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 Table of Contents 
ORIGINAL ARTICLE
Year : 2015  |  Volume : 38  |  Issue : 1  |  Page : 2-10  

Dose assessment to public due to exposure to natural radioactivity at the Bibiani gold mine


1 School of Nuclear and Allied Sciences, University of Ghana, Atomic-Campus;, Accra, Ghana; Ministry of Energy and Mineral Development, Kampala, Uganda
2 School of Nuclear and Allied Sciences, University of Ghana, Atomic-Campus; Radiation Protection Institute, Ghana Atomic Energy Commission, LG 80, Legon, Accra, Ghana
3 School of Nuclear and Allied Sciences, University of , Atomic-Campus, Accra, Ghana

Date of Web Publication14-Aug-2015

Correspondence Address:
Alex Twesigye
Ministery of Energy and Mineral Development, P.O Box 7270, Kampala, Uganda

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Source of Support: Nil., Conflict of Interest: None


DOI: 10.4103/0972-0464.162818

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  Abstract 

Studies have been carried out to assess exposure of the public due to natural radioactivity associated with soil, tailing and water at the Bibiani gold mine in Ghana. Concentrations of radionuclides in samples were determined by γ-ray spectrometry using high purity germanium detector. Gross-α and gross-β activity concentrations were also determined for the water samples using a low background gas-less automatic α/β counter. The mean activity concentrations of 238U, 232Th and 40K in soil/tailing samples were 16.1 ± 3.6, 16.6 ± 6.0 and 380 ± 15 bq/kg, respectively. The mean activity concentrations in water samples were 0.20 ± 0.04, 0.38 ± 0.03 and 3.05 ± 0.11 Bq/L for 226Ra, 232Th and 40K respectively. The total annual effective dose to the public was estimated to be 0.125 mSv. The radium equivalent activity, internal hazard index, and external hazard index for all soil/tailing samples are lower than the accepted safety limit value of 370 Bq/kg and 1.0 respectively. The gross-α and gross-β activity concentrations for the water samples were below the World Health Organisation recommended guideline values for drinking water quality. The results indicate an insignificant radiation exposure to the public.

Keywords: Heavy metal, Oke-Ogun, powdered milk, radionuclide


How to cite this article:
Twesigye A, Darko EO, Faanu A, Schandorf C. Dose assessment to public due to exposure to natural radioactivity at the Bibiani gold mine. Radiat Prot Environ 2015;38:2-10

How to cite this URL:
Twesigye A, Darko EO, Faanu A, Schandorf C. Dose assessment to public due to exposure to natural radioactivity at the Bibiani gold mine. Radiat Prot Environ [serial online] 2015 [cited 2019 Jun 25];38:2-10. Available from: http://www.rpe.org.in/text.asp?2015/38/1/2/162818


  Introduction Top


All living organisms are exposed to ionizing radiation comprising of cosmic rays coming from outer space, terrestrial nuclides occurring in the earth's crust, building materials, air, artificial radionuclides, water and foods and in the human body itself.[1][2][3] Naturally occurring primordial radionuclides have been present in the environment since the formation of the earth. Predominant part of the radioactivity of soil and tailing are derived from the decay of the primordial radionuclides 238U and 232Th decay series and 40K. These radionuclides make their way into the food chain through uptake by plants and animals.[4] Mining and mineral processing have been identified as among the main natural sources of exposure to radiation, especially when the ore body contains uranium, thorium and potassium in significant quantities.[3]

Generally, the activity concentrations of these radionuclides in normal rocks and soil are variable and low. However, certain minerals, including those that are commercially exploited, contain uranium and/or thorium series radionuclides at significantly elevated activity concentrations.[5]

The primary objective of this study was to determine the activity concentrations of 226Ra, 232Th and 40K in soils, tailings and water at the Bibiani gold mine in the western region of Ghana and its surrounding communities in order to evaluate the radiological hazard to the public.


  Materials and Methods Top


Description of study area

Bibiani gold mine is located at approximately latitude 6°27' north and longitude 2°17' west in the Western Region of Ghana. The mine is 250 km North-west of Accra and approximately 90 km South-west of the Ashanti capital, Kumasi. The principal and most practical access to the mine is from the east, along the Kumasi–Bibiani–Sefwi Bekwi highway. [Figure 1] shows the location of the study area and the sampling points while [Figure 2] shows the geology of the study area and the neighboring areas.
Figure 1: The location of the study area and sampling points

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Figure 2: The geology of the study area and the neighbouring areas

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The mine concession area is approximately 49 km2 and is underlain by metasedimentary rocks of the Lower Birimian in the Eastern part and by intercalated metasedimentary and metavolcanic rocks of the Upper Birimian in the western part of the mine concession. Granites occur in the south-western corner of the concession.

