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TECHNICAL NOTE
Year : 2018  |  Volume : 41  |  Issue : 2  |  Page : 99-103  

Assessment of natural radioactivity and radiation index parameters in the coastal environment of Kerala


1 Department of Physics, Payyanur College, Kannur, Kerala, India
2 Department of Physics, Mangalore University, Mangalore, Karnataka, India

Date of Submission31-Jan-2018
Date of Decision09-Apr-2018
Date of Acceptance17-Jun-2018
Date of Web Publication24-Aug-2018

Correspondence Address:
Dr. V Prakash
Department of Physics, Payyanur College, Edat, Kannur - 670 327, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_14_18

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  Abstract 

Measurement of natural radioactivity concentration in sand samples collected from the coastal belt of Kerala has been carried out using NaI(Tl) detector, and the radiological parameters were evaluated from the measured activities. It is well reported that in certain situation, the natural radioactivity in the environmental matrices can reach reference levels or beyond. Hence, the study has been performed to understand the distribution and enrichment of natural radionuclides in the coastal environment of Kerala, and thereby assessing dose to the inhabitants. The mean activity concentration of naturally occurring radionuclides, namely 40K, 226Ra, and 232Th in the samples were 53.2, 7.0, and 101.3 Bq/kg, respectively. The radiological parameters, namely absorbed dose rate, the radium equivalent activity (Raeq), the annual effective dose (indoor and outdoor), and reference indices were evaluated and compared to the recommended safety limits. The present 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 and 232Th, 40K, natural radioactivity, radiation index parameters


How to cite this article:
Vineethkumar V, Kaliprasad C S, Prakash V. Assessment of natural radioactivity and radiation index parameters in the coastal environment of Kerala. Radiat Prot Environ 2018;41:99-103

How to cite this URL:
Vineethkumar V, Kaliprasad C S, Prakash V. Assessment of natural radioactivity and radiation index parameters in the coastal environment of Kerala. Radiat Prot Environ [serial online] 2018 [cited 2019 May 22];41:99-103. Available from: http://www.rpe.org.in/text.asp?2018/41/2/99/239679


  Introduction Top


Radioactive nuclides that are part of air, water, soil, sand, rocks, and vegetation and the like-contain varying amounts of radioactivity.[1] Hence, site-specific variation in the dose received by the inhabitants will be expected. Coastal environment of Kerala is a well-reported high background radiation area, especially the places, namely Chavara, Neendakara, and Karunagappally [2] and dose to the population from these areas is expected to be high. The assessment of gamma-radiation dose from natural sources also assumes great significance as the natural radiation is the largest contributor to external dose to the world population.[3] Given this, a systematic investigation of radioactivity level in sand samples collected from coastal belt of Kerala has been carried out, and radiological parameters have been evaluated from the activities. The results are presented and discussed in detail in the manuscript.


  Materials and Methods Top


Sample collection and preparation

In the present study, sand samples were collected from nine different locations, namely Bekal (S1), Muzhappilangad (S2), Kappad (S3), Akalad (S4), Mararikulam (S5), Alappuzha (S6), Azheekal (S7), Kollam (S8), and Kovalam (S9) along the coastal belt of Kerala [Figure 1] following standard procedures and techniques,[4] and from each location, four samples were collected. The analysis of more number of samples from a specific location will give good representation of the activity level. About 2 kg of the samples were collected in a polythene bag and brought to the laboratory for further processing. The samples were cleaned and air dried at room temperature and then dried in an oven at 110°C for 24 h until constant dry weight is obtained. The dried samples were then sieved through 250-μm mesh and stored in airtight plastic containers for 30 days to prevent the escape of radiogenic gases radon (222 Rn) and thoron (220 Rn) and to allow radioactive equilibrium with their corresponding progeny.[5] The samples were then analyzed following standard techniques.
Figure 1: Map of sampling locations

