Radioactivity measurements and risk assessments in soil samples at south and middle of Qatar A. T. Al-Kinani*, M. A. Amr**, K. A. Al-Saad**, A. I. Helal***, and M. M. Al Dosari* *Radiation and Chemical protection Department, Ministry of Environment, P. O. Box 7634, Doha, Qatar. **Department of Chemistry and Earth Sciences, Qatar University, P. O. Box 2713, Doha, Qatar. ***Department of Nuclear Physics, NRC, Atomic Energy Authority, Cairo, 13759, Egypt. ABSTRACT Health risks associated with the exposure to the natural radioactivity present in soil materials have great concern all over the world. Thus soil samples were collected from an urban areas at south and middle of Qatar in order to measure natural radioactivity, 40 K, 226 Ra and 232 Th and the artificial 137 Cs using gamma-ray spectrometry method. The soil activity concentrations ranged from 25.01-40.31 for 226 Ra, 12.37-4.99 for 232 Th and 133.8-250.1 for 40 K with mean values of 57, 87 and 207 Bq/ kg, respectively. The concentrations of these radionuclides were compared with the available data from other countries. The average and ranges of activity concentration of 226 Ra in Qatar soil areas are very much comparable to the world figures. However, the concentration for 232 Th is comparable to other Gulf area and lower than that for Egypt and the world figures. The concentration for 40 K is lower as compared with Egypt and Kuwait figures, but it was comparable to Oman figures. The radium equivalent activity (Ra eq ) in these soil samples ranges from 74.45 Bq/kg to 41.21 Bq/kg) with a mean value of 57.4 Bq/kg which is far below the safe limit (permissible) limit (370 Bq/kg). The calculated values for external hazard index H ex for the soil samples ranged from 0.102-0.21 and the average concentration of 0.164 was lower than other values reported. However, these values are lower than unity; therefore, the soil from these regions is safe and can be used as a construction material without posing any significant radiological threat to population. The absorbed dose rate calculated from activity of 226 Ra, 232 Th and 40 K ranges between 11.529-21.446, 2.383-11.744, and 5.304-10.357 ngy/h, respectively, and the total average absorbed dose rate 28.915 ngy/h was lower than the world wide average absorbed dose rate 51 ngy/h. The total absorbed dose in the study areas ranged from 20.146-40.389 ngy/ h with an average value of 28.915 ngy/h. The corresponding outdoor and indoor annual effective doses ranged from 49.5 to 20.146 µsv and 198.31 to 24.78 µsv with an average value of 136.95 and 36.145 µsv, respectively. Whereas the world wide average annual effective dose is approximately 460 µsv. Key Words: Natural Radioactivity/ Hazard Index/ Risk Assessment. INTRODUCTION Human beings and other living matters have always been exposed to natural radiations arising from naturally occurring radioactive elements in the soil and rocks, cosmic rays from outer space and the internal exposure from radioactive elements in food, water and air. The natural radioactivity in soil primarily comes from 238 U and 232 Th series (and their decay progeny) and the primordial radionuclide 40 K. These radioisotopes contribute to the dose received by human being through external exposure -200-
and internal exposure due to their ingestion and inhalation. Because of the natural or artificial processes, radionuclides may accumulate and be concentrated in selected areas of the environment. The fallout resulting from the nuclear weapon tests are released into the atmosphere and they travels at great altitudes. They gradually fall to earth and are washed down in rain. 137 Cs, one of the fission products, is strongly absorbed and retained by soil particles as are the natural radionuclides. External gamma dose estimation due to the terrestrial sources is essential because it contributes to the collective dose. These doses vary depending upon the concentrations of the natural radionuclides