± 1.4 Bq/kg to 90.6 ± 3.4 Bq/kg with a mean value of 41.4 ± 2.1 Bq/kg. The activity of 40K was ranged from 278.2 ± 4.0 Bq/kg to 650.7 ± 6.7 Bq/kg with a mean value of 388.4 ± 5.1 Bq/kg. Relatively higher levels of thorium were observed in soil. The radium equivalent activity and other radiological parameters were also determined.

Keywords: External hazard index, internal hazard index, NaI (Tl) spectrometer, radium equivalent activity


How to cite this article:
Jagadeesha B G, Narayana Y. Natural radionuclide concentration in Hassan district of South India. Radiat Prot Environ 2018;41:37-41

How to cite this URL:
Jagadeesha B G, Narayana Y. Natural radionuclide concentration in Hassan district of South India. Radiat Prot Environ [serial online] 2018 [cited 2019 Jun 25];41:37-41. Available from: http://www.rpe.org.in/text.asp?2018/41/1/37/233645


  Introduction Top


Human beings are exposed to radiation by natural radionuclides present in air, water, rocks, plants, and soil.[1] The naturally occurring radionuclides present in soil include 226 Ra,232 Th, and 40 K.[2] These radionuclides contribute to the total absorbed dose through inhalation, ingestion, and external irradiation.[3] The distribution of radionuclides is not uniform in the earth's crust and their dispersion in the soil enables to assess radiation exposure potential to population of the region. It is important to determine the level of natural radioactivity in soil and to evaluate the gamma dose rate arising from the earth crust for the outdoor occupants. The radionuclide concentration in soil becomes toxic to the living organisms only if it exceeds permissible level. The global average radionuclide activity in soil is around 30, 35, and 400 Bq/kg for 238 U,232 Th, and 40 K, respectively.[4] The estimation of external gamma dose due to terrestrial sources is essential to assess the individual doses. The dose variation depends on the concentration of natural radionuclides present in the soil and also local geology of the region.

Hassan district of South Karnataka is one of the highly mineralized districts in India and the second largest producer of chromite in the country.[5] The most important rock formations of the district are chromate, titaniferous magnetite, chalcopyrite, asbestos, and quartz.[6] Due to the presence of different rock formations in this region, it is necessary to determine the level of natural radioactivity in the soil. The main objective of the study is to determine the activity concentration of natural radionuclides in the soil samples collected from Hassan district of South Karnataka and to evaluate the radiological hazard due to natural radioactivity in the soil.


  Materials and Methods Top


The soil samples were collected from 32 locations of Hassan district, keeping in view of different rock formations and mineral deposits in the region. The study area covers about 6826 km 2. [Table 1] presents the longitude and latitude of the sampling locations of the region. The samples were collected following standard protocol.[7],[8],[9] To ensure a representative sample of the location, four soil samples from 100 m apart were collected, pooled together, and mixed thoroughly. Then, the soil samples were divided into four equal parts; one part of the sample was taken as the representative sample of the location. The collected soil samples were taken to the laboratory and dried at about 110°C to remove the moisture content. The samples were crushed and sieved in a 250-μ sieve. The samples were sealed in 300 ml plastic container and kept for a period of 30 days for secular equilibrium between radon and its progeny.[10]
Table 1: Sampling locations

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The analysis was carried out using NaI (Tl)-based gamma ray spectrometer. The detector having the size 5” × 5” with multichannel analyzer was used. The calibration was done using standard sources such as RG-U, RG-Th, and RG-K, procured from the International Atomic Energy Agency, Vienna. To reduce counts from background radiation, the detector was provided with graded shielding. The GSPEC software (Para Electronics- Manufacturing Division of Electronic Enterprises, Mumbai, India) was used to obtain gamma ray spectrum with acquisition time of 80,000 s.[11] Around 98.5% of the total dose was received from the U-238 series, especially from the 226 Ra and its progenies.[11] The choice of radionuclides to be detected as a reference was made based on the fact that the NaI (TI) detector used in this study had a modest energy resolution. The photons emitted by them would be sufficiently discriminated if their emission probability and their energy were high enough and the surrounding background continuum was low enough. Therefore,214 Bi having gamma peaks at 1764 keV (15.9%) was used to estimate 226 Ra in the sample, while that of the daughter radionuclide 208 Tl having an energy 2614 keV (35.8%) was used to estimate 232 Th because the secular equilibrium was achieved between the daughter nuclides and their parent nuclides. The 40 K concentration was estimated from 40 K itself having energy 1460.8 keV (10.7%).[11],[12],[13],[14]

Several methods are available in the literature for the analysis of the spectra from Nal (TI) detector. The simultaneous equation method, Spectrum stripping, and method of least squares are the commonly used methods.[15] The simple stripping method can be used if the spectrum has two gamma energies such as 40 K and 137 Cs which are well separated. The simultaneous equation method can be employed for the analysis, where samples contain more than two radionuclides. In the present study, simultaneous equation method [15],[16] was used for the analysis of the spectrum and to determine the activity concentration of various radionuclides. In this method, the Compton corrected and background subtracted count rates were used for the three radionuclides of interest.[15],[16],[17] The following simultaneous equation method was used to analyze the spectrum and calculate the concentration of 232 Th,226 Ra, and 40 K.[15],[18]







Where C1, C2, and C3 are Compton corrected and background subtracted counts of the photo peaks of 232 Th,226 Ra, and 40 K, respectively. The Compton contribution factors are F1, F2, and F3. The total integral counts are T2.61, T1.76, and T1.46 under the photo peaks of 208 Tl,214 Bi, and 40 K, respectively.

