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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 4  |  Page : 203-209  

Assessment of radioactivity concentration of natural radionuclides and radiation index parameters in soil samples of East Khasi Hills District, Meghalaya, India


1 Department of Physics, North Eastern Hill University, Shillong-793022, Meghalaya, India
2 Umthlong Nongthliew Government Secondary School, Shillong-793120, Meghalaya, India

Date of Submission04-Oct-2018
Date of Decision08-Oct-2018
Date of Acceptance13-Oct-2018
Date of Web Publication6-Feb-2019

Correspondence Address:
Bobbyland Lyngkhoi
Department of Physics, North Eastern Hill University, Shillong - 793 022, Meghalaya
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_70_18

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  Abstract 


The activity concentrations of the natural radionuclides were measured from the soil samples of East Khasi Hills District of Meghalaya, India, using gamma-ray spectroscopy. The range of the activity concentrations determined for 40K, 226Ra, and 232Th in Bq/kg was 96.6–518.0, 27.2–122.1, and 39.2–153.4, respectively, and their respective average values were 221.0, 45.9, and 68.7. The range of outdoor annual effective dose was 53.1–209.3 μSv/y, with an average value of 88.2 μSv/y. The values of radium equivalent activity (Raeq) ranged from 96.5 to 265.4 Bq/kg. Values of Raeq from all the sampling locations were less than the recommended limit of 370 Bq/kg set by the Organization for Economic Cooperation and Development except in one location, Mylliem village. The study reveals that the soil samples from the selected, sampling villages, except Mylliem village, are safe and do not pose any significant radiological threat to population exposure.

Keywords: Activity concentrations, East Khasi Hills District, gamma-ray spectroscopy, natural radionuclides, radiation index


How to cite this article:
Lyngkhoi B, War S A, Nongkynrih P. Assessment of radioactivity concentration of natural radionuclides and radiation index parameters in soil samples of East Khasi Hills District, Meghalaya, India. Radiat Prot Environ 2018;41:203-9

How to cite this URL:
Lyngkhoi B, War S A, Nongkynrih P. Assessment of radioactivity concentration of natural radionuclides and radiation index parameters in soil samples of East Khasi Hills District, Meghalaya, India. Radiat Prot Environ [serial online] 2018 [cited 2019 Jun 16];41:203-9. Available from: http://www.rpe.org.in/text.asp?2018/41/4/203/251684




  Introduction Top


Radioactivity is a natural phenomenon, and natural sources of radiation are features of the environment.[1] Human beings are continuously exposed to ionizing radiation arising from sources including cosmic rays, terrestrial radiations, natural radionuclides in water, air, soil, and plants, and artificial radioactivity from fallout in nuclear testing and medical applications.[2],[3] The radiation exposure received by individuals in the environment from the primordial sources is known to constitute about 80% of natural background radiation, the remaining 20% being from cosmic rays and nuclear processes.[4] The main sources of radiation exposure to human are that of natural radiation mainly from primordial radionuclides consisting of uranium (238 U), thorium (232Th), and potassium (40K), mostly from the decay products of 235,238U and 232Th and the single decay radionuclide of 40K.[5],[6] In 238U series, the decay chain from 226 Ra is radiologically important, and therefore, reference is often made to 226Ra instead of 238U.[7] External outdoor exposures arise from terrestrial radionuclides present at trace levels in all soils. The specific levels are related to the types of rock from which the soils originate. Higher radiation levels are associated with igneous rocks, such as granite, and lower level with sedimentary rocks.[8] Natural radioactivity is widely spread in the earth's environment and therefore depends primarily on the geological and geographical conditions and appears at different levels in the soils of each region in the world.[9],[10]

East Khasi Hills (EKH) is one of the 11 districts of the state of Meghalaya, which is known as “The Abode of Clouds.” The natural beauty of the district where foaming waterfalls, caves, and gorges around it attracts tourists from different parts of the country and outside. The capital of the state Shillong is located in EKH district (EKHD).