Sampling and sample preparation for gamma spectrometry analysis

A total of 24 samples were collected from several different locations for analysis. They included 14 soil/tailing samples and 10 water samples. The sampling strategy that was adopted for the soil/tailing samples was random.[6] At each identified location samples were randomly collected within defined borders of the area of concern. Each location was divided into grids and samples taken at different points and mixed together to give a composite sample. It is expected that the mining site should have higher values of activity concentrations of 238U, 232Th and 40K as compared to the communities if the study area has significant levels of radionuclides. Samples taken from the communities were to serve as controls. At each location, the samples were taken using a shovel. The samples were obtained at a typical depth of 5 cm from the top surface layer to produce approximately 2 kg wet weight per sample. Each sample was then packed into its own secure water-tight bag to prevent cross contamination, labeled and transported to the laboratory for analysis.[6][7][8][9]

The soil/tailing samples were air dried for 7 days and subsequently oven dried at a temperature of 105°C for 3–4 h until a constant weight was attained, thus ensuring complete removal of any residual moisture.[8] The dried samples were pulverized into a fine powder using a ball mill grinder and sieved through a standard 2 mm mesh size. The homogenized samples were then filled into 1 L Marinelli beakers and hermetically sealed with the aid of masking tape to prevent the escape of airborne 222Rn and 220Rn from the samples. All samples were weighed and stored for at least 1 month prior to measurement in order to attain radioactive secular equilibrium between the long-lived parent radionuclides and their short-lived daughter radionuclides in the 238U and 232Th decay series and counted using a high purity germanium detector (HPGD) for 36,000 s.[10],[9]

For the water samples, no special preparation was carried out. They were homogenized and filled into 1 L Marinelli beaker and stored for 1 month prior to measurements. The samples were then counted on a gamma detector (HPGD) for 36,000 s.

Instrumentation and calibration

The activity concentrations of the radionuclides in the samples were measured using a HPGD in a low background configuration[11]. The gamma spectrometry system consists of an n-type HPGD (ORTEC) coupled to a computer based multi-channel analyzer mounted in a cylindrical lead shield (100 mm thick) and cooled in liquid nitrogen at 77K (−196°C). The relative efficiency of the detector was 20% with energy resolution of 1.8 keV at γ-ray energy of 1332 keV of60 Co. The resultant spectrum of each sample was acquired via the Maestro-32 software developed by ORTEC.

The background radioactivity level in the environment around the detector was established by counting 1 L Marinelli beaker filled with distilled water for 36,000 s and in the same geometry as the samples. The background spectrum was used to correct the net peak area of γ-ray of measured isotopes. The minimum detectable activities were 0.12, 0.33 and 2.43 Bq/kg for 226Ra ( 238U), 232Th and 40K respectively.

The energy and efficiency calibrations were performed using multi-nuclide standard in the energy range of 60 keV to 2000 keV contained in a 1 L Marinelli beaker. The standard radionuclides (NW146) were uniformly distributed in solid water with volume and density of 1000 ml and 1.0 g/m3, respectively and manufactured by QSA Global GmbH, Germany. The gamma emitting radionuclides used for the calibration in the Marinelli beaker geometry were57 Co (122 keV), 137Cs (662 keV) and60 Co (1173 and 1333 keV).

Calculation of activity concentration and estimation of doses

The radioactivity concentration of 226Ra ( 238U) was determined from γ-ray energies of its daughters 214Pb (351.92 and 295.21 keV) and 214Bi (609.31 and 1764.50 keV) and the 232Th was determined from γ-ray energies of its daughters 212Pb (238.63 keV), 208Tl (583.14 and 2614.53 keV) and 228Ac (911.07 and 969.11 keV). The radioactivity concentration of 40K was determined from its γ-ray energy of 1460.80 keV.

