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 Table of Contents 
TECHNICAL NOTE
Year : 2018  |  Volume : 41  |  Issue : 2  |  Page : 94-98  

Assessment of natural background gamma radiation levels in and around Loktak Lake of Manipur, India


1 Department of Radiotherapy, RCC, RIMS, Imphal, Manipur, India
2 Department of Physics, Oriental College, Imphal, Manipur, India

Date of Submission24-Dec-2017
Date of Decision12-Feb-2018
Date of Acceptance28-Feb-2018
Date of Web Publication24-Aug-2018

Correspondence Address:
Dr. Arunkumar B Sharma
Department of Radiotherapy, RCC, RIMS, Imphal, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_32_17

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  Abstract 

Natural pollution level due to terrestrial gamma radiation was measured at about seventy different sites in and around the Loktak Lake of Manipur, India, using NaI (Tl) scintillator-based Micro-R-survey meter, and high purity germanium detector. The observed annual effective dose in this study area ranges from 0.3 to 1.2 mSvy−1 with an average value of 0.7 ± 0.1 mSvy−1, which is higher than world average value of about 0.4 mSy−1 reported by the UNSCEAR 2000. The soil analysis for 226Ra, 232Th, and 40K of this area gives an average concentration of 74.6 (38.9-126.0) Bq/kg, 112.1 (30.2–224.3) Bq/kg, and 792.9 (287.4–1194.8) Bq/kg, respectively.

Keywords: Effective dose, Loktak Lake, natural radioactivity


How to cite this article:
Sharma AB, Singh NS. Assessment of natural background gamma radiation levels in and around Loktak Lake of Manipur, India. Radiat Prot Environ 2018;41:94-8

How to cite this URL:
Sharma AB, Singh NS. Assessment of natural background gamma radiation levels in and around Loktak Lake of Manipur, India. Radiat Prot Environ [serial online] 2018 [cited 2019 Dec 12];41:94-8. Available from: http://www.rpe.org.in/text.asp?2018/41/2/94/239684


  Introduction Top


The assessment of radiation doses in humans from natural sources is important because natural ionizing radiation is the largest contributor to the collective effective dose received by the world's population. About 80% of the annual effective dose received by the general population is from the natural background radiation.[1] Natural radionuclides of significance in soil, air, water, and living organisms include 40 K and the isotopes of the 238 U and 232 Th decay chains. The concentrations of natural radionuclides depend primarily on the geological formation and soil type of the location; these two factors (geology and soil type) greatly influence the dose distribution from natural terrestrial radiation.[1],[2] Most of the general population spends their time (about 80%) in indoors, and there is a high chance of getting both external as well as internal radiation exposure from natural radionuclide present in the building materials.[3] The main radionuclides present in the indoor and outdoor environment which contributes natural background radiation includes 238 U,232 Th,40 K,222 Rn, and 220 Rn.[4],[5] Assessment of natural radioactivity is necessary not only because of their radiological hazards to general public but also they act as excellent biochemical and geochemical tracers in the environment. Uranium-series radionuclides present in nature have been of particular interest due to their relatively high biological mobility.[6] Although natural radioactivity is available through the earth, the accession in specific areas varies relatively within narrow limit.[7]

The study area under investigation covered in and around Loktak Lake of Manipur, India. Loktak Lake is the largest freshwater lake in North-East India and is famous for the phumdis (heterogeneous mass of vegetation, soil, and organic matters at various stages of decomposition) floating over it. Keibul Lamjao, the only floating national park in the world, is located in this lake. It is connected with two districts of Manipur, namely Bishnupur and Imphal West districts. Nearly 100,000 people depend on the lake for their livelihood. The lake is home to 233 species of aquatic plants, more than 100 species of birds, and 425 species of animals, including the Indian python and sambhar.[8] More than thousands of peoples are visiting this lake every year from outside and inside the country.

