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World Applied Sciences Journal 13 (11): 2338-2343, 2011 ISSN 1818-4952 © IDOSI Publications, 2011
Natural Radioactivity Concentrations and Dose Assessment in Shore Sediments along the Coast of Greater Accra, Ghana A. Amekudzie, G. Emi-Reynolds, A. Faanu, E.O. Darko, A.R. Awudu, O. Adukpo, L.A.N. Quaye, R. Kpordzro, B. Agyemang and A. Ibrahim Radiation Protection Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra, Ghana Abstract: Radioactivity concentration of Naturally Occurring Radioactive Materials (NORMs) in shore sediments (samples) collected along the coast of Accra were analyzed using a gamma spectrometry with a high purity germanium detector. The specific activity concentrations range from 8.60 Bqkg -1 to 61.01 Bqkg -1 with a mean of 29.78 Bqkg -1 for 40 K, 0.62 Bqkg -1 to 148.80 Bqkg -1 with a mean of 22.04 Bqkg -1 for 226 Ra and 0.17 Bqkg -1 to 732.60 Bqkg -1 with a mean of 108.60 Bqkg -1 for 232 Th. The 40 K concentration compares well with the world average value while that for 226 Ra and 232 Th does not. The average absorbed dose rate was estimated to range from 0.75 nGyh-1 to 509.38 nGyh-1 with an average of 77.02 nGyh-1 , while the average annual effective dose also ranged from 0.00 mSvy-1 to 0.63 mSvy-1 with an average of 0.09 mSvy-1 was also estimated. The average radium equivalent activity and average external hazard index were also estimated to be 9.00 Bqkg -1 and 0.48 respectively and they both compare well with internationally acceptable values. Key words: Shore sediments radioactivity concentration gamma spectrometry annual effective dose •
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INTRODUCTION Beach sand or soil is mineral deposits formed through the weathering and erosion of rocks. These deposits found at different levels within the sand contain natural radionuclides that contribute to ionizing radiation exposure on earth. Studies carried out on radiation hazards arising from the use of sand or soil has shown that natural radiation is the largest contributor to external dose to the world population [1-3]. Research findings have shown that natural radioactive elements present in soil are primordial radionuclides from the 238 U series, 232 Th series and 40 K normally known as Naturally Occurring Radioactive Materials (NORMs) [4]. Greater Accra, the capital of Ghana is where most economic activities take place. It has its capital to be Accra and is populated by 2,905,726 citizens within six districts and it covers an area of 3245km2 . Greater Accra has a coastline of approximately 225km stretching from Kokrobite on the west to Ada on the east [5]. The coastline of the Greater Accra Region is habited by individuals who survive by producing bricks for building purposes from the beach sand. The coastline is also a very popular location for holiday makers and tourists.
•
For these reasons, it is important to verify the extent of any abnormality along the coast with respect to NORMs. There has been very little work done on the distribution of radionuclides and their concentration along the coastal belt of Ghana [6]. The aim of this work is to assess the activity concentrations of natural radionuclides in shore sediment and also the annual effective dose rate to Ghanaians living along the coast of the Greater Accra Region and additional information for the evaluation of a baseline activity concentrations trend of along the coast of Ghana. MATERIALS AND METHOD Sample collection: Thirty-five (35) samples consisting of shore sediments were collected from seven (7) different beach sites along the coast of the Greater Accra Region of Ghana (Fig. 1). At each site, samples of shore sediments were randomly collected from three (3) different locations at a depth of 0.20m (20cm) from the surface at intervals of 2m and transferred into polyethylene bags and labeled accordingly. The background gamma dose rates at 1m above ground level were measured using Rados Universal Radiation Survey Meter, model RDS-200 and manufactured in Finland.