series, and their daughter products and 40 K, present in the soils and rocks, which depend upon the local geology of the selected region. Background radiation due to local geology and geochemical effects cause enhanced levels of terrestrial radiation (UNSCEAR 2000). Therefore, measurements of natural radioactivity in soil are of a great interest for many institutions throughout the world. To evaluate the terrestrial gamma dose rate for outdoor occupation, it is very important to estimate the natural radioactivity level in soils. The present work was conducted to ascertain the overall contributions of fallout 137 Cs and natural 40 K, 226 Ra and 232 Th in the Quatrain soil samples. Which include measurement of radium equivalent activity (Ra eq ), the absorbed dose rate in the air (D) at height of 1m above the surface of ground and Radium Equivalent, External Hazard Index (H ex ), to assess possible radiological hazards to human health. Material and methods: Sampling EXPERIMENTAL Twenty one soil samples were collected from middle and south parts of Qatar. Most of investigated soil samples were collected near Saudi border. The geographic location of the study areas are between the latitude 155000 230000 and longitude 320000 470000, as shown in Figure (1). The soil was collected at a depth of about 20 cm from the ground surface to get the natural soil. Soil samples were filled into labeled polyethylene bags and sealed. Fine quality of the sample is obtained using scientific sieve of 150 micron-mesh size. The samples were crushed into fine powder by using Mortar and Pestle to get rid of stones. Before measurement samples are dried in an oven at a temperature of 353 K for 24 h to ensure that any significant moisture was removed from the samples. Each soil sample is packed and sealed in an airtight PVC 200 or 1000 ml Marinelli beaker and kept inside refrigerator for about 4 weeks period to allow secular radioactive equilibrium among the daughter products of radon ( 222 Ra), thoron ( 220 Ra) and their short lived decay products. Table (1) shows the number, ID, coordinate, and weight of soil samples collected from investigated area. Measurements: Using coaxial hyper-pure germanium detector (HPGe) (Canberra) based on high-resolution gamma spectrometry system, the activity of samples is counted. Each soil samples was placed on the crystal detector and analyzed using a high-resolution, low-background gamma-ray spectroscopy. The detector has a resolution of 2.5 kev and relative efficiency of 20% for 1.332 MeV gamma energy of 60 Co. The output of the detector is connected to PC. The spectral data is analyzed using the Genie 2000 Gamma Analysis Software package. Each spectrum was acquired for 86400 seconds (24 hours). The detector was surrounded by a lead shield lead on all sides to reduce the background radiation level of the system, and lined inside with copper sheets to minimize the X-rays emitted due to interaction of cosmic radiation with lead. The absolute photo-peak efficiency calibration of the system were carried out using standard 152 Eu because of its suitable half-life and the wide range of gamma ray energies produced during its decay process (121.8 kev - 1409.1 kev) and mixed source containing ( 57 Co, 60 Co, 137 Cs, and 203 Hg) which have an identical geometry to the soil samples. -201-
Fig (1). Locations of samples collection. A Marinelli beaker with the same geometry filled with de-ionized water was used to determine the background spectrum observed by the germanium detector. The counting time of the ambient background spectrum was also for 24 hours. The samples were placed, in their Marinelli beakers, directly on to the front face of the detector. The counting geometry of the samples and the standard sources used for efficiency calibration were kept constant. A gamma-ray energy transitions lines ranging from ~100 kev up to 2.614 MeV, associated with the decay products of the natural series were then analyzed independently to obtain more statistically significant overall results. These data were combined under the assumption of secular equilibrium of the radionuclides within these samples. The natural radioactivity in soil primarily comes from U and Th series and natural potassium. They are essential parameters in determining the natural radioactivity concentration levels and their behavior in -202-
