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 Table of Contents 
ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 3  |  Page : 154-158  

Study of natural radioactivity (226Ra, 232Th, and 40K) in soil samples for the assessment of average effective dose and radiation hazard parameters


1 Department of Physics, Oriental College, Imphal, Manipur, India
2 Department of Radiotherapy, RIMS, Imphal, Manipur, India
3 Department of Physics, Environmental Radiation Dosimetry Laboratory, Oriental College, Imphal, Manipur, India

Date of Submission07-Nov-2017
Date of Decision25-Nov-2017
Date of Acceptance18-Dec-2017
Date of Web Publication16-Feb-2018

Correspondence Address:
B Arunkumar Sharma
Department of Radiotherapy, RIMS, Imphal - 795 003, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_29_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 measured in soil samples collected from Thoubal and Bishnupur districts, Manipur, 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 28.7 Bq/kg (Thoubal College) to 126.0 Bq/kg (Kakching Khunou College), 27.2 Bq/kg (Ikop Pat) to 203.5 Bq/Kg (Kakching Khunou College), and 335.6 Bq/kg (Nongpok Sekmai) to 1953.9 Bq/kg (Mantak) with overall mean values of 69.8, 76.6, and 1028.0 Bq/kg, respectively. The radium equivalent activities (Raeq) have been evaluated in soil samples for the assessment of the radiation hazard. The corresponding absorbed dose rate, annual effective dose, and external radiation hazard index in the study area range from 54.5 to 239.3 nGyh−1, 0.3 to 1.5 mSv, and 0.3 to 1.4 with an average value of 121.4 nGyh−1, 0.7 mSv, and 0.7, respectively.

Keywords: High purity germanium gamma spectrometry, natural radioactivity (226Ra, 232Th, and 40K), radiation hazard index, radiation hazards


How to cite this article:
Singh S N, Sharma B A, Devi TP. Study of natural radioactivity (226Ra, 232Th, and 40K) in soil samples for the assessment of average effective dose and radiation hazard parameters. Radiat Prot Environ 2017;40:154-8

How to cite this URL:
Singh S N, Sharma B A, Devi TP. Study of natural radioactivity (226Ra, 232Th, and 40K) in soil samples for the assessment of average effective dose and radiation hazard parameters. Radiat Prot Environ [serial online] 2017 [cited 2019 Aug 17];40:154-8. Available from: http://www.rpe.org.in/text.asp?2017/40/3/154/225587




  Introduction Top


External gamma radiation exposure to public arises from terrestrial radionuclides present at trace levels in all soil samples. The specific levels of particular radionuclide are related to the types of rock from which the soils originate. Igneous rocks, such as granite usually, show higher radiation levels as compared with sedimentary rocks. However, some shale and phosphate rocks have relatively high content of radionuclide as some exceptions.[1] 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.[2],[3],[4] There have been many radiological surveys to determine the background levels of radionuclides in soil samples.[5],[6],[7],[8],[9] Extensive studies of the background exposure levels in different countries indicate that the three main radionuclides, namely,238 U and 232 Th series and 40 K make approximately equal contributions to the externally incident gamma radiation dose to individual in both outdoors and indoors.[4] Studies of natural radioactivity are necessary not only because of their radiological impact to public but also they act as excellent biochemical and geochemical tracers in the environment. U-series radionuclides present in nature have been of particular interest due to their relatively high biological mobility.[1] Although natural radioactivity is available through the earth, the accession in specific areas varies relatively within narrow limit.[10]

Thoubal and Bishnupur are the only remaining valley districts next to Imphal districts, Manipur, India. It is in the North Eastern part of the country and is about 300 km away from Domiasiat, Meghalaya, a well-known place for its heavy-mineral deposit area for the country. Thoubal district is having an area of 514 m 2 with a total population of 420,517 (2011 census), whereas Bishnupur district is having an area of 496 km 2 with a population of 240,363 (2011 census).

Evaluation of natural radioactivity concentration in 12 sites across the Thoubal and Bishnupur Valley was done using high purity germanium (HPGe) detector. The absorbed gamma dose rate, annual effective dose, radium equivalent activities, and external radiation hazard index were evaluated and compared to the guidance levels proposed by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR2008)[4] as well as reports of different countries on a radiological survey of soil samples.