Corresponding Author: Dr. A. Amekudzie, Radiation Protection Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra, Ghana
2338
World Appl. Sci. J., 13 (11): 2338-2343, 2011
Fig. 1: Geological location of sampling sites Sample preparation: At the laboratory, the samples were air dried for one week then oven dried for 24 hours at 105o C. They were then grinded into powder and sieved through a 2mm wire mesh. The samples prepared into a 1L Marinelli beaker and weighed, sealed and labeled. The prepared samples were allowed to stay for four (4) weeks to allow secular equilibrium to be established between the long-lived parent and daughter nuclides. The samples were each counted on a High Purity Germanium Detector for ten hours (36000s). Sample analysis and calculation Calibration of the gamma spectrometry system: Before analysis of the sample, the detector was
calibrated with respect to energy and efficiency calibration. The energy calibration related channel number of the spectrometer to the energy of the standard reference material. The calibration was performed by matching principal gamma rays in the spectrum of the standard to the channel numbers. The general relationship between energy and channel number is given as: (1) Where E is energy in keV, Cn is channel number, i = 0, 1, 2, 3… n and A i is calibration constant [7]. The calibration was carried out using a mixed standard containing the following radionuclides 139 Ce, 2339
World Appl. Sci. J., 13 (11): 2338-2343, 2011 137
Cs, 88 Y, 57 Co and 60 Co homogenously mixed in a solid 1L marinelli beaker with corresponding characteristic energies of 122 keV, 662 keV, 1836 keV, 1172 keV and 1332 keV respectively. The expression of the energy (in keV) calibration was represented by the relationship below (2) Similarly, the efficiency calibration was performed using the mixed radionuclide standard in the 1L Marinelli beaker by acquiring spectrum for ten hours (10 hrs). The net count rate was determined at photopeaks for all energies used for determining the efficiency. The efficiency is related to the count rate of standard [8] as: (3) Where PE is the gamma-ray emission probability for energy E, ε (E) is the efficiency of the detector, NT is the total counts under a photopeak, NB is background count, ASTD is the activity (Bq) of calibration standard at the time of measurement and TSTD is the counting time of the standard [9]. The efficiency of the detector is related to energy by the expression: (4) Where ε(E) is the efficiency, a, b and c are polynomial constants and x is the energy (keV) of a particular photopeak. Below is a table showing the radionuclides used for the efficiency calibration (Table 1) and a graph showing the efficiency curve (Fig. 4) with the coefficient of regression (R2 ). Thus the efficiency energy relationship expression which was used for the efficiency calculation was obtained as
+0.0433 For x>100 keV
(5)
Specific activity calculation: The gamma lines of 238.63, 351.92 and 1460.83 were used to determine 232 Th, 226 Ra and 40 K respectively. The specific activity concentrations of 232 Th, 226 Ra and 40 K in Bq/kg were determined using the following expression
probability, Td is decay time between the sample preparation and counting, Tc is the counting time, ε(E) is the total counting efficiency of the detector system, M is the mass of sample and the expression exp -(λ.Td ) is the correction factor for decay between sampling and counting [10]. Dose rate calculation: The absorbed dose rates at 1m (meter) above the ground were determined by applying the expression [1, 11]
(7) Where, A (in nGy/Bq is A sp (Specific Activity Concentration) for Th, K and Ra with dose conversion factors of 0.604, 0.0417 and 0.462 respectively. The annual Effective dose rate in an outdoor environment was calculated by the following formula: (8) Where, ED is the annual effective dose in Sv, D is the absorbed dose rate in nGyh-1 , T is the annual exposure time in hours of 8760, TC is the outdoor time conversion factor of 0.2 and F is the dose conversion factor of 0.7 [1, 12, 13]. External hazard index: Shore sediments and other raw materials are used in building and other construction works usually by the coastal dwellers. The natural radioactivity in these raw materials are usually determined from 226 Ra, 232 Th and 40 K contents. Minerals from beach sand as well as soils and rejected light sands are used by industries and building construction firms. Hence the gamma-ray radiation hazards due to the specified radionuclides can be assessed using indices [14]. The radium equivalent activity is an index that represents the specific activities of 226 Ra, 232 Th and 40 K by a single quantity which takes into account the radiation hazards associated with them. This can be calculated using the equation below: (9)
where ARa, ATh and AK are the specific activities of 226 Ra 232 Th and 40 K in Bqkg -1 respectively. The Ra eq is related to the external gamma dose and internal dose due to radon and its daughters. The maximum (6) value of Ra eq in building materials must be < 370Bqkg -1 for safe use. The external hazard index is derived from Ra eq Where, Asp is the Specific Activity Concentration, λ is the decay constant, Nsam is total net counts sampling expression through the supposition that its maximum in the peak range, PE is the gamma-ray emission value allowed corresponds to the upper limit of Ra eq 2340
World Appl. Sci. J., 13 (11): 2338-2343, 2011
(i.e. 370 Bqkg -1 ). The index value must be less than unity in order to keep the radiation hazard insignificant. This means the radiation exposure due to the radioactivity from construction material is limited to 1.0 mSvy-1 . The external hazard index was determined from the equation below: (10) where, ARa, ATh and AK are the specific activities of 226 Ra 232 Th and 40 K in Bq kg -1 respectively [15]. Table I: Radionuclides used for the efficiency calibration Nuclide
