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ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 3  |  Page : 149-153  

Assessment of radioactivity in the soil samples from Imphal city, India, and its radiological implication


1 Department of Radiotherapy, RIMS, Imphal, Manipur, India
2 Department of Physics, Oriental College, Imphal, Manipur, India
3 Department of Physics, Environmental Radiation Dosimetry Laboratory, Oriental College, Imphal, Manipur, India
4 Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Submission31-Oct-2017
Date of Decision02-Dec-2017
Date of Acceptance26-Dec-2017
Date of Web Publication16-Feb-2018

Correspondence Address:
Nabadwip Sarangthem Singh
Department of Physics, Oriental College, Takyel, Imphal - 795 001, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_28_17

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  Abstract 


The radioactivity concentration and the natural gamma absorbed dose rates of the terrestrial radionuclides (226Ra, 232Th, and 40K) have been determined in soil samples collected from twenty different locations of Imphal city, India, using high-purity germanium detector. The range of activity concentration of 226Ra, 232Th, and 40K in the soil from the studied areas varies from 30.3 Bq/kg (Phayeng) to 155.5 Bq/kg (Lamphel), 21.2 Bq/kg (Yairipok) to 257.1 Bq/kg (Lamphel), and 287.4 Bq/kg (Uchiwa) to 2209.3 Bq/kg (Changangei) with overall mean values of 94.1 Bq/kg, 146.5 Bq/kg and 1222.9 Bq/kg, respectively. The absorbed dose rate and annual effective outdoor dose in the study area range from 61.0 nGyh−1 and 0.4 mSv (Laphupat Tera) to 294.4 nGyh−1 and 1.1 mSv (Lamphel) with an average value of 182.9 nGyh−1 and 1.1 mSv. The external hazard index ranged from 0.4 to 1.8 with an average of 1.1. It was significant in 12 locations as it exceeded unity.

Keywords: Absorbed gamma dose rates, alluvial soil, hazard index, Imphal, natural radioactivity


How to cite this article:
Sharma B A, Singh NS, Devi PT, Basu H, Saha S, Singhal R K. Assessment of radioactivity in the soil samples from Imphal city, India, and its radiological implication. Radiat Prot Environ 2017;40:149-53

How to cite this URL:
Sharma B A, Singh NS, Devi PT, Basu H, Saha S, Singhal R K. Assessment of radioactivity in the soil samples from Imphal city, India, and its radiological implication. Radiat Prot Environ [serial online] 2017 [cited 2020 Apr 8];40:149-53. Available from: http://www.rpe.org.in/text.asp?2017/40/3/149/225586




  Introduction Top


Naturally occurring radioactivity in soils comes from U and Th series and natural K. The presence of this natural radioactivity in soil and building materials results in internal and external exposure to the dwellers. Accumulation of these natural radionuclides in soil could pose potential health hazard.[1],[2] Therefore, the assessment of gamma radiation dose from natural sources is of particular importance as natural radiation is the largest contributor to the external dose of the world population.[3],[4],[5] The natural gamma dose rates vary depending on the concentration of the natural radionuclides,238 U,232 Th, their daughter products, and 40 K present in soil, sand, and rocks, which, in turn, depends on the local geological and geographical conditions. Many investigations were reported on natural radioactivity and natural gamma radiation level by in situ measurement or by analysis of radionuclide concentration in soil samples.[6],[7],[8],[9],[10],[11]

Imphal, the capital of Manipur state, India, is at the northeastern part of the country and is about 300 km away from Domiasiat, Meghalaya, a well-known placed for its heavy-mineral deposit area for the country. Imphal city is having two districts, namely, Imphal East and Imphal West with a total population of about 10 lakhs (2011 census) and area of about 1300 sq km. Every year, thousands of tourist visitors are travelling this place from inside and outside the country. Thus, the evaluation of the natural dose rate behavior in this area is important to understand doses from natural radiation as well as establishing a baseline reference.