EKH occupies an area of 2748 km 2 and lies between 25°07” and 25°41” N Latitude and 91°21” and 92°09” E Longitude. The district is situated at an altitude of 1496 m above sea level. The district is an integral part of the Meghalaya plateau which comprises rocks of Achaean basement complex in the central and northern parts comprising gneiss, quartzite, and schist. According to the National Institute of Disaster Management, the soils of the hills are derived from gneissic complex parent materials; they are dark brown to dark reddish-brown, varying in depth from 50 to 200 cm with texture of soils varies from loamy to fine loamy.[11]

The objective of the study is to determine the natural radionuclides concentrations in soil samples collected from different locations of EKHD of Meghalaya, India. Knowing their activities, the different radiological parameters, such as radium equivalent activity (Raeq), absorbed dose rate (ADR), and annual effective dose (AED), are to be determined which would enable to estimate the exposure to radiation from the natural radionuclides present in soil samples of the study area.


  Materials and Methods Top


Sampling areas and sample preparation

Soil samples (0–60 cm depth) were collected from 31 different villages of EKHD, Meghalaya. The geographical locations including latitudes and longitudes of the sampling areas are shown in [Figure 1]. Sampling was carried out from available open areas which were least disturbed. The collected samples were then placed in labeled polythene bags and transferred to the laboratory for preparation and analysis. Stones and grass were removed from the samples and then dried to remove moisture using an oven at 100°C–110°C for about 24 h. They were then homogenized and were stored in an air-tight cylindrical plastic container (3” diameter × 5” height) for 1 month (~28 days) so that secular equilibrium was attained between 226 Ra,228 Th, and their daughter products.[12],[13] The containers were completely filled up to the brim to allow uniform distribution of 220 Rn and 222 Rn progenies and to avoid any of their accumulation on the top.
Figure 1: Sampling locations of East Khasi Hills District

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Radioactivity determination

Measurements of 40K,226 Ra, and 232Th activity levels in soil samples were determined by using a flat type 3” × 3” NaI (Tl) detector housed in a graded lead shield, PC coupled MCA card (PHAST PC-8K MCA, TYPE: MC 1008) and associated relevant electronics. Gamma-ray spectrometer coupled with NaI (Tl) detector was calibrated for 0–3148 keV energy range. Energy calibration and the percentage efficiency evaluation of the gamma-spectrometer were determined using standards, RGK-1, RGU-1, and RGTh-1 obtained from the International Atomic Energy Agency. The standards were packed in similar plastic containers which were used for soil samples' storage. Counting was carried out after allowing sufficient time for attaining secular equilibrium. The samples were then placed on top of the detector, and the spectrum was acquired for 50,000 s for each one of them. The activity of 40K in the samples was evaluated from the 1460 keV peak, for 226 Ra from 1764.6 keV gamma line of 214 Bi and for 232Th from 2614.5 keV gamma line of 208 Tl.

The minimum detection limits (MDLs) of our instrument for 40K,226 Ra, and 232Th were estimated using statistical laws and the background count rate of the instrument for each specific energies of the radionuclides.[14] The MDLs of the used gamma-ray spectrometer for 650 g sample size and 50,000 s counting time are presented in [Table 1].
Table 1: Percentage efficiency (η) and minimum detection limits of the detector

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The activities per unit mass (specific activity) of potassium, radium, and thorium in the soil samples were determined using the following equation:[10]



Where A is the specific activity (Bq/kg) of a radionuclide, N is the net peak area of a peak at energy E, B is the background count, T is the sample counting time (seconds), η is the detector efficiency, γd is the number of gammas per disintegration of the nuclide at energy E, Ms is the mass of the measured sample in kg, and σ is the standard deviation.

In situ ambient (external) gamma-radiation levels in air at each site was also measured using a Micro-R-Survey Meter (TYPE: UR 705, Nucleonix) whose minimum traceable limit is 8.8 nGy/h. The survey meter was kept at a height of 1 m above the ground while measuring the dose rate.