Energy (KeV)
Cobalt -57 Caesium-137
Half-life (days)
122
Efficiency
271.83
0.04
RESULTS AND DISCUSSION Table 1 shows the radionuclides used for the efficiency calibration. The summary of the research carried out are tabulated in Table 2 and 4 below. Table 2: Mean specific activity concentration of 40K, 232Th and 226Ra in the various shore sediment samples
Samples
Mean specific Activity concentration (Bqkg-1) -------------------------------------------------------40 226 232 K Ra Th
Chorkor Beach
21.31
1.42
1.49
James Town Beach Labadi Beach Nungua Beach Kokrobite Beach
14.67 43.97 41.17 17.76
0.82 140.80 4.05 3.74
1.04 732.60 8.64 6.63
662
137.66
0.01
Cobalt -60
1173
1925.30
0.01
Cobalt -60
1333
1925.30
0.01
Teshie Beach Weija Beach
61.01 8.60
2.85 0.62
9.66 0.17
Yttrium-88
1836
106.63
0.01
Average
29.78
22.04
108.60
Table 3: Comparison of specific activities of K, Th and Ra in Bqkg-1 in shore sediment samples from greater accra region and other studies in different beaches of the world Specific activity (BqKg-1 ) --------------------------------------------------------------------------------40 226 232 K Ra Th
Location SS-1 SS-2 SS-3 SS-4 SS-5 SS-6 SS-7 Preta beach Southeastern Brazil Visakhapatnam, India Northeast Coast, Spain Red seashore sediment, Egypt Beach sand, Al-Maidan, North Sinai, Seabed sand, Tuen Mun Hong Kong Coastal Karnataka Zircon, Bangladesh Global recommended Value range Average value
21.31 14.67 43.97 41.17 17.76 61.01 8.60 47-283 136-1087 98-1011 77 1210 55 472 100-700 370
1.42 0.82 140.80 4.05 3.74 2.85 0.62 54-180 100-400 5-19 95.3-105.6 108.0 27.7 249.2 6439.0 7-50 35.0
1.49 1.04 732.60 8.64 6.63 9.66 0.17 128-349 300-600 5-44 2.3-221.9 146.0 29.8 489.6 1324.0 10-50 40
Reference Current Current Current Current Current Current Current [18] [19] [20] [21] [22] [23] [24] [2]
Work Work Work Work Work Work Work
[16]
Table 4: Absorbed dose rate, annual effective dose rate, radium equivalent activity and external hazard index of studied shore sediment samples Absorbed dose Annual effective Radium equivalent External Samples rate (nGyh-1 ) dose rate (mSv) activity (Bqkg-1 ) hazard Index Chorkor beach James-Town Beach Labadi beach Nungua beach Kokrobite beach Teshie beach Weija beach Average
2.44 1.62 509.38 8.80 6.47 9.69 0.75 77.02
0.00 0.00 0.63 0.01 0.01 0.01 0.00 0.09
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0.11 3.03 56.41 0.67 0.51 0.74 1.52 9.00
0.01 0.01 3.22 0.05 0.04 0.06 0.00 0.48
World Appl. Sci. J., 13 (11): 2338-2343, 2011
Potassium-40 (40 K) content along the coast ranged between an average of 8.60 Bqkg -1 and 61.01 Bqkg -1 . Thorium-232 (232 Th) and Raduim-226 (226 Ra) contents ranged from an average of 0.17 Bqkg -1 to 732.60 Bq kg 1 and 0.62 Bqkg -1 to 140.80 Bqkg -1 respectively. The mean absorbed dose rate in nGyh-1 , the mean annual effective dose in mSvy-1 , the radium equivalent activity in Bqkg -1 and the external hazard index for the various beaches are also shown in Table 4. The mean absorbed dose rate due to the presence of 226 Ra, 232 Th and 40 K in the shore sediment samples along the coast of the Greater Accra Region of Ghana was found to range between 0.75 nGyh-1 and 509.38 nGyh-1 with an average of 77.02 nGyh-1 . The mean annual effective dose rate was found to range between 0.00 mSvy-1 and 0.63 mSvy-1 with an average of 0.09 mSvy-1 while the radium equivalent activity was ranging between 0.11 Bqkg -1 and 56.41 Bqkg -1 with an average of 9.00 Bqkg -1 . The external hazard index also ranged from 0.00 to 3.22, with an average of 0.48.
CONCLUSION The study of the radionuclide concentration levels in shore sediments along the coast of the Greater Accra Region in Ghana has been carried out. It is therefore envisaged that the levels of radioactivity associated with the used of the beaches (shore sediments) would not pose any hazards. The annual effective dose may not have any effect on the natural background ionization radiation of the environment; however it may increase over longer periods especially that of Labadi beach. REFERENCES 1.
2.
DISCUSSION The average specific activity concentration of Ra, 232 Th and 40 K in this work compares well with similar results from other parts of the world as seen in Table 3. With the exception of the sample taken from Labadi beach, the averaged absorbed dose rates due to the presence of 226 Ra, 232 Th and 40 K in the rest of the samples studied were found to be much lower than the world average value of 60 nGyh-1 and the average annual effective dose also compares well with the worldwide average of 0.07 mSvy-1 [1]. The average annual effective dose rate along the coast was estimated to be 0.09 mSvy-1 . This is lower than the International Commission for Radiological Protection value of 1.0 mSvy-1 for practices for members of the public. The average absorbed dose rate and average annual effective dose estimates from the Labadi beach sample are 509.38 nGyh-1 and 0.63 mSvy-1 respectively. These values are due to the presence of high activity concentrations from 232 Th and 226 Ra which is attributed to a significant difference in sand properties, like density, humidity and porosity. The experimental results of radium equivalent activity which indicate radiation hazards arising from the various beach samples studied show that the average Ra eq values are below the internationally acceptable value of 370 Bqkg -1 . The estimated external hazard indices were all also less than unity, i.e. Hex=1.0. This implies that activities involving the use of shore sediments (beach sand) are safe and do not attract any high levels of radiation exposure. 226
3.
4.
5. 6.
7.
8. 9.