Evaluation of natural radioactivity concentration in an extensive selection of sites across the Imphal Valley was done using high-purity germanium (HPGe) detector. The radiation absorbed dose rate, annual effective dose, radium equivalent activities, and external radiation hazard index were evaluated and compared to nationwide reports and the guidance levels proposed by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2008).[5]


  Materials and Methods Top


Sample collection and radionuclide measurement

Soil samples were collected from twenty sites from Imphal city (ten each from Imphal East and Imphal West 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. Imphal Valley is fertile place and is mainly made up of alluvial soil. The texture of soil varies from sandy to loam to clayey. It is not hard soil and 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 was then packed in plastic bottle/container with predefined geometry, weighed, and carefully sealed to prevent the escaped of radon gaseous 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 radionuclide.[12] Radionuclide activity concentration of 226 Ra,232 Th, and 40 K in the samples was measured using a HPGe detector with a relative efficiency of 35% and resolution of 1.8 keV for the gamma emission of the 1333.0 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, 1332.5 keV) was used. For efficiency calibration, Uranium-ore standard (IAEA-RGU-1, Uranium ore) having activity of 4940.0 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 4th-order polynomial which was later used to evaluate the efficiencies or the radionuclides of interest. The activity of 40 K was evaluated from 1460.0 keV photopeak; the activity of 226 Ra from 1766.0 keV gamma line of 214 Bi and 352.0 keV of 214 Pb and 232 Th from the most prominent gamma lines of 510.6, 583.2, and 2610.0 keV of 208 Tl,212 Bi (727.2 keV), and 212 Pb (238.6 keV).

Assessment of radiological parameters

Radium equivalent activity (Raeq)

Raeq is most widely used radiation hazard index associated with material that contains 226 Ra,232 Th, and 40 K. It is estimated on the assumption that 370.0 Bq/kg of 226 Ra, 259.0 Bq/kg of 232 Th, and 4810.0 Bq/kg of 40 K produce similar γ-ray dose rates [13],[14] and is given as

Raeq = 226 Ra + 1.43 232 Th + 0.077 40 K(1)

A value of 370.0 Bq/kg gives annual dose rate of 1.0 mSv/y. This radium equivalent concept express in single index or number is widely used radiation hazard index associated with the gamma output from different mixture of uranium, thorium, and potassium in the soil samples.

External hazard index (Hex)

The external hazard index is an assessment of hazard of the natural gamma radiation for samples under investigation and evaluated using the relation of Hex by Beretka and Matthew [13] as

Hex = 226 Ra/370 + 232 Th/259 + 40 K/4810(2)

The maximum value of Hex to be less than unity, which corresponds to the upper limit of Raeq(370.0 Bq/kg).[15] The main objective of this index is to limit natural gamma radiation dose to effective dose of 1.0 mSv/y ICRP 65 report.[16]

Absorbed dose rate (D)

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[5] as

D (nGyh −1) =0.427CRa + 0.623CTh + 0.043Ck(3)

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 (Deff) and conversion coefficient of adult (adult: 0.7Sv/Gy and children/infant: 0.8 Sv/Gy)[16] are taken into account from the absorbed dose in air to the effective dose.


  Results Top


The measured mean values of activity concentrations of radionuclides 226 Ra,232 Th, and 40 K obtained from twenty different sites as indicated in [Figure 1] are given in [Table 2]. The activity concentration of 226 Ra,232 Th, and 40 K are in the range from 30.3 to 155.5 Bq/kg, 21.2–257.1 Bq/kg, and 287.1–2209.3 Bq/kg with a mean value of 94.2 ± 36.6, 146.5 ± 82.2, and 1222.9 ± 603.3 Bq/kg, respectively.
Figure 1: Map of Manipur (India) showing the Imphal city (Imphal East and Imphal West districts). Hollow and solid circles are the sites of soil samples in Imphal East District and Imphal West district

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Table 2: Activity concentration of 226Ra, 232Th, and 40K in soil samples along with their corresponding radium equivalent activity, external radiation hazard index, absorbed dose rate, and annual effective dose

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Radium equivalent activity (Raeq) owing to activity concentration of the three natural radionuclides, namely,226 Ra,232 Th, and 40 K from all twenty sites varies from 127.9 to 647.4 Bq/kg. The mean value of Raeq is 397.7 ± 172.4 Bq/kg. The mean value of absorbed gamma radiation dose rate in air (D), annual effective dose rate (Deff), and external radiation hazard index (Hex) evaluated in this study are shown in [Table 2]. It shows that mean (range) value D, Deff, and Hex as 182.9 (61.0–294.4) nGy/h, 1.1 (0.4–1.8) mSv/y, and 1.1 (0.4–1.8), respectively.


