Assessment of radioactivity level and radiological parameters in soil samples of Akalad, Thrissur District, Kerala

V Vineethkumar; V Prakash; Y Narayana

doi:10.4103/rpe.RPE_45_18

ORIGINAL ARTICLE

Year : 2018 | Volume : 41 | Issue : 3 | Page : 128-131

Assessment of radioactivity level and radiological parameters in soil samples of Akalad, Thrissur District, Kerala

V Vineethkumar¹, V Prakash¹, Y Narayana²
¹ Department of Studies and Research in Physics, Payyanur College, Kannur, Kerala, India
² Department of PG Studies in Physics, Mangalore University, Mangalagangotri, Karnataka, India

Date of Submission	20-Jun-2018
Date of Decision	23-Jul-2018
Date of Acceptance	28-Aug-2018
Date of Web Publication	19-Nov-2018

Correspondence Address:
Dr. V Prakash
Department of Studies and Research in Physics, Payyanur College, Kannur - 670 327, Kerala
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/rpe.RPE_45_18

Abstract

Systematic studies on radiation level and distribution of radionuclides have been carried out using NaI(Tl) detector in different locations of Akalad region, Thrissur District, Kerala. The activity of naturally occurring radionuclides viz. ⁴⁰K, ²²⁶Ra and ²³²Th in the samples collected from the region were measured and found to vary in the range 9.99±0.69 Bq kg^-1 to 32.11±0.77 Bq kg^-1 with a mean value 19.64 Bq kg^-1; 2.15±0.21Bq kg^-1 to 11.28±0.39 Bq kg^-1 with a mean value 7.73 Bq kg-1 and 20.80±0.67 Bq kg-1 to 122.40±1.16 Bq kg-1 with a mean value 79.91 Bq kg-1 respectively. The radium equivalent activity varies within the range 39.26 Bq kg^-1 to 178.85 Bq kg^-1 with a mean value 123.51 Bq kg^-1. The absorbed dose rate varies within the range 16.49 nGyh^-1 to 79.14 nGyh^-1 with a mean value 51.91 nGyh^-1. The present systematic investigation indicates that the data are comparable with the reported values elsewhere and in most of the cases observed values were well within the permissible limit.

Keywords: ²²⁶Ra, ²³²Th, ⁴⁰K, natural radioactivity, radiological parameters

How to cite this article:
Vineethkumar V, Prakash V, Narayana Y. Assessment of radioactivity level and radiological parameters in soil samples of Akalad, Thrissur District, Kerala. Radiat Prot Environ 2018;41:128-31

How to cite this URL:
Vineethkumar V, Prakash V, Narayana Y. Assessment of radioactivity level and radiological parameters in soil samples of Akalad, Thrissur District, Kerala. Radiat Prot Environ [serial online] 2018 [cited 2023 Jun 2];41:128-31. Available from: https://www.rpe.org.in/text.asp?2018/41/3/128/245798

Introduction

Natural radionuclides are widely distributed in soil, rocks, and air. Assessment of natural radioactivity level in the environmental matrices has great importance because natural radiation is the largest contributor to the external dose to the world population.^[1] The distribution and enrichment of radionuclides in various environmental matrices will also be different. There are few regions in the world, the radiation levels were found to be high and termed as high background radiation areas. Coastal Kerala is one of the important parts of the south-west coast of India and a known high background radiation area. In Kerala, especially the places in Kollam district, such as Chavara, Neendakara, Karunagapally, are reported as high background radiation areas.^[2] Akalad is situated in Thrissur district, part of the coastal environment of Kerala. In view of this, an attempt was made to assess the radionuclide concentration of the soil samples collected from the environment of Akalad, and the results are discussed in the paper. Radium equivalent activity and radiological parameters due to natural radiation exposure were also identified and discussed in the light of reported literature values.

Materials and Methods

Sample collection and preparation

For the present study, different sampling locations from Akalad region that showed high activity compared with the normal background area read on a portable plastic scintillometer were chosen. Soil samples were collected from 10 different locations in and around the Akalad region following standard procedures.^[3] About 2 kg of samples were collected from a depth of 10–30 cm in a polythene bag from each sampling locations and were brought to the laboratory for further analysis. The samples were cleaned and air dried at room temperature in open air. Then, the samples were dried in an oven at 110°C for 24 h to remove the moisture contents. The processed samples were then sieved through 250 μm mesh and kept in airtight standard geometry plastic containers of 250 ml capacity, for 30 days to prevent the escape of radiogenic gases radon (²²²Rn) and thoron (²²⁰Rn) and to allow radioactive equilibrium with their corresponding progeny.^[4] The samples were then analyzed following standard techniques.