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Experimental details

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 standard sources for uranium, thorium, and potassium procured from the International Atomic Energy Agency, Vienna. The full-width half-maximum was 60.78 KeV with a resolution of 8.46%, for the 137 Cs (661 KeV) peak. The detector was shielded with lead blocks of size 6” × 3” × 1.75”, to reduce counts due to terrestrial gamma-ray radiations. The samples were counted for sufficiently long time (40,000 s) using GSPEC software (Amcrys, Nauki Ave, Kharkov, Ukrine.) to obtain the gamma-ray spectrum with good statistics. The 226 Ra radionuclide was estimated from 1764 KeV (15.9%) gamma peaks of 214 Bi,232 Th was estimated using 2614 KeV (35.8%) gamma transition of 208 Tl, and 40 K radionuclide was estimated using 1460.8 KeV (10.7%) gamma peak from 40 K itself to determine the concentration of 40 K in different samples.[6],[7] The spectra are analyzed for the photopeak of radium, thorium daughter products, and potassium.

In the present work, simultaneous equation method was used for the analysis of the spectrum and to determine the activity concentrations of radionuclides.[8]

C1 = T2.61

C2 = T1.76 − F1C1

C3 = T1.46 − F2C3 − F3C2

Here, C1, C2, and C3 are the Compton-corrected and background-subtracted counts under the photopeaks of 232 Th,226 Ra, and 40 K, and T2.61, T1.76, and T1.46 are the total integral count under the photopeaks of 208 Tl,214 Bi, and 40 K, respectively, and F1, F2, and F3 are Compton contribution factors of 232 Th on 226 Ra,232 Th on 40 K, and 226 Ra on 40 K, respectively.[9]

From the Compton-corrected count, the activity (A, Bq/kg) of 40 K,226 Ra, and 232 Th 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 (Raeq) from the given relation.[10]

Raeq = ARa + 1.43 ATh + 0.077 AK

Where, ARa, ATh, and AK are the specific activities of 226 Ra,232 Th, and 40 K expressed in Bq/kg. Radium equivalent concept allows a single index or number and is widely used index to describe the gamma output from different mixers of uranium, thorium, and potassium in the samples.[11]

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.[12]

D = 0.462 CRa + 0.604 CTh + 0.0042 CK

Where D is absorbed dose rate in nGyh −1 and CRa, CTh, and CK are the activities of 226 Ra,232 Th, and 40 K 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.[13] 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 hazard index, due to the emitted gamma rays, from the sand samples were calculated using the relation.[14]

Hex= (ARa/370 Bq/kg) + (ATh/259 Bq/kg) + (AK/4810 Bq/kg) ≤1

The internal hazard index factor should be calculated to assess the radiation hazard risk due to radon and its short-lived daughter products, which are hazardous to respiratory organs.[15] The internal exposure to radon and its daughter products is quantified by internal hazard index which is given by the equation:

Hix= (ARa/185 Bq/kg) + (ATh/259 Bq/kg) + (AK/4810 Bq/kg) ≤1

Where, ARa, ATh, and AK are activity concentrations of 226 Ra,232 Th, and 40 K, in Bq/kg.


  Results and Discussion Top


The activity concentrations of the natural radionuclides, namely 40 K,226 Ra, and 232 Th in the samples collected from nine sites have been summarized in [Table 1]. The activity of 40 K, collectively for all the 36 sand samples, found to be ranged from 8.7 to 173.9 Bq/kg with an average value of 53.2 Bq/kg. The 226 Ra activity varied from 0.3 to 23.2 Bq/kg with an average value of 7.0 Bq/kg. The 232 Th activity ranged from 1.5 to 511.6 Bq/kg with an average value of 101.3 Bq/kg. S7 (4) and S7 (1) have higher values of 40 K and 226 Ra, respectively, while S8 (1) has higher value of 232 Th concentration. The lowest activity concentration of 40 K and 226 Ra observed in S3 (1) and 232 Th in the samples S3 (2). The variation of activity concentration of 40 K,226 Ra, and 232 Th at different locations along the coastal belt of Kerala is shown in [Figure 2].
Table 1: Activity of 40K, 226Ra, and 232Th