  Discussion Top


The measured activity concentration of 226 Ra,232 Th, and 40 K was compared with nationwide reported values as shown in [Table 3]. It is observed that mean values of the concentration of three radionuclides in the soil of the twenty samples are much higher than the mean values reported worldwide.[3],[4],[5],[11] National reported values of concentration of the radionuclides in the soil are in the range from 18.0 Bq/kg (Punjab) to 445.8 Bq/kg (Kollam) for 226 Ra, 8.0 Bq/kg (Northern Rajasthan) to 2022.0 Bq/kg (Kollam) for 232 Th, and 78.3 Bq/kg (Kaiga) to 1894.0 Bq/kg (Northern Rajasthan) for 40 K, respectively. The concentrations of 226 Ra and 232 Th in this study area are in medium range compare with nationwide reports.[2],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33] The concentration of potassium usually lower in basaltic rock than in acidic rock, but more variable in basaltic rock region.[35] Soil available in Imphal Valley is acidic with pH ranging between 4.5 and 6.8.[36]40 K in this study area ranges from 287.4 to 2209.3 Bq/kg with mean value of 1222.9 Bq/kg, which is moderately high compared with reports [17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33] of this country. Soil of this valley region commonly contains small fragments of rocks, loam, sandy clay, and sand, and they are quite diverse. Moreover,40 K binds more tightly to loam and clay soils. Thus, the large increment of 40 K may influence the increase of the dose as we observed in many cases of this study. The mean values of the three radionuclide concentrations are well within the above national ranges. However, the mean dose rate level of 182.9 nGyh −1 and annual effective dose of 1.1 mSv are much higher compared with the world average dose rate of 59.0 nGyh −1 reported by UNSCEAR 2000,[4] 55.0 nGyh −1 by Butt et al.[37] and 52.8 nGyh −1 by Alaamer [11] and annual effective dose of 0.4 mSv by UNSCEAR in 1998,[3] and world average of about 0.5 mSv per year by Alaamer.[11] However, the mean absorbed gamma dose rate is well within the nationwide range of 37.7 nGyh −1 (Kaiga) to 351.1 nGyh −1 (Garhwal Himalaya). The national average absorbed dose rate as observed from [Table 3] shows of about 120.0 nGyh −1. The majority (18/20) of the annual effective doses are observed higher than the world average of about 0.5 mSv per year [11] since the enhanced radioactivity presence of heavy mineral deposits in these study sites. Twelve soil samples have radium equivalent activities more than the limit set (370.0 Bq/kg) in the Organization for Economic Co-operation and Development (OECD) report.[38] The external hazard index (Hex) for the area under study ranges from 0.4 to 1.8 with mean values of 1.1. Out of 20 study sites, 12 locations exceed the threshold value unity for hazard index.[15] It is also observed that the mean value of the annual effective dose rate for all twenty sites is less than the average worldwide exposure of 2.4 mSvy -1 due to natural sources [4] and the limit proposed by OECD [38] if the soil samples were used as building material.
Table 3: Activity concentration of 226Ra, 238U, 232Th, and 40K and their corresponding absorbed dose rate as measured nationwide

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  Conclusions Top


The mean values for annual effective gamma radiation dose rates, radium equivalent dose, and external hazard index were evaluated for the twenty different places distributed in the Imphal city. The mean values of absorbed gamma dose rate, annual effective dose, radium equivalent, and external radiation hazard index are 182.9 nGyh −1, 1.1 mSv/y, 397.7 Bq/kg, and 1.1, respectively. The majority of the effective doses of study sites are observed higher than the world average of about 0.5 mSv/year owing to the enhanced radioactivity presence of heavy mineral deposits in these sites. This study suggests further investigation. The data generated in this study may be useful to form as a baseline reference for comparison with nationwide as well as worldwide data.

Acknowledgment

We would like to thank Health Physics Division, BARC, namely, Dr. Chinna Esakki, SO/E, Shri S. K. Sahoo, SO/E, Shri Anil Gupta, SA/E, and Pradyumna Lenka, SA/E, for their valuable contribution in soil analysis of our soil samples at their laboratory.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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[PUBMED]  [Full text]  
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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3]



 

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