the environment. Artificial radionuclides can also be present such as 137 Cs from the fallout of nuclear weapons testing and from accidents such as Chernobyl. Such data can be used as a baseline from which to detect any future artificial release of radioactive radionuclides. Table (1): Number, ID, Coordinate, and Weight of Sample collected from investigated area. NO Sample ID Weight of Sample (g). Longitude x Latitude y 1 130 676.9 22500 445000 2 164 899 205000 435000 3 202 760.9 205000 425000 4 204 200 215000 425000 5 346 990.6 155000 385000 6 2601 989.8 175000 330000 7 2605 200 195000 330000 8 2617 818 195000 335000 9 2628 1050 180000 340000 1 0 2645 726.3 20000 345000 11 2672 1000 195000 355000 12 2673 760.5 200000 355000 13 2674 800.7 205000 355000 14 2688 777.2 195000 360000 15 2689 200.1 200000 360000 16 2705 200.5 200000 365000 17 2714 1172 165000 370000 18 2743 200.2 225000 375000 19 2755 200 200000 380000 20 2766 780.2 185000 385000 21 2770 816.6 205000 385000 The activity levels for radionuclides in the measured samples are computed using the following equation: where A is the activity level of a certain radionuclide expressed in Bq/Kg dry weight, C is the net counting rate of sample subtracted from background (counts per seconds), ɛ is the counting efficiency of the used detector, P is the absolute transition probability of γ-decay and W is the dried sample weight expressed in Kg. The specific radioactivity of 226 Ra under the peak energy of 186.21 kev are the sum of 235 U under the peak energy of 185.7 kev and peak energy of 226 Ra alone. Thus the radioactivity of 226 Ra alone calculated by subtracting the specific radioactivity of 235 U which is calculated from the peak -203-
energy of 143.76 kev from the total specific radioactivity calculated for 226 Ra. The measured activity concentrations for 232 Th series in the samples were calculated from 228 Ac and 212 Pb decay assuming equilibrium decays of 228 Ac and 212 Pb in the 232 Th series. The external hazard index is an evaluation of the hazard of the natural gamma radiation. The prime objective of this index is to limit the radiation dose to the admissible dose equivalent limit of 1 msv/y. The external hazard index should be below the unity for the radiation hazard to be negligible The external hazard index H ex can be calculated by the following equation cited by (Beretka and Mathew 1985): H ex = C Ra /370 + C Th /259 + C K /4810 1 where C Ra, C Th and C K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg, respectively. The values of this index must be less than one in order to keep the radiation hazard without posing any significant radiological threat to the population. The terms of radium equivalent activity (Ra eq ) in Bq/kg are used to compare the radiological effect or activity of material that contain 226 Ra, 232 Th and 40 K in soil. Ra eq is used to assess the radiation hazards associated with these materials that provide a guideline in regulation safety standards on radiation protection for general public living in the area under investigation. Ra eq is calculated by the following equation (Yu et al 1992): Ra eq = C Ra + 1.43C Th + 0.07C K where C Ra, C Th and C K are the activity concentrations of 226 Ra, 232 Th and 40 K, respectively in Bq/kg. Ra eq was calculated assuming that 370 Bq/kg of 226 Ra or 259 Bq/kg of 232 Th or 4810 Bq/kg of 40 K produce the same gamma dose rate. The total external terrestrial gamma radiation absorbed dose rate (D) in air due to gamma rays emitted by the 214 Pb, 214 Bi, 