Evaluation of natural background gamma radiation dose in and around Loktak Lake is performed using NaI scintillation-based Micro-R-Gamma survey meter (SM) at seventy sites. A few soil samples (six number) were collected at random around the lake for analysis of natural radioactivity concentration using high-purity germanium (HPGe) detector. The observation of the study was compared with nationwide reports and the guidance level proposed by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR-2008).[9]


  Materials and Methods Top


Study site

The coordinates of Loktak is 24°33′N 93°47′E and 24°33′N 93°47′E. The catchment and surface area of this lake are about 980.0 and 287.0 km 2, respectively. The lake surface elevation is about 768.5 m, the maximum length is 35.0 km, and maximum width is 13.0 km with an average depth of about 2.7 m. The largest of all the phumdis covers an area of 40.0 km 2 and is situated on the southeastern shore of the lake. Manipur river, Nambul river, and many small rivulets are flowing into this lake. The main islands are Thanga, Ithing, and Sendra. Furthermore, many floating islands called phumdis or phumshongs are available in this lake. Seventy sites, numbering 1–70 as shown in [Figure 1], were selected for the evaluation of natural gamma radiation dose rates.
Figure 1: Map of Loktak Lake and its main surrounding dwellers, numbering 1–70. Δ Soil sample collection sites

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Environmental radiation dosimetry

Measurement of gamma radiation dose rate (survey meter)

NaI (Tl) Scintillator-based micro-Roentgen SM manufactured by Nucleonix Systems Pvt. Ltd., Hyderabad, India, was used for instantaneous measurements.[5] The sensitivity of the SM is 1 μR/h. The dose rate at the respective site of the study was measured by keeping at a height of about 1 m from the ground. Repeated measurements of five times were taken from each location and average of all these five measurements is considered as the dose rate for that particular location.[10],[11]

Soil analysis

Soil samples were collected from six sites around the lake, 3 samples each from Imphal West and Bishnupur district. Samples were collected from about 30 cm deep from the surface of the soil, each weighing approximately 1.5 kg, and it was considered representative of the sampling sites. Each sample was packed inside a plastic bag, labeled, and carried to the laboratory. Soils available in this area are alluvial type and soft, may easily crush to get powder form. These samples were oven dried overnight (~15 h) at 110°C, homogenized, ground, and screened with a sieve of about 1 mm mesh. About 200 g of the homogeneous sample material were then packed in plastic bottle/container with predefined geometry, weighed, and carefully sealed to prevent the escaped of radon gases from the sample. Then, they were stored for at least 4 weeks to allow time for 234 U and 232 Th to reach equilibrium with their respective daughter radionuclides.[12] Radionuclide activity concentration of 226 Ra,232 Th, and 40 K in the samples was measured using HPGe detector with a relative efficiency of 35% and resolution of 1.8 keV for the gamma emission of the 1333 keV of 60 Co. For the energy calibration of the HPGe detector system, a mixed point source of 241 Am (59.5 keV),137 Cs (661.7 keV), and 60 Co (1173.2 and 1332.5 keV) was used. For efficiency calibration, uranium-ore standard (IAEA-RGU-1, Uranium ore) having activity of 4940 Bq/kg was used after it reached equilibrium with its daughter products. The daughter products and their energies used for efficiency calibration are given in [Table 1].
Table 1: Radionuclide daughter products and their energies used for efficiency calibration

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The energy versus efficiency plot was fitted to a fourth-order polynomial which was later used to evaluate the efficiencies or the radionuclides of interest. The activity of 40 K was evaluated from 1460 keV photopeak; the activity of 226 Ra was determined from 1766 keV gamma line of 214 Bi and 352 keV of 214 Pb and 232 Th from the most prominent gamma lines of 510.6, 583.2, and 2610 keV of 208 Tl, 727.2 KeV of 212 Bi, and 238.6 keV of 212 Pb.

Absorbed dose rate D (nGyh−1) and annual effective dose E (mSv)

The absorbed gamma dose rates D (nGyh −1) in air at 1 m above the ground surface for uniform distribution of 226 Ra,232 Th, and 40 K in the soil were calculated with Monte Carlo method provided by UNSCEAR 2008[9] as

D (nGyh −1) = 0.427CRa + 0.623CTh + 0.043Ck(1)

Where CRa, CTh, and Ck are the activity concentrations (Bq/kg) of 226 Ra,232 Th, and 40 K, respectively.

