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ORIGINAL ARTICLE
Year : 2020  |  Volume : 43  |  Issue : 1  |  Page : 13-20  

Measurement of terrestrial gamma radiation dose rate and radioactivity levels in the southern hilly regions of Manipur, India


1 Department of Radio-Imaging Technology, DMCC, D. M. College of Science, Imphal, Manipur, India
2 Department of Zoology, D. M. College of Science, Imphal, Manipur, India
3 Department of Physics, Oriental College (Autonomous), Imphal, Manipur, India
4 Department of Radiation Oncology, Regional Institute of Medical Sciences, Imphal, Manipur, India
5 Department of Physics, Moirang College, Imphal, Manipur, India

Date of Submission11-Dec-2019
Date of Decision20-Jan-2020
Date of Acceptance26-Feb-2020
Date of Web Publication12-May-2020

Correspondence Address:
B Arunkumar Sharma
Department of Radiation Oncology, Regional Institute of Medical Sciences, Imphal, Manipur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_40_19

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  Abstract 


The terrestrial gamma radiation levels and associated dose rates were estimated at 120 different sites with 12 soil samples in the southern hilly regions of Manipur, India, consisting of Churachandpur and Chandel district, by 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 0.37 to 1.51 mSv/y, with a mean value of 0.85 ± 0.16 mSv/y. Whereas Churachandpur and Chandel districts showed a mean value of 0.88 ± 0.18 (ranges: 0.67 to 1.51) mSv/y and 0.83 ± 0.14 (ranges: 0.37–1.24) mSv/y, respectively. This southern region of Manipur shows slightly higher values of annual effective dose as compared with the earlier reported value of about 0.7 mSv/y for the central valley region of Manipur, and world average value of about 0.4 mSv/y, reported by United Nations Scientific Committee on the Effects of Atomic Radiation (2000). The soil analysis for226Ra,232Th, and40K of this area gives an average activity concentration of 44.3 (ranges: 23.8–78.6) Bq/kg, 169.1 (ranges: 83.6–305.1) Bq/kg, and 1489.1 (ranges: 752.5–2426.8) Bq/kg, respectively.

Keywords: Annual effective dose, Chandel, Churachanpur, external exposure index, natural radioactivity


How to cite this article:
Joyshangkar M, Suranjit S, Singh S N, Sharma B A, Singh RT. Measurement of terrestrial gamma radiation dose rate and radioactivity levels in the southern hilly regions of Manipur, India. Radiat Prot Environ 2020;43:13-20

How to cite this URL:
Joyshangkar M, Suranjit S, Singh S N, Sharma B A, Singh RT. Measurement of terrestrial gamma radiation dose rate and radioactivity levels in the southern hilly regions of Manipur, India. Radiat Prot Environ [serial online] 2020 [cited 2020 May 29];43:13-20. Available from: http://www.rpe.org.in/text.asp?2020/43/1/13/284230




  Introduction Top


It is always a need to have baseline background radiation level information and natural radionuclides data because natural ionizing radiation is the largest contributor to the effective dose received by the world's population. The contribution of natural background radiation to the annual effective dose received by the general population is about 80%.[1] Natural radionuclides of significance in soil, air, water, and living organisms include long-lived radioisotopes40 K and the isotopes of238 U and232 Th decay chains. The concentration of natural radionuclides depends primarily on the geological formation and soil type of the location; these two factors (geology and soil type) greatly influence the dose distribution from the natural terrestrial radiation.[2] Igneous rocks are observed to be associated with higher radiation levels as compared with sedimentary rocks. Most of the general population spend their maximum time (about 80%) in indoors, and there is a high chance of getting both external and internal radiation exposures from the natural radionuclide present in the building materials.[3] The main radionuclides present in indoor and outdoor environment, which contributes natural background radiation, includes238 U,232 Th,40 K,222 Rn, and220 Rn.[4],[5] Hence, the study of long-lived radioisotopes of238 U,232 Th, and40 K and their daughter radioisotopes with their derived radiological parameters are the crucial aspects for public awareness and environmental safety.[6] Although the natural radioactivity is available through the earth, the accession in specific areas varies relatively within the narrow limit.[7]

The study area under investigation covered two southern hill districts of Manipur, namely Churachandpur (C. C. Pur) and Chandel district. The evaluation of natural background gamma radiation dose in these southern hill districts of Manipur, India, aims to determine the dose of radiation exposure from the environmental radiation sources and to evaluate the annual dose rates from the natural terrestrial radiation. Similar studies have been conducted in the valley regions (which includes all four valley districts) of Manipur.[8],[9],[10] The observation of this study was compared with nationwide reports and the guidance level proposed by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).[11]