226 Ra, 232 Th decay chain and 40 K at 1 m above the ground level. Using the conversion factor of 0.0414 ngyh -1 /Bqkg -1 for 40 K, 0.461 ngyh -1 /Bqkg -1 for 226 Ra, and 0.623 ngyh - 1 /Bqkg -1 for 232 Th (UNSCEAR 1993). These conversion coefficients were originally determined from Monte Carlo calculation using mathematical phantoms UNSCEAR (1993). In the above conversion factors, it is assumed that all the decay products of 226 Ra and 232 Th are in radioactive equilibrium. The published maximal admissible (per missible) dose rate is 55 ngy/h. The following equation can be calculated by (Mehra et al 2007): D (ngy/h)=0.461 C Ra + 0.623 C Th + 0.0414 C K where, C Ra, C Th andc k are the activity concentrations (Bq/kg) of radium, thorium and potassium in the samples. Assuming that 137 Cs and the 235 U decay series can be neglected as they contribute very little to the total dose from environmental background. Calculation of annual effective dose in the outdoor and indoor, account must take the conversion coefficient from absorbed dose in air to effective dose and (0.7 Sv/Gy) which is used to convert the absorbed rate to human effective dose equivalent. Annual estimated average effective dose equivalent received by a member is calculated using an indoor and outdoor occupancy factor of 80% and 20% respectively. The annual effective doses are determined as follows Mehra et al (1997): Indoor (nsv) = (Absorbed Dose) / ngy x 8760h x 0.8 x 0.7 Sv/Gy Outdoor (nsv) = (Absorbed Dos) / ngy x 8760h x 0.2 x 0.7 Sv/Gy -204-
RESULTS AND DISCUSSION Figure (2) shows the background-subtracted spectrum for sample number 2605 and clearly shows transitions associated with decays from the 226 Ra, 214 Pb, and 214 Bi isotopes associated with the 238 U decay chain, as well as 228 Ac and 212 Pb decay lines from the 232 Th series. Fig. (2): Background-subtracted gamma-ray spectrum associated with sample number 2605. Table (2) summarizes the results of measurements of natural radionuclide ( 226 Ra, 232 Th, and 40 K) concentrations in the collected soil samples. Analysis of the results indicates that there is some degree of a positive correlation between the activity concentrations of 232 Th and 40 K since the concentration of 40 K increases as the concentration of 232 Th increases in the soil samples. Table (2) shows the radium equivalent activity (Ra eq ) in these soil samples ranging from 74.45 Bq/kg to 41.21 Bq/ kg) with mean value of 57.4 Bq/kg, which is far below the safe (permissible) limit (370 Bq/kg) recommended by Bereka and Mathew (1985). Table (3) shows the calculated values for external hazard index H ex for the soil samples ranging from 0.102 to 0.21 and average concentration was 0.164. The absorbed dose rate calculated from activity concentration ranged between 11.529 and 21.446 ngy/h for 226 Ra and between 2.383 and 11.744 ngy/h for 232 Th, and between 5.304 and 10.357 ngy/h for 40 K. The average absorbed dose rate 28.915 ngy/h was lower than the world wide average absorbed dose rate, 51 ngy/h UNSCEAR (2000). -205-
Table (2): Activity concentration of 226 Ra, 232 Th, 40 K and 137 Cs using gamma ray spectrometry and H ex in the soil samples from Qatar. Sample No ID 226 Ra Radionuclide concentration Bq/kg 137 Cs 232 Th 40 K Ra eq (H ex ) 1 130 40.31±1.3 1.04±0.06 11.97±0.55 243.22± 5.9 74.45 0.21 2 164 28.43±0.8 1.56±0.08 7.519±0.41 200.09± 4.8 53.18 0.189 3 202 35.61±1.1 0.62±0.03 9.379±0.47 210.93± 4.8 63.79 0.176 4 204 28.61±0.7 1.34±0.07 9.01±0.496 209.60± 4.7 56.15 0.156 5 346 27.02±0.7 0.43±0.01 3.77±0.025 125.71± 3.5 41.21 0.114 6 2601 25.01±0.6 1.25±0.06 7.751±0.42 201.48± 4.1 50.19 0.14 7 2605 29.83±0.8 1.85±0.09 9.919±0.5 218.94± 4.2 59.34 0.102 8 2617 27.32±0.8 1.11±0.06 8.39±0.42 203.9± 4.91 53.58 0.149 9 2628 31.92±1.0 0.80±0.05 11.44±0.52 241.23± 5.8 65.17 0.180 1 0 2645 30.0±1.09 1.16±0.06 9.809±0.41 215.2±5.6 59.09 0.164 11 2672 34.94±1.6 0.44±0.02 8.41±0.461 205.2± 5.07 61.33 0.171 12 2673 