The radionuclide activity (A) was determined using the following formula.[19]



Where C is the Compton corrected net counts under the photo peak, SD is the standard deviation, E is the photo peak efficiency of the detector, A is the abundance of the characteristic gamma ray,[7] and W is the weight of the samples in grams.


  Results and Discussion Top


The range and mean activity of 232 Th,226 Ra, and 40 K are presented in [Table 2]. The activity of 232 Th varied from 20.4 ± 1.3 Bq/kg to 136.3 ± 3.3 Bq/kg with a mean value of 56.9 ± 2.0 Bq/kg, and the range and mean activity of 226 Ra ranged from 22.5 ± 1.4 Bq/kg to 90.6 ± 3.4 Bq/kg with a mean value of 41.4 ± 2.1 Bq/kg. The range and mean activity of 40 K ranged from 278.2 ± 4.0 Bq/kg to 650.7 ± 6.7 Bq/kg with a mean value of 388.4 ± 5.0 Bq/kg. The 232 Th and 40 K vary widely in the region, but the activity of 226 Ra is almost constant in all sampling locations. This is due to the fact that 226 Ra does not migrate in soil unless it is deposited in ground. The high activity of 232 Th may be due to the presence of metamorphic rocks, Gneiss, and Quartz in these areas.[20],[21] From the results, it clearly shows that the mean activity of 40 K was highest as compared to 232 Th and 226 Ra. The variation of activity of radionuclides is shown in [Figure 1]. The obtained mean activity of 40 K was within the world average value of 420 Bq/kg. The obtained activity of 226 Ra and 232 Th was higher than the world average value of 33 Bq/kg and 45 Bq/kg, respectively.[22] The obtained radionuclide concentrations varied from place to place depending on the geological rock formation in the region. The comparison of the activity of natural radionuclide with other regions of the world are presented in [Table 4].
Table 2: Measured gamma activity of 232Th, 226Ra, and 40K in soil samples

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Figure 1: Activity of 232Th, 226Ra, and 40K in soil samples

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Table 4: Comparison of the activity of 226Ra, 232Th, and 40K

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Radium equivalent activity

Radiation exposure for uniform scale, for all natural radionuclides, is defined in terms of radium equivalent activity (Raeq) and calculated using the following formula.[23]



Where ARa is the 226 Ra activity concentration in Bq/kg, ATh is the 232 Th activity concentration in Bq/kg, and AK is the 40 K activity concentration in Bq/kg. The above equation is the estimate of 0.7 Bq/kg of 232 Th, 13 Bq/kg of 40 K, and 1 Bq/kg of 226 Ra which produces the same gamma dose rate. The obtained Raeq is presented in [Table 3] and is varied from 106.36 Bq/kg to 257.41 Bq/kg with a mean value of 152.67 Bq/kg. The observed Raeq is less than the recommended limit of 370 Bq/kg.[24]
Table 3: Radiological hazardous parameters

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External hazard index

The external exposure to natural radionuclide in soil is quantified by the external hazard index (Hex) and calculated using the following formula.[25]



It is found that Hex varied from 0.29 to 0.70 with a mean value of 0.41 and is less than unity for the radiation hazard to be negligible.

Internal hazard index

The internal exposure to radon and its daughter products is quantified by the internal hazard index (Hin) and is calculated using the following formula.[26]



The obtained Hin varied from 0.35 to 0.91 with a mean value of 0.52. The obtained Hin is less than unity for the radiation hazard to be negligible. The Hin and Hex values are presented in [Table 3].

Absorbed gamma dose rate

Absorbed gamma dose rate (nGy/h) in air above the ground surface due to the activity of 226 Ra,232 Th, and 40 K is calculated using the following formula.[1]



The obtained absorbed gamma dose rate varied from 49 to 116 nGy/h, with a mean value of 71 nGy/h.

Representative level index

Representative level index (Iϒr) is used to estimate the level of gamma radiation hazard associated with natural radionuclide concentration which is calculated using the following formula.[27]



The obtained Iϒr varied from 0.77 to 1.84 with a mean value of 1.10. The Iϒr value is less than unity for the radiation hazard to be negligible.


  Conclusions Top


The gamma activity of 232 Th,226 Ra, and 40 K in soil samples was measured using NaI (Tl)-based gamma ray spectrometer. The radionuclide concentration varied from place to place due to the presence of different geological formations of the region. Relatively higher activity of 232 Th was observed in soil. There is no significant implication due to the observed higher 232 Th activity in soil. The Raeq was found below the recommended limit of 370 Bq/kg and the radiological parameters were also found within the safety limits. [37]

Acknowledgments

One of the authors Jagadeesha B.G gratefully thank Mangalore University for providing financial support.

Financial support and sponsorship

This study was financially supported by Mangalore University.

Conflicts of interest

There are no conflicts of interest.

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



 

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