The analytical expression used in the calculation of the activity concentrations is given by Equation (1) in Bq/kg for soil/tailing samples and Bq/L for water samples.[12]



Where, Aspec is the specific activity concentration in the samples, Nsam is total net counts for the sample in the peak range, λ is the decay constant of the parent nuclide, Td is decay time between the sampling and counting, PE is the γ-ray emission probability, ε(E) is the total counting efficiency of the detector system, Tc is the counting time, m is the mass of sample (kg) or volume (l) and the expression exp(λTd) is the correction factor for decay between sampling and counting.

Determination of natural radioactivity in water samples using gross-α and gross-β counter

Ten water samples were taken from bore-holes, tap water, water treatment plants, streams and waste water from the gold treatment plant and analyzed for gross-α and gross-β radioactivity. Three hundred milliliters of each water sample were acidified with 2 ml of concentrated HNO3 and evaporated to near dryness on a hot plate in a fume hood.

The residue in the beaker was rinsed with 1M HNO3 and evaporated again to near dryness. The residue was dissolved in the minimum amount of 1M HNO3 and transferred into a weighed 25 mm stainless steel planchet. The planchet with its content was heated until all moisture evaporated. It was then stored in a desiccator and allowed to cool and prevented from absorbing moisture.

The prepared samples were counted to determine α and β activity concentration using low background gas-less automatic α/β counting system (Canberra iMatic) calibrated with α ( 241Am) and β (90 Sr) standards. The system uses a solid state silicon (passivated implanted planar silicon) detector for α and β detection. The α and β efficiencies were determined to be 36.39 ± 2.1% and 36.61 ± 2.2% respectively. The background readings of the detector for α and β activity concentrations were 0.04 ± 0.01 and 0.22 ± 0.03 cpm respectively.

Radiation hazard indices

The radiological hazard of the NORM was determined by calculating the radium equivalent activity (Raeq), the external and internal hazard indices. For the purpose of comparing the radiological effect or activity of materials that contain 226Ra, 232Th and 40K by a single quantity, which takes into account the radiation hazards associated with them, a common index termed the Raeq activity was used. This activity index provides a useful guideline in regulating the safety standards on radiation protection for the general public residing in the area under investigation. The Raeq index represents a weighted sum of activities of the above mentioned natural radionuclides and is based on the estimation that 370 Bq/kg of 226Ra, 259 Bq/kg of 232Th and 4810 Bq/kg of 40K produce the same gamma radiation dose rates. It is the most widely used index to assess the radiation hazards and can be calculated using Equation 2.[7],[13]



Where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K in Bq/kg, respectively. The permissible maximum value of the Raeq activity is 370 Bq/kg which corresponds to an effective dose of 1 mSv for the general public.[7],[3]

The external hazard index (Hex) and internal hazard index (Hin) were calculated from Equations 3 and 4 respectively.





where Hex is the external hazard index, Hin is the internal hazard index, CRa, CTh and CK are the concentrations of 226Ra, 232Th and 40K (in Bq/kg), respectively. Hex must not exceed the limit of unity for the radiation hazard to be negligible. Hin must also be less than unity to have negligible hazardous effects of carcinogenic radon and its short-lived progeny to the respiratory system. The maximum value of Hex equal to unity and corresponds to the upper limit of Raeq activity of 370 Bq/kg.[14],[9],[3]

Absorbed dose rate in air (D)

Absorbed dose rate in the air at a height of 1 m above the ground was calculated from Equation 5.[13],[3]



Where, D is the absorbed dose rate (in nGy/h), CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K (in Bq/kg), respectively.

Estimation of annual effective dose equivalent

To estimate the annual effective dose (Eγ, ext) for soil/tailing samples, the conversion coefficient from the absorbed dose in air to effective dose of 0.7 Sv/Gy and the outdoor occupancy factor of 0.2 proposed by UNSCEAR[3] were used as shown in Equation 6.

, ext due to external gamma irradiation was calculated using the formula:



Where Dγ, ext is the average outdoor external gamma dose rate (µGy/h), Texp is the exposure duration per year, 8760 h and applying an outdoor occupancy factor of 0.2, DCFext is the effective dose to absorbed dose conversion factor of 0.7 Sv/Gy for environmental exposure to γ-ray.[3]

For the water samples, the committed effective doses (Eing) were estimated from the activity concentration of each individual radionuclide using Equation (7). A yearly water consumption rate for adults of 730 L/y was applied as well as the dose conversion factors of 238U, 232Th and 40K.[15],[16]



Where, Asp (w) is the activity concentration of the radionuclides in a sample in Bq/L, Iw is the intake of water in litres per year, and DCFing is the ingestion dose coefficient in Sv/Bq.[15]

Estimation of total annual effective dose

The total annual effective dose (ET) to each member of the public was calculated using ICRP dose calculation method.[17] The expression for the total annual effective dose is provided in Equation (8).