  Materials and Methods Top


Sample collection and radionuclides measurement

Soil samples were collected from 12 sites from Thoubal and Bishnupur districts (six each from each district) as shown in [Figure 1]. Samples were collected from about 30 cm deep from the surface of the soil; each weighing approximately 1.5 kg and were considered representative of the sampling sites. Each sample was packed inside a plastic bag, labeled, and carried to the laboratory. Soils available in this valley area are alluvial type and soft and may easily be crushed 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 standard polypropylene container, weighed, and carefully sealed to prevent the escape of gases from the sample. Then, they were stored for at least 4 weeks to allow time for 238 U and 232 Th to reach equilibrium with their respective daughter radionuclides.[11] 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, 1332.5 keV) was used. For efficiency, 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].
Figure 1: Map of Manipur and the radionuclide assessment districts. Solid circles: Study sites in Bishnupur district, Hollow circles: Study sites in Thoubal district

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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 gamma lines of 1766 keV 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.

Assessment of radiological parameters

Radium equivalent activity (Raeq)

Raeq is most widely used radiation hazard index parameter associated with material that contains 226 Ra,232 Th, and 40 K. It is estimated on the assumption that 370 Bq/kg of 226 Ra, 259 Bq/kg of 232 Th, and 4810 Bq/kg of 40 K produce similar γ-ray dose rates [12],[13] and is given as

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

A value of 370 Bq/kg gives annual dose rate of 1 mSv/y. This radium equivalent concept expressed as single index number is widely used radiation index associated with the gammas from a different mixture of uranium, thorium, and potassium in the soil samples.

External hazard index (H)

The external hazard index is an assessment of external hazard of the natural gamma radiation for samples under investigation and evaluated using the relation given by Beretka and Matthew [12] 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 Bq/kg) so as to keep the annual radiation dose below 1.5 mGy/y.[14]

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[4] 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), conversion coefficient of adult as 0.7Sv/Gy [15] is taken into account from the absorbed dose in air to the effective dose.


  Results and Discussion Top


The measured activity concentrations of radionuclides 226 Ra,232 Th, and 40 K obtained from 12 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 28.7 to 126.0 Bq/kg, 27.2 to 203.5 Bq/kg, and 335.6 to 1953.9 Bq/kg, with a mean value of 69.8 ± 32.3, 76.6 ± 48.3, and 1028.0 ± 551.0 Bq/kg, respectively. The activity concentration of 226 Ra,232 Th, and 40 K reported values from some countries are shown in [Table 3]. It is observed that the measured values of activity concentrations of 226 Ra,232 Th, and 40 K in this study are moderately high as compared with the most of the reported values from other countries but almost double to the world's average value. The present study indicates that mean value of 226 Ra (69.7 ± 32.3 Bq/kg) <232 Th (76.6 ± 48.3 Bq/kg) <40 K (1028.0 ± 551.0 Bq/kg).
Table 2: Activity concentration of 226Ra, 232Th, and 40K in soil samples along with their corresponding radium equivalent activity, radiation absorbed dose rate external radiation hazard indices

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Table 3: A review of radiological survey in some countries

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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 12 sites varied from 118.4 to 524.4 Bq/kg with a mean value of 258.5 ± 111.2 Bq/kg which is lower than the threshold value of 370 Bq/kg. Two soil samples have radium equivalent activities more than the limit set (370 Bq/kg) in the Organization for Economic Co-operation and Development report.[27]

The absorbed gamma radiation dose rate in air (D), annual effective dose rate (Deff), and external radiation hazard index (Hex) evaluated in this study ranges from 54.5 to 239.3 nGyh -1, 0.3 to 1.5 mSv, and 0.3 to 1.4 with mean values 121.4 nGyh -1, 0.7 mSv, and 0.7, respectively. However, the mean dose rate level of 121.4 nGyh -1 and annual effective dose of 0.7 mSv are higher compared with the world average dose rate of 59 nGyh -1 reported by UNSCEAR 2000,[16] 55 nGyh -1 by Butt et al.[28] and 52.76 nGyh -1 by Alaamer [24] and annual effective dose of 0.38 mSv by UNSCEAR in 1998[10] and the world average of about 0.5 mSv per year by Alaamer.[24] The majority (10/12) of the annual effective doses are observed higher than the world average of about 0.5 mSv,[24] since the enhanced radioactivity presence of heavy mineral deposits in these study sites. Two locations exceed the threshold value unity for external hazard index.[14]


  Conclusions Top


The mean values for annual effective gamma radiation dose rates, radium equivalent dose, and external hazard index were evaluated for the 12 different places distributed in the Thoubal and Bishnupur districts. The mean values of effective dose rate, radium equivalent, and external hazard index are 0.7 mSv/y, 258.5 Bq/kg, and 0.7, respectively. The majority of the annual effective doses of study sites are observed higher than the world average of about 0.5 mSv owing to the enhanced radioactivity presence of heavy mineral deposits in these sites. The data generated in this study may be useful to form as a baseline reference of this study area and for comparison with nationwide as well as radiological reports of worldwide.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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