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UNSCEAR, 2000. Sources and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, Report to general assembly. Annex B exposure from natural radiation sources. United Nations, New York. Alam, M.N., M.I. Chowdhury, M. Kamal, S. Ghose, M.N. Islam and M.N. Mustafa et al., 1999. The 226 Ra 232 Th and 40 K activities in beach sand minerals and beach soils of Cox’s Bazar, Bangladesh. Journal Environmental Radioactivity, 46: 243-250. Singh, S., A. Rani and R.K. Mahajan, 2005. 226 Ra, 232 Th and 40 K analysis in soil samples from some areas of Punjab and Himachal Pradesh, India using gamma ray spectrometry. Journal of Radiation Measurements, 39: 431-439. El-Kameesy, S.U., A.S. El-Ghany, S.M. ElMinyawi, Miligy and El-Mabrouk E.M., 2008. Natural Radioactivity of Beach Sand Samples in the Tripoli Region, Northwest Libya. Turkish Journal of Environmental Science, 32: 245-251. Ghana Statistical Service, 2002. The year 2000 population census. Yeboah, J., M. Boadu and E.O. Darko, 2001. Natural Radioactivity in soils and Rocks within the Greater Accra region of Ghana. Journal of Radioanalytical and Nuclear Chemistry, 249: 629-632. Darko, E.O. and C. Schandorf, 1998. Performance Characteristics of a low-level Gamma Spectrometry System. Journal of Applied Science and Technology, 3: 45-54. Pan, Z., 1999. Communication to the UNSCEAR Secretariat. Darko, E.O. and A. Faanu, 2007. Baseline Radioactivity Measurements in the vicinity of a gold processing plant. Journal of Applied Science and Technology, 12: 18-24.
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10. Oresengun, M.O., K.M. Decker and C.G. Sanderson, 1993. Determination of Self Absorption Correction by Computation in routine Gamma-ray Spectrometry for typical Environmental Samples. Journal of Radioactivity and Radiochemistry, 4: 38-45. 11. Veiga, R., N. Sanches, R.M. Anjos, K. Macario, J. Bastos and M. Iguatemy et al., 2006. Measurement of Natural Radioactivity in Brazilian Beach Sands. Journal of Radiation Measurement, 41: 189-196. 12. Navas, A., J. Soto and J. Machín, 2002. 238 U, 226 Ra, 210 Pb, 232 Th and 40 K activities in soil profiles of the Flysch sector (Central Spanish Pyrenees). Journal of Applied Radiation Isotopes, 57: 579-589. 13. Yang, Y.X., X.M. Wu, Z.Y. Jiang, W.X. Wang, J.G. Lu and J. Lin, 2005. Radioactivity Concentrations in Soils of the Xiazhuang Granite area. China. Journal Applied Radiation Isotopes, 63: 225-259. 14. NEA-OECD, 1979. Exposure to radiation from natural radioactivity in building materials. Report by NEA Group of Experts, OECD: Paris. 15. Beretka, J. and P.J. Mathew, 1985. Natural radioactivity of Australian building materials, industrial wastes and by-products. Journal of Health Physics, 48: 87-95. 16. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation), 1993. Sources, effects and risks of ionizing radiation. Report to the General Assembly with Scientific Annexes A and B. New York United Nations. 17. Ajayi, I.R., O.S. Ajayi and A.S. Fusuyi, 1995. The Natural Radioactivity of Surface Soils in Ijero. Ekiti Nigeria. Nigeria Journal of Physics, 7: 101-103.
18. Freitas, A.C. and A.S. Alencar, 2004. Gamma dose rates and distribution of natural radionuclides in sand beaches. Ilha Grande Southeastern Brazil. Journal of Environmental Radioactivity, 75: 211-223. 19. Kalyani, V.D.M.L., M.V.S. Chandrasekhar Rao, G. Sree Krishna Murty, G. Satyanarayana and D.L. Sastry, Sahasrabhude et al., 1990. Analysis of 232 Th and 238 U in the beach sands and the ocean sediments. Indian Journal of Protection, 10: 931-934. 20. Rosell, J.R., X. Ortega and X. Dies, 1991. Natural and artificial radionuclides on the northeast coast of Spain. Journal of Health Physics, 60: 709-712. 21. El-Mamoney, M.H. and A.E.M. Khater, 2004. Environmental characterization and radioecologicalimpacts of non-nuclear industries on the Red Sea coast. Journal of Environmental Radioactivity, 73: 151-168. 22. Seddeek, M.K., H.B. Badran, T. Sharshar and T. Elnimr, 2005. Characteristics, spatial distribution and vertical profile of gamma-ray emitting radionuclides in the coastal environment of North Sinai. Journal of Environmental Radioactivity, 84: 21-50. 23. Yu, K.N., Z.J. Guan, M.J. Stokes and E.C.M. Young, 1992. The assessment of the natural radiation dose committed to the Hong Kong people. Journal of Environmental Radioactivity, 17: 31-48. 24. Narayana, Y., H.M. Somashekarappa, A.P. Radhakrishna, K.M. Balakrishna and K. Siddappa, 1994. External gamma radiation dose rates in coastal Karnataka. Journal of Radiological Protection, 14: 257-264.