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Figure 2: The mean activity concentration of 40K, 226Ra, and 232Th at different locations

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It is also observed that the contribution of 226 Ra to the total dose is very low compared to the contribution of 232 Th and 40 K. The result obtained in the present study reveals that the activity concentration of 40 K and 226 Ra is lower, and 232 Th is higher than the Indian average values (Indian average values of 40 K,226 Ra, and 232 Th is 400, 28.67, and 63.83 Bq/kg, respectively).[1] The activity concentration of 40 K and 226 Ra is lower, and activity concentration of 232 Th is higher than the world average values (World average values of 40 K,226 Ra, and 232 Th is 420, 33, and 45 Bq/kg, respectively).[1] The radium equivalent activity ranges from 4.1 to 744.8 Bq/kg with an average value of 156.0 Bq/kg and is low compared to the recommended maximum value of 370 Bq/kg.

The samples follow the activity concentration order as 226 Ra <40 K <232 Th. The variation in the natural radioactivity levels is due to variations in concentrations of the radionuclides in the geological formations. The highest activities of the natural radionuclides, namely,40 K,226 Ra, and 232 Th, were observed in locations situated in Kollam District, one of the well-known high background radiation areas.

The radiological parameters calculated from the activity concentration were summarized in [Table 2]. The absorbed dose ranges from 6.48 to 329.19 nGyh −1 with an average value of 85.81 nGyh,−1 and is high compared to the world average value of 57 nGyh −1. The indoor annual effective dose varies in the range 0.03–1.61 mSv with an average value of 0.42 mSv, and the outdoor annual effective dose varies in the range 0.01–0.40 mSv with an average value of 0.11 mSv. The observed values for the indoor and outdoor annual effective doses were of the same order as that of world average value of 0.48 mSv.
Table 2: Radiological parameters

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The external and internal hazard indices for all the samples were calculated. The external hazard index varies from 0.01 to 2.01, with an average value of 0.42, and thus external hazard index was found to be less than unity. The internal hazard index for the gamma rays emitted from sand samples ranged from 0.01 to 2.04 with an average value of 0.45, and thus internal hazard index was found to be less than unity. The variation of external hazard index (Hex) and internal hazard index (Hin) is shown in [Figure 3]. The highest values of external and internal hazard indices were observed in the high background radiation areas. In those locations, the hazard indices were found to be higher than unity, which are expected for a high background region.
Figure 3: Variation of average Hexand Hinin the samples at different locations

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  Conclusions Top


The systematic analysis of the activities in the samples collected clearly indicates that 232 Th is the major contributor to the activity concentration, especially in the high background radiation areas. The activity concentration of 232 Th is found higher compared to the world and Indian average values. The higher activity of 232 Th is found in the samples collected from Kollam region and may be attributed to the presence of monazite bearing black sands. The activity concentration of 40 K,226 Ra is well within the permissible limit. The wide variation in the radionuclides activity concentration along the coastal belt is expected as these are influenced by geological and geochemical processes. The sand weathered from the rocks, which are rich in heavy metals, and radioactive minerals can contribute to the enhanced level of 232 Th activity. Industrial activities present in the region may also affect the radioactivity levels in the region. The contribution of 226 Ra to the total dose is very low compared to 40 K and 232 Th for the activity concentration in the samples. The absorbed dose, indoor and outdoor effective doses were slightly higher compared to the world average values. The hazard indices were found to be within the safety limits except locations of high background radiation areas. However, radium equivalent activity was well below the maximum permissible limit. More systematic analysis is needed to understand the various processes influencing the activity concentration, and a theoretical model may be established accordingly.

Acknowledgements

The first author would like to acknowledge UGC, New Delhi, for awarding JRF.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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