  Materials and Methods Top


Measurement of gamma radiation dose rate (Survey Meter)

NaI (Tl) scintillator-based microroentgen Survey Meter (SM) manufactured by the Nucleonix Systems Pvt. Ltd., Hyderabad, India, having a sensitivity of 1 μR/hr was used for the instantaneous measurement of dose rate by keeping the SM at a height of about 1 m distance from the ground surface. Repeated measurements not <10 times for each spot were taken. The average value for all these ten measurements is assumed to be the dose rate for that particular area.[9] One hundred and twenty sites, numbering 1–60 each for the two districts, as shown in [Figure 1], were selected for the evaluation of natural gamma radiation dose rates.
Figure 1: Map showing the southern hilly regions (Churachandpur and Chandel districts of Manipur, India. Solid triangles are the sites of soil samples in Churachandpur, and hollow triangles indicate soil samples in Chandel district

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Soil analysis

Soil samples were collected from 12 different sites [6 each from each district, as shown in [Figure 1] of the study area, from a depth of about 30 cm from the soil surface; each weighing approximately 1.5 kg, which represents the sampling sites. The samples were then packed inside a thick black polythene plastic bag, making it airtight and carefully handled to protect from any mechanical damages. Each sample is then properly labeled and carried for laboratory processing. These samples were then dried overnight (~20 h) at 110°C, homogenously grounded and finally sieved into a particle size of about 0.45 mm mesh. Approximately 250 g of the homogeneous fine mesh of each sample material was then packed inside a plastic container with predefined geometry, weighed, and properly sealed to restrict the escape of radon gas from the packed. The processed samples were then stored for about 1 month to achieve the equilibrium condition of226 Ra and232 Th along with their respective daughter nuclides.[9],[12] The activity concentrations of226 Ra,232 Th, and40 K were measured using the high-purity germanium (HPGe) detector having a relative efficiency of 35% and resolution of 1.8 keV for the gamma emission spectra of60 Co (1333.0 keV). A mixed point sources of241 Am (59.5 keV),137 Cs (661.7 keV), and60 Co (1173.2 keV and 1332.5 keV) were utilized for the energy calibration of HPGe detector system. Uranium ore standard (IAEA-RGU-1, uranium ore) having an activity of 4940.0 Bq/kg was utilized for the purpose of efficiency calibration after achieving the equilibrium condition along with their corresponding daughter radionuclides. The energies of radionuclides used for efficiency calibration are shown in [Table 1].
Table 1: Daughter radionuclides and their energies used for efficiency calibration

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The activity of40 K was calculated from 1460.0 keV gamma spectrum,226 Ra from 1766.0 keV gamma line of214 Bi, and 352.0 keV of214 Pb;232 Th from the gamma lines of 510.6 keV, 583.2 keV, and 2610.0 keV of208 Tl; and 727.2 keV of212 Bi and 238.6 keV of212 Pb. Similar methodology was carried out by Sharma et al.[8],[9],[10]

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 of40 K was evaluated from 1460 keV photopeak of40 K. The activity of226 Ra was determined from 1766 keV gamma line of214 Bi and 352 keV gamma line of214 Pb and the activity of232 Th was determined from the most prominent gamma lines of 510.6, 583.2, and 2610 keV of208 Tl, 727.2 KeV gamma line of212 Bi, and 238.6 keV gamma line of212 Pb.

Radiological parameters

Radium equivalent

Raeq is most commonly used single quantity to represent the natural radioactivity associated to those materials containing226 Ra,232 Th, and40 K. The estimation of the activity was made on the assumption that 370.0 Bq/kg of226 Ra, 259.0 Bq/kg of232 Th, and 4810.0 Bq/kg of40 K produce similar gamma dose rate as mention by the following equation:[13]

Raeq = 226 Ra +1.43232 Th + 0.07740 K (1)

Raeq value of 370.0 Bq/kg gives an annual effective dose rate (Doseeff) of approximately 1.0 mSv/y.