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Natural Radioactivity Concentrations and Dose Assessment in Shore Sediments along the Coast of Greater Accra, Ghana A. Amekudzie, G. Emi-Reynolds, A. Faanu, E.O. Darko, A.R. Awudu, O. Adukpo, L.A.N. Quaye, R. Kpordzro, B. Agyemang and A. Ibrahim Radiation Protection Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra, Ghana Abstract: Radioactivity concentration of Naturally Occurring Radioactive Materials (NORMs) in shore sediments (samples) collected along the coast of Accra were analyzed using a gamma spectrometry with a high purity germanium detector. The specific activity concentrations range from 8.60 Bqkg -1 to 61.01 Bqkg -1 with a mean of 29.78 Bqkg -1 for 40 K, 0.62 Bqkg -1 to 148.80 Bqkg -1 with a mean of 22.04 Bqkg -1 for 226 Ra and 0.17 Bqkg -1 to 732.60 Bqkg -1 with a mean of 108.60 Bqkg -1 for 232 Th. The 40 K concentration compares well with the world average value while that for 226 Ra and 232 Th does not. The average absorbed dose rate was estimated to range from 0.75 nGyh-1 to 509.38 nGyh-1 with an average of 77.02 nGyh-1 , while the average annual effective dose also ranged from 0.00 mSvy-1 to 0.63 mSvy-1 with an average of 0.09 mSvy-1 was also estimated. The average radium equivalent activity and average external hazard index were also estimated to be 9.00 Bqkg -1 and 0.48 respectively and they both compare well with internationally acceptable values. Key words: Shore sediments radioactivity concentration gamma spectrometry annual effective dose •
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INTRODUCTION Beach sand or soil is mineral deposits formed through the weathering and erosion of rocks. These deposits found at different levels within the sand contain natural radionuclides that contribute to ionizing radiation exposure on earth. Studies carried out on radiation hazards arising from the use of sand or soil has shown that natural radiation is the largest contributor to external dose to the world population [1-3]. Research findings have shown that natural radioactive elements present in soil are primordial radionuclides from the 238 U series, 232 Th series and 40 K normally known as Naturally Occurring Radioactive Materials (NORMs) [4]. Greater Accra, the capital of Ghana is where most economic activities take place. It has its capital to be Accra and is populated by 2,905,726 citizens within six districts and it covers an area of 3245km2 . Greater Accra has a coastline of approximately 225km stretching from Kokrobite on the west to Ada on the east [5]. The coastline of the Greater Accra Region is habited by individuals who survive by producing bricks for building purposes from the beach sand. The coastline is also a very popular location for holiday makers and tourists.
•
For these reasons, it is important to verify the extent of any abnormality along the coast with respect to NORMs. There has been very little work done on the distribution of radionuclides and their concentration along the coastal belt of Ghana [6]. The aim of this work is to assess the activity concentrations of natural radionuclides in shore sediment and also the annual effective dose rate to Ghanaians living along the coast of the Greater Accra Region and additional information for the evaluation of a baseline activity concentrations trend of along the coast of Ghana. MATERIALS AND METHOD Sample collection: Thirty-five (35) samples consisting of shore sediments were collected from seven (7) different beach sites along the coast of the Greater Accra Region of Ghana (Fig. 1). At each site, samples of shore sediments were randomly collected from three (3) different locations at a depth of 0.20m (20cm) from the surface at intervals of 2m and transferred into polyethylene bags and labeled accordingly. The background gamma dose rates at 1m above ground level were measured using Rados Universal Radiation Survey Meter, model RDS-200 and manufactured in Finland.