The estimation of annual effective dose (E) and conversion coefficient of adult (adult: 0.7Sv/Gy) is taken into account from the absorbed dose in air to the effective dose.


  Results and Discussion Top


The natural background radiation levels were measured using SM at seventy dwellings in and around Loktak Lake. The location-wise measured average values collected in 1 year are presented in [Table 2]. Significant changes in gamma radiation dose rate due to seasonal variation were not observed in the present study, similar with the earlier work of Reddy et al.[10] The effective dose rate in and around Loktak Lake from natural gamma radiation ranges from 0.3 to 1.2 mSvy −1, with an average value of 0.7 ± 0.1 mSvy −1. The annual effective dose of the world as reported by the UNSCEAR, 2000,[2] is about 0.4 mSv, world average of about 0.5 mSv per year by Alaamer,[14] and national average of about 0.3 mSv per year by the UNSCEAR, 2008.[9] The observed data of annual effective dose of the presents study are higher than 0.5 mSv except two sites (site number 6 and 30). A close scrutiny was made for possible contributing factors to higher levels of radiation and it is observed that Manipur river, Nambul rivers, and many small rivulets originating from hills and valley areas with erosion containing heavy minerals are flowing into this lake and only one river (Manipur river) is flowing out from this lake. Soil samples were collected from six sites as indicated in [Figure 1], surrounding the Loktak Lake, and analyzed. The concentration of the radionuclides, namely,226 Ra,232 Th, and 40 K, absorbed dose rates, and annual effective dose are given in [Table 3]. [Figure 2] is a graph of annual effective doses at six sites (soil samples collection sites) measured by the two measurement techniques, and it may be fitted with a simple linear equation (y = 0.9744x) with a coefficient of determination R2 = 0.91 for any statistical conclusion. It shows a good correlation between the two types of measurement (i.e., SM and HPGe). The soil analysis for 226 Ra,232 Th, and 40 K observed an average concentration of 74.6 (range: 38.9–126.0) Bq/kg, 112.1 (range: 30.2–224.3) Bq/kg, and 792.9 (range: 287.4–1194.8) Bq/kg, respectively. This radionuclide concentration under present investigation is relatively higher compared with global [9],[11],[15],[16],[17],[18],[19],[20] and national [9],[10],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30] average values [Table 4]. It is also observed that the mean value of the annual effective dose is on par with the average worldwide exposure of 0.41 mSvy −1 due to terrestrial natural gamma sources [2] and the limit proposed by OECD,[31] if the soil samples were used as building material.
Table 2: Average annual absorbed dose (Davg) for each location measured using survey meter (mSv/year) with standard deviation

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Table 3: Measured annual average activity level of the three Radio-isotopes (226Ra, 232Th and 40K) in the soil of 6 sites around Loktak Lake in Bq/kg, Natural absorbed gamma radiation level, D (nGyh-1), and annual effective dose, E (mSvy-1) along with survey meter values

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Figure 2: Survey meter reading versus soil analysis (high purity germanium) reading for annual effective dose rate (mSv/y)

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Table 4: A review of radionuclides (226Ra, 232Th, 40K) reported in soil analysis of various country and different places of this country

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  Conclusions Top


The mean values for annual effective gamma radiation dose rates for seventy different places surrounding the Loktak Lake were evaluated. The mean value of the annual effective dose is observed as 0.7 ± 0.1 mSv. The effective dose rates of the study sites are observed to be slightly higher than the world average of about 0.5 mSv per year owing to the enhanced radioactivity presence of heavy mineral deposits in this area. The average concentration of 226 Ra,232 Th, and 40 K in soil of this study area is 74.6 (range: 38.9–126.0) Bq/kg, 112.1 (range: 30.2–224.3) Bq/kg, and 792.9 (range: 287.4–1194.8) Bq/kg, respectively. The data generated in this study may be used to form as a baseline reference for comparison with nationwide as well as worldwide data.

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]
 
 
    Tables

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



 

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