Where ET is the total effective dose in Sievert (Sv), Eγ, ext is the external gamma effective dose from the soil/tailing samples; Eing (w) is the effective dose from the consumption of water.

Estimate of stochastic risk

Estimate of trivial cancer risk due to nSv level doses of natural origin is not appropriate. It is well established that even in the high background regions the risks are not higher than the risk in control population.

[TAG:2]Results and Discussion[/TAG:2]

[Table 1] shows the sample locations and their respective co-ordinates at the Bibiani gold mine and its environs. The measured radioactivity concentrations of 238U, 232Th and 40K for soil/tailing samples are shown in [Table 2]. The results of the absorbed dose rates, annual effective dose, Raeq activity and hazard indices are shown in [Table 3]. The results show that the activity concentration of 238U ranged from 10.89 ± 0.69 to 23.15 ± 2.19 with an average value of 16.06 ± 3.56 Bq/kg. The highest value of activity concentration was measured in sample S11 and the lowest from sample S13. The activity concentration of 232Th ranged from 10.31 ± 0.63 to 35.84 ± 1.47 with an average value of 16.56 ± 5.96 Bq/kg. The highest value of activity concentration was measured in the sample from S11 and the lowest from sample S13. The activity concentration of 40K ranged from 20.98 ± 0.65 to 483.86 ± 14.91with an average value of 379.76 ± 14.58 Bq/kg. The highest value of activity concentration was measured in the sample from S11 and the lowest from sample S08. The results of the activity concentrations in this study compared quite well with similar studies that have been carried out in other countries as shown in [Table 4]. [Figure 3] shows the average activity concentrations of 238U, 232Th and 40K in the soil/tailings samples in comparison with the UNSCEAR 2000 report estimated values.
Table 1: Sample locations with coordinates for soil/tailing and water samples

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Table 2: Distribution of 238U, 232Th and 40K activity concentrations in the soil/tailing samples

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Table 3: The calculated absorbed dose rate (D), AEDE, Raeq, Hin and Hex obtained from all the soil/tailing samples measured in the current work

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Table 4: Comparison of 226Ra, 232Th and 40K activity concentrations in the soil/tailing samples in the study area with published data

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Figure 3: Comparison of the average activity concentrations of 238U, 232Th and 40K in the soil/tailing samples with UNSCEAR 2000 report

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The results of the calculated absorbed dose rate in the soil/tailing samples ranged from 12.14 to 40.17 nGy/h with a mean value of 33.32 nGy/h. It can be observed that all the absorbed dose rate values obtained in this study fall within the worldwide range of 18–93 nGy/h. The mean absorbed dose rate estimated in this study is lower than the worldwide mean value of 59 nGy/h.[3]

The corresponding estimated annual effective dose was 0.042 mSv/y. This value was compared with the UNSCEAR 2000 report estimated value as shown in [Figure 4]. In order to assess if the soil/tailing in the study area could be a source of public radiation exposure if used for building purposes, the following hazard indices were used; Raeq activity in Bq/kg, Hex and the Hin indices. The Raeq activity is related to the external gamma dose from the terrestrial radionuclides and the internal dose due to radon and its decay products of 210Pb and 210Po.
Figure 4: Comparison of annual effective dose due to exposure in soil/tailing and UNSCEAR

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The values of Raeq activity in the samples ranged from 27.26 to 82.72 Bq/kg with an average value of 69.09 Bq/kg. The average external and internal indices were 0.187 and 0.231, respectively. The maximum acceptable value of Raeq in building materials must be less than 370 Bq/kg for the material to be considered safe for use. The results show that the Hin and Hex values for all the soil/tailing samples are below the limit of unity, meaning that the radiation dose is below the permissible limit of 1 mSv/y recommended by ICRP.[18],[3] It can be concluded that the soil/tailing in the study area may not pose any radiological health risk to the people using these materials for building.