Corresponding Author: Dr. A. Amekudzie, Radiation Protection Institute, Ghana Atomic Energy Commission, P.O. Box LG 80, Legon-Accra, Ghana
2338
World Appl. Sci. J., 13 (11): 2338-2343, 2011
Fig. 1: Geological location of sampling sites Sample preparation: At the laboratory, the samples were air dried for one week then oven dried for 24 hours at 105o C. They were then grinded into powder and sieved through a 2mm wire mesh. The samples prepared into a 1L Marinelli beaker and weighed, sealed and labeled. The prepared samples were allowed to stay for four (4) weeks to allow secular equilibrium to be established between the long-lived parent and daughter nuclides. The samples were each counted on a High Purity Germanium Detector for ten hours (36000s). Sample analysis and calculation Calibration of the gamma spectrometry system: Before analysis of the sample, the detector was
calibrated with respect to energy and efficiency calibration. The energy calibration related channel number of the spectrometer to the energy of the standard reference material. The calibration was performed by matching principal gamma rays in the spectrum of the standard to the channel numbers. The general relationship between energy and channel number is given as: (1) Where E is energy in keV, Cn is channel number, i = 0, 1, 2, 3… n and A i is calibration constant [7]. The calibration was carried out using a mixed standard containing the following radionuclides 139 Ce, 2339
World Appl. Sci. J., 13 (11): 2338-2343, 2011 137
Cs, 88 Y, 57 Co and 60 Co homogenously mixed in a solid 1L marinelli beaker with corresponding characteristic energies of 122 keV, 662 keV, 1836 keV, 1172 keV and 1332 keV respectively. The expression of the energy (in keV) calibration was represented by the relationship below (2) Similarly, the efficiency calibration was performed using the mixed radionuclide standard in the 1L Marinelli beaker by acquiring spectrum for ten hours (10 hrs). The net count rate was determined at photopeaks for all energies used for determining the efficiency. The efficiency is related to the count rate of standard [8] as: (3) Where PE is the gamma-ray emission probability for energy E, ε (E) is the efficiency of the detector, NT is the total counts under a photopeak, NB is background count, ASTD is the activity (Bq) of calibration standard at the time of measurement and TSTD is the counting time of the standard [9]. The efficiency of the detector is related to energy by the expression: (4) Where ε(E) is the efficiency, a, b and c are polynomial constants and x is the energy (keV) of a particular photopeak. Below is a table showing the radionuclides used for the efficiency calibration (Table 1) and a graph showing the efficiency curve (Fig. 4) with the coefficient of regression (R2 ). Thus the efficiency energy relationship expression which was used for the efficiency calculation was obtained as
+0.0433 For x>100 keV
(5)
Specific activity calculation: The gamma lines of 238.63, 351.92 and 1460.83 were used to determine 232 Th, 226 Ra and 40 K respectively. The specific activity concentrations of 232 Th, 226 Ra and 40 K in Bq/kg were determined using the following expression
probability, Td is decay time between the sample preparation and counting, Tc is the counting time, ε(E) is the total counting efficiency of the detector system, M is the mass of sample and the expression exp -(λ.Td ) is the correction factor for decay between sampling and counting [10]. Dose rate calculation: The absorbed dose rates at 1m (meter) above the ground were determined by applying the expression [1, 11]
(7) Where, A (in nGy/Bq is A sp (Specific Activity Concentration) for Th, K and Ra with dose conversion factors of 0.604, 0.0417 and 0.462 respectively. The annual Effective dose rate in an outdoor environment was calculated by the following formula: (8) Where, ED is the annual effective dose in Sv, D is the absorbed dose rate in nGyh-1 , T is the annual exposure time in hours of 8760, TC is the outdoor time conversion factor of 0.2 and F is the dose conversion factor of 0.7 [1, 12, 13]. External hazard index: Shore sediments and other raw materials are used in building and other construction works usually by the coastal dwellers. The natural radioactivity in these raw materials are usually determined from 226 Ra, 232 Th and 40 K contents. Minerals from beach sand as well as soils and rejected light sands are used by industries and building construction firms. Hence the gamma-ray radiation hazards due to the specified radionuclides can be assessed using indices [14]. The radium equivalent activity is an index that represents the specific activities of 226 Ra, 232 Th and 40 K by a single quantity which takes into account the radiation hazards associated with them. This can be calculated using the equation below: (9)
where ARa, ATh and AK are the specific activities of 226 Ra 232 Th and 40 K in Bqkg -1 respectively. The Ra eq is related to the external gamma dose and internal dose due to radon and its daughters. The maximum (6) value of Ra eq in building materials must be < 370Bqkg -1 for safe use. The external hazard index is derived from Ra eq Where, Asp is the Specific Activity Concentration, λ is the decay constant, Nsam is total net counts sampling expression through the supposition that its maximum in the peak range, PE is the gamma-ray emission value allowed corresponds to the upper limit of Ra eq 2340
World Appl. Sci. J., 13 (11): 2338-2343, 2011
(i.e. 370 Bqkg -1 ). The index value must be less than unity in order to keep the radiation hazard insignificant. This means the radiation exposure due to the radioactivity from construction material is limited to 1.0 mSvy-1 . The external hazard index was determined from the equation below: (10) where, ARa, ATh and AK are the specific activities of 226 Ra 232 Th and 40 K in Bq kg -1 respectively [15]. Table I: Radionuclides used for the efficiency calibration Nuclide