External exposure index

Formerly used term “Hazard index, ” the word “Hazard ” is replaced with the word “External Exposure Index (EEI), ” which indicates the level of exposure from the natural gamma radiation. The value when exceeds unity, it does not cause any significant exposure which might cause adverse health effects. To cause any adverse health effects, the index value should approximately exceed about a million. Thus, the use of the word “Hazard ” in exposure indices may be considered inappropriate for ambient background exposures as suggested by Rao.[14] It is evaluated by using the following equation given as:

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

The maximum value of EEI should be lower than unity.[15]

Absorbed dose rate (nGy/h)

The external absorbed dose rate due to terrestrial gamma radiation at 1 m above the ground surface was evaluated from the activity concentration of226 Ra,232 Th, and40 K present in the soil using the Monte Carlo method given by the UNSCEAR, 2008[11] as:

Dose (nGy/h) = 0.462CRa + 0.604CTh + 0.0417 CK (3)

Where CRa, CTh, and CK are the activity concentrations in Bq/kg for226 Ra,232 Th, and40 K, respectively.

Annual effective dose rate

For the evaluation of annual Doseeff, the conversion coefficient (0.7 Sv/Gy) was taken into account for converting the external absorbed dose in air to effective dose received by the adults and indoor occupancy factor of 0.8 since majority of the general population greatly spend 80% of their time in indoors.[2] The dose rate expressed in the unit of mSv/y was estimated using the following formula given as:

Doseeff (mSv/y) = Dose (nGy/h) × 8760 h × 0.8 × 0.7 Sv/Gy × 10−6 (4)


  Results Top


The average annual Doseeff measured by the survey meter at 120 different sites (60 each for both the districts) is shown in [Table 2]. The data were collected within a period of about 6 months. Significant variation in gamma radiation dose rate due to seasonal changes was not observed in this present study, similar to the earlier work of Sharma and Singh.[9] The Doseeff for Churachandpur district ranges from 0.67 to 1.51 mSv/y, with a mean value of 0.88 ± 0.18 mSv/y, whereas for Chandel district, it ranges from 0.37 to 1.24 mSv/y, with a mean of 0.83 ± 0.14 mSv/y, respectively. The Doseeff of the entire study area is observed as 0.85 ± 0.16 (ranges: 0.37–1.51) mSv/y.
Table 2: Average annual absorbed dose rate (Doseavg) for each sixty sites of Churachandpur and Chandel district measured by survey meter given in (mSv/y) along with their standard deviation

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Soil analysis of this region for radionuclides226 Ra,232 Th, and40 K found an average value of 44.3 (range: 23.8–78.6) Bq/kg−1, 169.1 (range: 83.6–305.1) Bq/kg−1, and 1489.1 (range: 752.5–2426.8) Bq/kg−1, respectively. The radium equivalent activity (Raeq) owing to the activity concentrations of226 Ra,232 Th, and40 K present in soil from all 12 sites varies from 237.0 to 681.6 Bq/kg. The mean value of Raeq is 400.7 ± 112.4 Bq/kg. The mean values of absorbed gamma radiation dose rate in air (Doseab), annual Doseeff, and EEI are shown in [Table 3]. It shows that the average (range) value of Doseab, Doseeff, and EEI as 184.66 (108.84–312.49) nGy/h, 0.91 (0.53–1.53) mSv/y, and 1.08 (0.64–1.84), respectively.
Table 3: Measured average activity concentration of level of226Ra,232Th, and40K in the soil samples of the southern hilly regions of Manipur in Bq/kg with their corresponding absorbed dose rate, radium equivalent activity, external exposure index, annual effective dose rate, and survey meter effective dose

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  Discussion Top


The natural background radiation levels of 120 different sites (60 each of the two districts) of these southern hilly regions of Manipur were measured using the SM [Table 2]. The mean (range) of Doseeff of this study area is observed as 0.85 ± 0.16 (0.37–1.51) mSv/y. The annual effective dose of the world as reported by the UNSCEAR, 2000,[2] is about 0.4 mSv; the world average of about 0.5 mSv/y;[16] and the national average of about 0.3 mSv/y.[11] The observed data of annual effective dose of the presents study are higher than 0.5 mSv except two sites in Chandel district (site number 42 and 43). Fifteen sites, out of 120 locations have Doseeff more than 1 mSv/y (equivalent to Raeq of 370.0 Bq/kg−1), a limit set in the Organization for Economic Cooperation and Development (OECD).[17] The concentration of the radionuclides, namely226 Ra,232 Th, and40 K for the 12 sites, as shown in [Figure 1], along with Raeq, Doseab, Doseeff, and EEI are given in [Table 3]. [Figure 2] is a graph of annual Doseeffs at 12 sites (soil samples collection sites) measured by two measurement techniques. The plot of respective Doseeffs (SM and HPGe) may be fitted with a linear equation and the coefficient of determination (R), which is essentially a measure of quantity of deviation from mean, is evaluated. The mathematical form of R is given as:
Figure 2: Survey meter reading versus soil analysis (high-purity germanium) reading for annual effective dose rate (mSv/y)

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Y and n being variable and number of observations.