The mean activity concentrations of 226Ra, 232Th and 40K in the water samples collected from the study area are shown in [Table 5]. The activity concentrations of 226Ra, 232Th, and 40K are in the range of 0.150 ± 0.069–0.279 ± 0.096 Bq/L, 0.336 ± 0.075–0.438 ± 0.714 Bq/L, and 2.871 ± 0.176–3.179 ± 0.201 Bq/L with a mean value of 0.203 ± 0.037, 0.382 ± 0.031, and 3.047 ± 0.105 Bq/L respectively. The annual committed effective dose contributed by 226Ra, 232Th and 40K varied from 74.87 to 90.96 μSv/y with an average value 82.85 μSv/y. The highest value was recorded from sample W03, collected from the tailings storage facility (TSF). The lowest value was recorded from sample W02, collected from the water treatment plant and is used by the staff for domestic purposes.
Table 5: Distribution of 226Ra, 232Th and 40K activity concentrations in the water samples

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The mean activity concentrations recorded from the present study were also compared with the World Health Organization (WHO) guideline values of activity concentration for radionuclides in drinking water. The activity concentrations of 226Ra and 232Th of all the water samples were lower than the WHO guideline value of 1.0 Bq/L.[16]

World Health Organization, in its guidelines for radionuclide intake in drinking water recommends an annual committed effective dose limit of 0.1 mSv/y (100 μSv/y).[16] The average annual effective dose due to intake of radionuclides in water was compared with UNSCEAR estimated value and the average value recommended by WHO as shown in [Figure 5]. It can be observed that the calculated annual committed effective dose from drinking water in the study area is lower than the limit set by the WHO.
Figure 5: Comparison of the average annual effective dose due to intake of radionuclides in water with the average values recommended by World Health Organisation and UNSCEAR

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[Table 6] shows the gross-α and gross-β activity concentrations in water samples used in the various communities of the study area. Radionuclide concentrations in groundwater depend on the dissolution of minerals from rock aquifers. The gross-α activity concentrations in the water samples varied in a range of 0.004 Bq/L in the TSF to 0.032 Bq/L from Kiryanya stream in pipeline village with a mean value of 0.016 Bq/L. For the gross-β, the activity concentrations varied in a range of 0.026 Bq/L at the TSF to 0.283 Bq/L at mine water treatment plant with a mean value of 0.149 Bq/L. The WHO screening levels for drinking water are 0.5 Bq/L for gross-α and 1.0 Bq/L for gross-β.[16] The WHO guideline assumes a daily water consumption rate of 2 L that corresponds to an exposure lower than 0.1 mSv/y. Comparing these results with the WHO guideline values shows that all the values of the gross-α and gross-β are lower than the guideline values. This indicates that all the water sources in the study area which are designated for drinking and domestic purposes do not have significant natural radioactivity and may not pose any significant radiological hazard.
Table 6: Gross-α and gross-β activity concentrations in the water samples

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  Conclusions Top


The levels of natural radioactivity in soil/tailing and water samples collected from Bibiani gold mine were evaluated using high resolution γ-ray spectrometry. The average activity concentrations of 226U, 232Th and 40K were estimated to be 16.06 ± 3.56, 16.56 ± 5.96 and 379.76 ± 14.58 Bq/kg respectively. For the water samples, the mean activity concentrations of 238U, 232Th and 40K were 0.203 ± 0.037, 0.382 ± 0.031 and 3.047 ± 0.105 Bq/L respectively. The results in this study were compared well with studies carried out in other countries and with the worldwide average activity.[20],[24],[19],[7],[4],[2],[3] The estimated average absorbed dose rate based on soil/tailing radioactivity was found to be 33.32 nGy/h. The average annual effective doses estimated from direct external γ-ray exposure from natural radioactivity concentrations in soil/tailing and exposure from drinking water containing natural radioactivity were 0.042 and 0.083 mSv, respectively. The corresponding total annual effective dose for all the exposure pathways was 0.125 mSv.

In addition, the external and the internal hazard indices in all the samples were less than unity with average values of 0.187 ± 0.036 and 0.231 ± 0.044, respectively. From the results in this study, it can be concluded that these materials if used for construction of dwellings by the inhabitants of the study area might not pose any significant radiation hazard. The results in this study compare well with some studies in other countries.

The results indicate insignificant levels of the natural radionuclides, implying that the mining activities do not pose any significant radiological hazard to the communities in this area.



 
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    Figures

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

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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