Energy (KeV)
Cobalt -57 Caesium-137
Half-life (days)
122
Efficiency
271.83
0.04
RESULTS AND DISCUSSION Table 1 shows the radionuclides used for the efficiency calibration. The summary of the research carried out are tabulated in Table 2 and 4 below. Table 2: Mean specific activity concentration of 40K, 232Th and 226Ra in the various shore sediment samples
Samples
Mean specific Activity concentration (Bqkg-1) -------------------------------------------------------40 226 232 K Ra Th
Chorkor Beach
21.31
1.42
1.49
James Town Beach Labadi Beach Nungua Beach Kokrobite Beach
14.67 43.97 41.17 17.76
0.82 140.80 4.05 3.74
1.04 732.60 8.64 6.63
662
137.66
0.01
Cobalt -60
1173
1925.30
0.01
Cobalt -60
1333
1925.30
0.01
Teshie Beach Weija Beach
61.01 8.60
2.85 0.62
9.66 0.17
Yttrium-88
1836
106.63
0.01
Average
29.78
22.04
108.60
Table 3: Comparison of specific activities of K, Th and Ra in Bqkg-1 in shore sediment samples from greater accra region and other studies in different beaches of the world Specific activity (BqKg-1 ) --------------------------------------------------------------------------------40 226 232 K Ra Th
Location SS-1 SS-2 SS-3 SS-4 SS-5 SS-6 SS-7 Preta beach Southeastern Brazil Visakhapatnam, India Northeast Coast, Spain Red seashore sediment, Egypt Beach sand, Al-Maidan, North Sinai, Seabed sand, Tuen Mun Hong Kong Coastal Karnataka Zircon, Bangladesh Global recommended Value range Average value
21.31 14.67 43.97 41.17 17.76 61.01 8.60 47-283 136-1087 98-1011 77 1210 55 472 100-700 370
1.42 0.82 140.80 4.05 3.74 2.85 0.62 54-180 100-400 5-19 95.3-105.6 108.0 27.7 249.2 6439.0 7-50 35.0
1.49 1.04 732.60 8.64 6.63 9.66 0.17 128-349 300-600 5-44 2.3-221.9 146.0 29.8 489.6 1324.0 10-50 40
Reference Current Current Current Current Current Current Current [18] [19] [20] [21] [22] [23] [24] [2]
Work Work Work Work Work Work Work
[16]
Table 4: Absorbed dose rate, annual effective dose rate, radium equivalent activity and external hazard index of studied shore sediment samples Absorbed dose Annual effective Radium equivalent External Samples rate (nGyh-1 ) dose rate (mSv) activity (Bqkg-1 ) hazard Index Chorkor beach James-Town Beach Labadi beach Nungua beach Kokrobite beach Teshie beach Weija beach Average
2.44 1.62 509.38 8.80 6.47 9.69 0.75 77.02
0.00 0.00 0.63 0.01 0.01 0.01 0.00 0.09
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0.11 3.03 56.41 0.67 0.51 0.74 1.52 9.00
0.01 0.01 3.22 0.05 0.04 0.06 0.00 0.48
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Potassium-40 (40 K) content along the coast ranged between an average of 8.60 Bqkg -1 and 61.01 Bqkg -1 . Thorium-232 (232 Th) and Raduim-226 (226 Ra) contents ranged from an average of 0.17 Bqkg -1 to 732.60 Bq kg 1 and 0.62 Bqkg -1 to 140.80 Bqkg -1 respectively. The mean absorbed dose rate in nGyh-1 , the mean annual effective dose in mSvy-1 , the radium equivalent activity in Bqkg -1 and the external hazard index for the various beaches are also shown in Table 4. The mean absorbed dose rate due to the presence of 226 Ra, 232 Th and 40 K in the shore sediment samples along the coast of the Greater Accra Region of Ghana was found to range between 0.75 nGyh-1 and 509.38 nGyh-1 with an average of 77.02 nGyh-1 . The mean annual effective dose rate was found to range between 0.00 mSvy-1 and 0.63 mSvy-1 with an average of 0.09 mSvy-1 while the radium equivalent activity was ranging between 0.11 Bqkg -1 and 56.41 Bqkg -1 with an average of 9.00 Bqkg -1 . The external hazard index also ranged from 0.00 to 3.22, with an average of 0.48.
CONCLUSION The study of the radionuclide concentration levels in shore sediments along the coast of the Greater Accra Region in Ghana has been carried out. It is therefore envisaged that the levels of radioactivity associated with the used of the beaches (shore sediments) would not pose any hazards. The annual effective dose may not have any effect on the natural background ionization radiation of the environment; however it may increase over longer periods especially that of Labadi beach. REFERENCES 1.
2.
DISCUSSION The average specific activity concentration of Ra, 232 Th and 40 K in this work compares well with similar results from other parts of the world as seen in Table 3. With the exception of the sample taken from Labadi beach, the averaged absorbed dose rates due to the presence of 226 Ra, 232 Th and 40 K in the rest of the samples studied were found to be much lower than the world average value of 60 nGyh-1 and the average annual effective dose also compares well with the worldwide average of 0.07 mSvy-1 [1]. The average annual effective dose rate along the coast was estimated to be 0.09 mSvy-1 . This is lower than the International Commission for Radiological Protection value of 1.0 mSvy-1 for practices for members of the public. The average absorbed dose rate and average annual effective dose estimates from the Labadi beach sample are 509.38 nGyh-1 and 0.63 mSvy-1 respectively. These values are due to the presence of high activity concentrations from 232 Th and 226 Ra which is attributed to a significant difference in sand properties, like density, humidity and porosity. The experimental results of radium equivalent activity which indicate radiation hazards arising from the various beach samples studied show that the average Ra eq values are below the internationally acceptable value of 370 Bqkg -1 . The estimated external hazard indices were all also less than unity, i.e. Hex=1.0. This implies that activities involving the use of shore sediments (beach sand) are safe and do not attract any high levels of radiation exposure. 226
3.
4.
5. 6.
7.
8. 9.