  Determination of Radiation Index Parameters Top


The radiological assessment was carried out to know the levels of radiation associated with the soil samples to the population living in the district. The following radiological parameters were determined:

Absorbed dose rate

ADR due to the external terrestrial gamma radiation in air at a height of about 1 m above the ground was calculated using the conversion coefficients: 0.0417 nGy h −1/Bq kg −1 for 40K, 0.462 nGy h −1/Bq kg −1 for 226 Ra, and 0.604 nGy h −1/Bq kg −1 for 232Th.[15]

ADR (nGy/h -1) =0.0417AK + 0.462ARa + 0.604ATh (2)

Annual effective dose

AED received by an individual was calculated from ADR using a conversion coefficient 0.7 Sv/Gy, with an outdoor occupancy factor of 0.2.[15]

AED (μSv/y) = ADR (nGy/h) ×8760 h × 0.2 × 10−3 × 0.7 Sv/Gy (3)

Radium equivalent activity

Raeq is a single quantity that has been introduced as an index to represent the specific activities of 40K,226 Ra, and 232Th. It is the weighted sum of the activities of the radionuclides based on the estimation that 4810 Bq/kg of 40K, 370 Bq/kg of 226 Ra, and 259 Bq/kg of 232Th produce the same gamma-ray dose rate. This index takes into account the radiation hazards associated with the natural radionuclides. The recommended limit value for Raeq is 370 Bq/kg as recommended by the Organization for Economic Cooperation and Development (1979).[16] Raeq is calculated using the following equation:[17],[18]

Raeq = 0.077 AK + ARa + 1.43 ATh (4)

Where AK, ARa, and ATh are the specific activities of 40K,226 Ra, and 232Th in Bq/kg.


  Results and Discussions Top


[Table 2] shows the range and specific activities of 40K,226 Ra, and 232Th in the soil samples from 31 different locations of EKHD. As can be seen from [Table 2], the minimum average activity concentrations of 40K was 96.6 Bq/kg (Sohra), of 226 Ra was 27.2 Bq/kg (Nongkynrih), and of 232Th was 39.2 Bq/kg (Mawngap). The maximum activity concentrations of 40K, 226Ra, and 232Th were measured from soil samples of Mylliem with average values of 518.0 Bq/kg for 40K, 122.1 Bq/kg for 226 Ra, and 153.4 Bq/kg for 232Th. The concentrations of 40K and 226 Ra were observed to vary from one sampling village to another under our study area. This may be explained as these radionuclides exhibit some degree of mobility and are present in soils as cations and as such tend to be absorbed in soil minerals.[19] Higher activity concentrations of the above radionuclides were observed from soil samples of Mylliem village when compared with the rest of the sampling villages.
Table 2: Activity concentrations of 40K, 226Ra, and 232Th in Bq/kg and 232Th/226Ra ratio

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The highest concentration of 40K,226 Ra, and 232Th found from Mylliem village may be interpreted based on its geology. It is reported that the Mylliem village is known as the Mylliem granitoids of the Meghalaya plateau, Northeastern India. The Mylliem granitoids represent one of the disharmonic neoproterozoic igneous plutons which cover an area of about 40 km 2. According to the International Union of Geological Science system of classification, Mylliem granitoids have been characterized as granite and granodiorite.[20],[21] Therefore, the presence of the granitic rock might have contributed to the higher activity concentrations.[8],[3]

The world average global concentration levels of 40K,226 Ra, and 232Th in the soil are 400, 35, and 30 Bq/kg, respectively.[8] The soils samples from all sampling locations exhibited 40K activity concentration lower than the global average value, except Mylliem (EK29). Soil samples from Tyrna (EK6), Mawryngkneng (EK11), Lawbah (EK15), Mawngap (EK16), and Nongkynrih (EK21) showed average values of 226 Ra lower than the global average value while the rest showed their values to be higher. The soil samples from all the sampling locations showed their average values of 232Th concentration to be more than 30 Bq/kg. In all samples, the activity concentrations of the natural radionuclides were of the order 40K > 232Th > 226 Ra. The activity concentration of 232Th higher than 226 Ra in all the sampling locations may be due to the absorbing of thorium by soil in a very immobile form.[20] In addition,232Th/238 U ratio [Table 2] indicated a higher abundance of 232Th over 226 Ra.40K is found to dominate over 226 Ra and 232Th in all the locations as it is one of the most abundant elements found in nature.