Experimental details

The exact net weight of the samples were determined using electronic balance before taken for the analysis. Each sample weighing about 275 g was subjected to gamma spectrometric analysis using high efficiency 5 cm × 5 cm NaI (Tl) based gamma-ray spectrometer. The spectrometer was calibrated using RG-U, RG-Th, and RG-K, as standard sources. These are standards sources for Uranium, Thorium, and Potassium, procured from the International Atomic Energy Agency, Vienna. The full-width half-maximum was 60.78 KeV with resolution 8.46%, for the ¹³⁷Cs (661 KeV) peak. The detector was shielded with lead blocks of size 6” × 3” × 1.75”, to reduce the counts due to terrestrial gamma-ray radiations. The samples were counted for sufficiently long time (40,000 s) to reduce the counting error using GSPEC software (Amcrys, Nauki Ave, Kharkov, Ukrain) to obtain the gamma-ray spectrum with good statistics. The concentration of radionuclides, namely, ⁴⁰K, ²²⁶Ra and ²³²Th in the samples was determined from the counts obtained. The ²²⁶Ra radionuclide was estimated from 1764 KeV (15.9%) gamma peaks of ²¹⁴Bi. ²³²Th was estimated using 2614 KeV (35.8%) gamma transition of ²⁰⁸Tl. ⁴⁰K radionuclide was estimated using 1460.8 KeV (10.7%) gamma peak from ⁴⁰K itself to determine the concentration of ⁴⁰K in different samples.^[5],[6] The spectra are analyzed for the photopeak of radium, thorium daughter products, and potassium.

In the present study, simultaneous equation method was used for the analysis of the spectrum and to determine the activity concentrations of radionuclides.^[7]

C₁ = T_2.61

C₂ = T_1.76-F₁C₁

C₃ = T_1.46-F₂C₃-F₃C₂

Here, C₁, C_2, and C₃ are the compton-corrected and background-subtracted counts under the photopeaks of ²³²Th, ²²⁶Ra, and ⁴⁰K and T_2.61,T_1.76 and T_1.46 are the total integral count under the photopeaks of ²⁰⁸Tl, ²¹⁴Bi ⁴⁰K, respectively, and F_1,F_2, and F₃ are Compton contribution factors of ²³²Th on ²²⁶Ra, ²³²Th on ⁴⁰K and ²²⁶Ra on ⁴⁰K, respectively.^[8]

From the compton-corrected count, the activity (A, Bq/kg) of ⁴⁰K, ²²⁶Ra and ²³²Th were estimated, using the given relation.

Where, C is the compton-corrected count rate under the photopeak, SD is the standard deviation, E is the photopeak efficiency (%) of the detector, a is the abundance of characteristic gamma-ray, and W is the weight of the sample in grams.

In the coastal regions of Kerala, a wide variation in the radiation level has been observed. To obtain uniformity in the exposure, total activity was calculated regarding radium equivalent activity (Ra_eq) from the given relation.^[9]

Ra_eq = A_Ra + 1.43 A_Th + 0.077 A_K

Where, A_Ra, A_Th, and A_K are the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K expressed in Bq/kg. Radium equivalent concept allows a single index or number and is widely used hazard index to describe the gamma output from different mixers of uranium, thorium, and potassium in the samples.^[10]

Absorbed dose rate due to gamma radiations in the air at 1 m above ground level for the uniform distribution of naturally occurring nuclides was calculated from the given equation.^[11]

D = 0.462 C_Ra + 0.604 C_Th + 0.0042 C_K

Where D is absorbed dose rate in nGyh⁻¹ and C_Ra, C_Th, and C_K are the activities of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples expressed in Bq/kg.

The annual effective dose rate is determined by considering the conversion coefficient from absorbed dose in air to effective dose as 0.7 SvGy⁻¹ with indoor and outdoor occupancy of 80% and 20%, respectively.^[12] It is calculated from the given equation.