The value of R in the plot fitting may be employed to study the reliability of fitting.[18] The fitted linear equation (y = 1.0077x) shows the coefficient of determination R = 0.90 for any statistical conclusion. It shows a good correlation between the two types of measurement (i.e. SM and HPGe).

The soil analysis for226 Ra,232 Th, and40 K concentration of these southern hilly regions of Manipur is observed relatively higher compared with the global and national average values [Table 4].40 K concentration value is observed to dominate over226 Ra and232 Th in all locations. The concentration of potassium usually lower in basaltic rock than in acidic rock, but more variable in basaltic rock region.[33] Soil available in these southern hilly regions is acidic with pH ranging between 4.4 and 6.8.[34],[35]40 K in this study area ranges from 752.5 to 2426.8 Bq/kg, with a mean value of 1489.1 Bq/kg, which is moderately high compared with other reports.[6],[28],[29],[30],[31],[32] Scatter plots between the Doseab and the activity concentrations of the radionuclides (226 Ra,232 Th, and40 K) are shown in [Figure 3],[Figure 4], [Figure 5]. It shows a good correlation between measured absorbed dose rates and activity concentration for232 Th with R = 0.89, moderate for40 K (R = 0.50), and least for226 Ra (R = 0.15). The radium equivalent of the radionuclide (226 Ra,232 Th, and40 K) for all 12 sites is observed as Raeq of232 Th > Raeq of40 K > 226 Ra. The contributing factors of gamma radiation absorbed dose rates of each site for all 12 different locations due to radionuclides226 Ra,232 Th, and40 K is also observed as maximum gamma absorbed dose contribution is from232 Th (average of 65%: ranges between 59% and 71%) followed by40 K (average of 29%: ranges between 21% and 44%), and least with226 Ra (average of 12%: ranges between 5% and 20%). Variation if any, with this small dose contribution from226 Ra, has the least impact to the total gamma absorbed dose and observed least correlation (R = 0.15) in this study. Moreover, the activity concentration of226 Ra and232 Th, as well as226 Ra and40 K, shows no correlation with R = 0.05 and 0.06. Whereas, the activity concentration of226 Ra and40 K shows the poor correlation with R = 0.21. EEI for the area under the study ranges from 0.64 to 1.84, with a mean value of 1.08. Out of 12 soil study sites, 8 locations exceed the threshold value unity for EEI. It is also observed that the mean value of the annual Doseeffs for all 120 sites measured by SM and 12 soil sample sites measured by HPGe are less than the average worldwide exposure of 2.4 mSv/y due to natural sources[36] and the limit proposed by OECD 1979[17] if the soil samples were used as the building material.
Table 4: A review of some reported activity concentrations of226Ra,232Th, and40K in soil samples for various country and different regions of this country

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Figure 3: Scatter plot of232Th versus measured absorbed dose rate

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Figure 4: Scatter plot of40K versus measured absorbed dose rate

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Figure 5: Scatter plot of226Ra versus measured absorbed dose rate

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  Conclusions Top


The mean values for annual effective gamma radiation dose rates for 120 different sites in the southern hilly regions of Manipur were evaluated. The mean value of the annual effective dose is observed as 0.85 ± 0.17 mSv, whereas the mean values of annual effective gamma radiation dose rates for the two districts are 0.88 ± 0.18 mSv (Churchandpur) and 0.83 ± 0.14 mSv (Chandel), respectively. The Doseeffs of the study sites are observed to be slightly higher than the world average of about 0.5 mSv/y owing to the enhanced radioactivity presence of heavy mineral and potassium deposits in this area. The average concentration of226 Ra,232 Th, and40 K in soil of this study area is 44.3 (range: 23.8–78.6) Bq/kg−1, 169.1 (range: 83.6–305.1) Bq/kg−1, and 1489.1 (range: 752.5–2426.8) Bq/kg−1, respectively. The data generated in this study may be used to form a baseline reference for comparison with nationwide as well as worldwide data.

Acknowledgments

The present article is the outcome of investigations carried out by the authors. The authors express their gratefulness to the Environment Radiation Dosimetry Laboratory, Oriental College (Autonomous), under Atomic Energy Regulatory Board, Mumbai, Government of India, New Delhi, India, for their valuable contribution and providing the laboratory support necessary for this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

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



 

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