[Table 3] shows the calculated ADRs (CADRs) in air resulting of gamma radiation from the natural radionuclides present in soil samples at 1 m above the ground surface from all sampling locations. The results show that the dose rate varies from 43.3 to 170.6 nGy/h with an overall average value of 72.0 nGy/h. The obtained overall average value from the present study area is found to be slightly higher than the reported Indian average value of 69 nGy/h [22] and the world average value of 59 nGy/h.[8] [Figure 2] shows a comparison of the average values of the ADR from the sampling villages under the study area with that of India, with the world, and also with the high background radiation areas (HBRAs).[8],[12] As can be seen from [Figure 2], the obtained overall average value of ADR from the present study area is found to be much less than the HBRAs.
Table 3: Radiation hazard indices (measured absorbed dose rate, calculated absorbed dose rates, annual effective dose and radium equivalent activity)

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Figure 2: Comparison of calculated absorbed dose rate from the study area and high background radiation areas; *Avg: Average

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The mean CADR and the mean measured ADR (MADR) are depicted in [Figure 3]. The difference between them might have resulted due to the treatment of the samples before spectrometry. The factors such as density, humidity, and compactness degree in situ are reported to be different between the treated samples and the dried samples.[23] In addition, the measured dose values obtained using the radiation dosimeter are higher as it responses to the high-energy beta-rays as well as contribution due to the cosmic radiation.[12]
Figure 3: Calculated absorbed dose rate and measured absorbed dose rate

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Scatter plots between the MADR and the activity concentrations of the radionuclides (40K,226 Ra, and 232Th) are shown in [Figure 4], [Figure 5], [Figure 6]. A strong positive correlation exists between 226 Ra and 232Th with the MADR, and the values are 0.74 and 0.85, respectively. This gives a picture that most of the contribution to the dose comes from the 226 Ra and 232Th series and less from 40K. The t-test at 95% confidence level confirms the significant relationship between the MADR and the activity concentration of the three measured radionuclides.
Figure 4: Scatter plot of 40K versus measured absorbed dose rate

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Figure 5: Scatter plot of 226Ra versus measured absorbed dose rate

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Figure 6: Scatter plot of 232Th versus measured absorbed dose rate

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The AED (outdoor) from the selected locations ranges from 53.1 to 209.3 μSv/y [Table 3], with an overall average value of 88.2 μSv/y. The value is higher than the world average value of 70.0 μSv/y.[8] [Figure 7] shows the AED from different sampling locations.
Figure 7: Annual effective dose

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The values of Raeq for the analyzed soil samples are given in [Table 3]. The Raeq values ranging from 96.5 to 381.3 Bq/kg with an average value of 161.2 Bq/kg show that Raeq is much less than the recommended safety limit of 370 Bq/kg.[16] The values of Raeq from all the sampling locations as illustrated in [Figure 8] are below the safety limits which is 370 Bq/kg. For Mylliem village, the value of Raeq is slightly higher than the recommended limit.
Figure 8: Radium equivalent activity

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  Conclusions Top


The results obtained from the analysis of gamma-ray spectroscopic data of soil samples collected from the selected areas of EKHD of Meghalaya, India, show that the average activity concentration of 40K (221.0 Bq/kg) is lower than the world average value (400 Bq/kg). However, the values of 226 Ra (45.9 Bq/kg) and 232Th (68.7 Bq/kg) are slightly higher than the world average values which are 35 Bq/kg and 30 Bq/kg, respectively. The calculated average ADR of 71.9 nGy/h from the selected locations is found to be slightly higher than the Indian average value of 69 nGy/h and more than the world average value of 59 nGy/h. However, the value is much less when compared with HBRAs. The average AED (outdoor) estimated is 88.2 μSv/y while that of the world average value is 70.0 μSv/y.

The values of Raeq as calculated from the measured activities of the radionuclides are found to be below the reported standard levels of 370 Bq/kg. The findings of our study show that the soils from all the selected, studied locations except Mylliem are safe without posing any significant radiological threat to population exposure. As for Mylliem village, more research work has to be carried out to have a better understanding of the high concentration of the natural radionuclides.

Acknowledgments

We are thankful to the Atomic Minerals Directorate, NER, Shillong, for the help rendered to us in getting the samples crushed homogeneously.

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], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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