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UNSCEAR, 2000. Sources and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, Report to general assembly. Annex B exposure from natural radiation sources. United Nations, New York. Alam, M.N., M.I. Chowdhury, M. Kamal, S. Ghose, M.N. Islam and M.N. Mustafa et al., 1999. The 226 Ra 232 Th and 40 K activities in beach sand minerals and beach soils of Cox’s Bazar, Bangladesh. Journal Environmental Radioactivity, 46: 243-250. Singh, S., A. Rani and R.K. Mahajan, 2005. 226 Ra, 232 Th and 40 K analysis in soil samples from some areas of Punjab and Himachal Pradesh, India using gamma ray spectrometry. Journal of Radiation Measurements, 39: 431-439. El-Kameesy, S.U., A.S. El-Ghany, S.M. ElMinyawi, Miligy and El-Mabrouk E.M., 2008. Natural Radioactivity of Beach Sand Samples in the Tripoli Region, Northwest Libya. Turkish Journal of Environmental Science, 32: 245-251. Ghana Statistical Service, 2002. The year 2000 population census. Yeboah, J., M. Boadu and E.O. Darko, 2001. Natural Radioactivity in soils and Rocks within the Greater Accra region of Ghana. Journal of Radioanalytical and Nuclear Chemistry, 249: 629-632. Darko, E.O. and C. Schandorf, 1998. Performance Characteristics of a low-level Gamma Spectrometry System. Journal of Applied Science and Technology, 3: 45-54. Pan, Z., 1999. Communication to the UNSCEAR Secretariat. Darko, E.O. and A. Faanu, 2007. Baseline Radioactivity Measurements in the vicinity of a gold processing plant. Journal of Applied Science and Technology, 12: 18-24.
World Appl. Sci. J., 13 (11): 2338-2343, 2011
10. Oresengun, M.O., K.M. Decker and C.G. Sanderson, 1993. Determination of Self Absorption Correction by Computation in routine Gamma-ray Spectrometry for typical Environmental Samples. Journal of Radioactivity and Radiochemistry, 4: 38-45. 11. Veiga, R., N. Sanches, R.M. Anjos, K. Macario, J. Bastos and M. Iguatemy et al., 2006. Measurement of Natural Radioactivity in Brazilian Beach Sands. Journal of Radiation Measurement, 41: 189-196. 12. Navas, A., J. Soto and J. Machín, 2002. 238 U, 226 Ra, 210 Pb, 232 Th and 40 K activities in soil profiles of the Flysch sector (Central Spanish Pyrenees). Journal of Applied Radiation Isotopes, 57: 579-589. 13. Yang, Y.X., X.M. Wu, Z.Y. Jiang, W.X. Wang, J.G. Lu and J. Lin, 2005. Radioactivity Concentrations in Soils of the Xiazhuang Granite area. China. Journal Applied Radiation Isotopes, 63: 225-259. 14. NEA-OECD, 1979. Exposure to radiation from natural radioactivity in building materials. Report by NEA Group of Experts, OECD: Paris. 15. Beretka, J. and P.J. Mathew, 1985. Natural radioactivity of Australian building materials, industrial wastes and by-products. Journal of Health Physics, 48: 87-95. 16. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation), 1993. Sources, effects and risks of ionizing radiation. Report to the General Assembly with Scientific Annexes A and B. New York United Nations. 17. Ajayi, I.R., O.S. Ajayi and A.S. Fusuyi, 1995. The Natural Radioactivity of Surface Soils in Ijero. Ekiti Nigeria. Nigeria Journal of Physics, 7: 101-103.
18. Freitas, A.C. and A.S. Alencar, 2004. Gamma dose rates and distribution of natural radionuclides in sand beaches. Ilha Grande Southeastern Brazil. Journal of Environmental Radioactivity, 75: 211-223. 19. Kalyani, V.D.M.L., M.V.S. Chandrasekhar Rao, G. Sree Krishna Murty, G. Satyanarayana and D.L. Sastry, Sahasrabhude et al., 1990. Analysis of 232 Th and 238 U in the beach sands and the ocean sediments. Indian Journal of Protection, 10: 931-934. 20. Rosell, J.R., X. Ortega and X. Dies, 1991. Natural and artificial radionuclides on the northeast coast of Spain. Journal of Health Physics, 60: 709-712. 21. El-Mamoney, M.H. and A.E.M. Khater, 2004. Environmental characterization and radioecologicalimpacts of non-nuclear industries on the Red Sea coast. Journal of Environmental Radioactivity, 73: 151-168. 22. Seddeek, M.K., H.B. Badran, T. Sharshar and T. Elnimr, 2005. Characteristics, spatial distribution and vertical profile of gamma-ray emitting radionuclides in the coastal environment of North Sinai. Journal of Environmental Radioactivity, 84: 21-50. 23. Yu, K.N., Z.J. Guan, M.J. Stokes and E.C.M. Young, 1992. The assessment of the natural radiation dose committed to the Hong Kong people. Journal of Environmental Radioactivity, 17: 31-48. 24. Narayana, Y., H.M. Somashekarappa, A.P. Radhakrishna, K.M. Balakrishna and K. Siddappa, 1994. External gamma radiation dose rates in coastal Karnataka. Journal of Radiological Protection, 14: 257-264.
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