Indoor (mSv) = D (nGyh⁻¹) × 8760 × 0.8 × 0.7 (SvGy⁻¹)

Outdoor (mSv) = D (nGyh⁻¹) × 8760 × 0.2 × 0.7 (SvGy⁻¹)

The external gamma index, due to the emitted gamma rays, from the sand samples were calculated, using the relation.^[13]

H_ex= (A_Ra/370 Bq/kg) + (A_Th/259 Bq/kg) + (A_K/4810 Bq/kg) ≤1

The internal gamma index factor was calculated to assess the radiation exposure due to radon and its short-lived daughter products, which are hazardous to respiratory organs.^[14] The internal exposure to radon and its daughter products is quantified by internal exposure index which is given by the equation:

H_in= (A_Ra/185 Bq/kg) + (A_Th/259 Bq/kg) + (A_K/4810 Bq/kg) ≤1

Where, A_Ra, A_Th, and A_K are activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K, in Bq/kg.

Results and Discussion

[Table 1] represents the activity concentration of ⁴⁰K, ²²⁶Ra, and ²³²Th in the collected samples. The activity of radionuclides, namely, ⁴⁰K, ²²⁶Ra, and ²³²Th found to vary in the range 9.99 ± 0.69–32.11 ± 0.77 Bq/kg with a mean value 19.64 Bq/kg; 2.15 ± 0.21–11.28 ± 0.39 Bq/kg with a mean value 7.73 Bq/kg and 20.8 ± 0.67–122.4 ± 1.16 Bq/kg with a mean value 79.9 Bq/kg, respectively. The result obtained in the present study reveals that the activity concentration of ⁴⁰K and ²²⁶Ra is lower and the activity concentration of ²³²Th is higher than the Indian average values (Indian average values of ⁴⁰K, ²²⁶Ra and ²³²Th is 400, 28.67, and 63.83 Bq/kg, respectively).^[1] The activity concentration of ⁴⁰K and ²²⁶Ra is lower and activity concentration of ²³²Th is higher than the world average values (World average values of ⁴⁰K, ²²⁶Ra and ²³²Th is 420, 33 and 45 Bq/kg, respectively).^[1] The radium equivalent activity varies within the range 39.26–178.85 Bq/kg with a mean value 123.51 Bq/kg. The average radium equivalent activity measured from the activity is well below the world average of 370 Bq/kg.

Table 1: Activity of ⁴⁰K, ²²⁶Ra and ²³²Th

Click here to view

[Table 2] represents the radiological parameters obtained from the radionuclide activities. The absorbed dose rate varies within the range 16.49–79.14 nGyh⁻¹ with a mean value 51.91 nGyh⁻¹ and is lower than the world average value of 57 nGyh⁻¹. The indoor annual effective dose varies in the range 0.08-0.388 mSv with an average value 0.254 mSv and outdoor in the range 0.02-0.097 mSv with an average value 0.063 mSv. The average indoor annual effective dose was higher and the outdoor annual effective dose was lower than the world average value of 0.07 mSv.^[1] The internal exposure index varies from 0.126-0.538 with an average value 0.354 and the external exposure index varies from 0.106 to o. 508 with a mean value 0.333. The values of internal and external exposure index were found to be less than unity.

Table 2: Radiological parameters

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Conclusions

The concentrations of natural radionuclides have been determined using gamma-ray spectrometry and the radiological parameters, namely, radium equivalent activity, absorbed dose rate, annual effective dose (indoor and outdoor), exposure indices (internal and external) were calculated from the activity and compared with the Indian and World average values. The activity concentration of ⁴⁰K and ²²⁶Ra is lower and the activity concentration of ²³²Th is higher than the permissible limit. The systematic analysis of the samples collected clearly indicates that ²³²Th is the major contributor for the activity concentration. The soils weathered from the rocks, which are rich in heavy metals and radioactive minerals can contribute to the enhanced level of ²³²Th activity. Industrial activities present in the region may also affects the radioactivity levels in the region. The contribution of ²²⁶Ra to the total dose is very low compared to ⁴⁰K and ²³²Th for the activity concentration in the samples. The wide variation in the radionuclides activity concentration is influenced by geological and geochemical processes. The values obtained for the radiological parameters were well within the recommended safety limits except for the indoor annual effective dose rates. More systematic analysis is needed to understand the various processes influencing the activity concentration.

Acknowledgments

The first author wishes to acknowledge the UGC, New Delhi, for providing financial support as JRF to carry out the present investigation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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