39.13±1.4 0.39±0.01 7.51±0.41 199.24±4.6 63.83 0.176 13 2674 33.89±1.3 0.551±0.0 7.75±0.42 201.89± 4.7 59.10 0.164 14 2688 46.52±1.7 0.53±0.02 7.86±0.45 202.59± 4.9 71.94 0.199 15 2689 30.64±1.2 1.18±0.07 11.22±0.51 238.5± 6.9 63.38 0.177 16 2705 30.80±1.2-7.297±0.42 193.5± 4.6 54.78 0.152 17 2714 28.61±1.1 0.18±0.01 4.99±0.031 133.8± 3.2 45.10 0.125 18 2743 30.29±1.2-8.77±0.441 206.8± 5.1 57.31 0.159 19 2755 31.88±1.3 3.29±0.09 10.99±0.49 230.6± 6.8 63.74 0.177 20 2766 32.90±1.3 0.82±0.06 10.32±0.49 223.6± 6.5 62.87 0.175 21 2770 28.86±1.1 1.24±0.06 12.37±0.61 250.1± 7.0 64.1 0.176 Average 32.02 1.04 8.88 207.34 57.4 0.164-206-
Table (3): Air-absorbed dose rates and annual effective doses at some locations of Qatar. Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 average 232 Th 11.744 8.283 5.928 5.694 2.383 4.899 6.269 5.302 7.230 6.199 5.315 4.747 4.899 4.965 7.091 4.612 3.154 5.54 6.946 6.522 7.818 Absorbed dose (ngy/h) 226 Ra 18.583 13.106 16.416 13.189 12.459 11.529 13.75 12.594 14.713 13.83 16.111 18.039 15.623 21.446 14.128 14.202 13.189 13.965 14.696 15.171 13.306 40 K 10.068 8.284 8.73 8.677 5.304 8.338 9.062 8.441 9.986 8.909 8.495 8.248 8.358 8.387 9.874 8.011 5.564 8.565 9.548 9.671 10.357 Total 40.398 29.673 31.074 27.56 20.146 24.766 29.081 26.337 31.929 28.938 29.921 31.034 28.88 34.798 31.093 26.825 22.683 28.07 31.19 31.364 31.481 28.915 Annual effective dose (µsv ) outdoor indoor 49.5 36.5 38.22 33.95 24.78 30.5 35.79 32.39 39.28 35.6 36.80 38.17 35.52 42.80 38.24 32.96 27.92 34.53 38.26 38.58 38.75 36.145 198.31 145.69 152.55 135.52 98.92 121.62 142.78 129.32 156.77 141.89 146.91 152.35 141.80 170.81 152.65 131.73 111.37 137.82 153.14 153.98 154.56 136.95-207-
The total absorbed dose in the study area ranged from 20.146 to 40.389 ngy/h with an average value of 28.915 ngy/h. The corresponding outdoor and indoor annual effective doses range from 49.5 to 20.146 µsv and 198.31 to 24.78µSv with an average value of 136.95 and 36.145 µsv respectively while the world wide average annual effective dose is approximately 460µSv UNSCEAR (2000). CONCLUSION The average activity concentration of 226 Ra in Qatar soil equal (32.02) is similar to the world figures (35 Bq/kg) reported in UNSCEAR (2000), and it is also very much similar to Oman, Spain, and Hungary concentration (29.7, 32, and 33 Bq/kg respectively). The average activity concentration of 232 Th in Qatar soil is equal 8.88 Bq/kg. This is similar to the activity concentration of Kuwait (10 Bq/kg) and lower than world figures ( 30 Bq/kg) depending on geological formation of soil. The average activity concentration of 40 K in Qatar soil (207.34 Bq/kg) are very much similar to that of Oman and Kuwait (225 and 370 Bq/kg respectively). The calculated average value of external hazard index (H ex ) for Qatar soil sample is 0.164, which is lower than values compared to figures reported by (Mehra et al 2007). However, these values are lower than unity; therefore, the soil from these regions is safe and can be used as a construction material without posing any significant radiological threat to population (European Commission. Radiation Protection 112 1999 report). The average of the indoor and outdoor effective dose due to natural radioactivity of soil samples is lower than the average national and world recommended value of 1.0 msv/y. Artificial radionuclides can also be presented including 137 Cs from the fallout of nuclear weapons testing and from accidents such as Chernobyl are essential parameters in determining the natural radioactivity concentration levels and their behavior in the environment. Such data can be used as a baseline from which to detect any future artificial release of radioactive radionuclides. ACKNOWLEDGMENT This article was made possible by NPRP award [NPRP 08-187-1-034] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. REFERENCES (1) A. T. Al-Kinani; Iran J. Radiat. Res.; 4, 143 (2006). (2) K. A. Al-Saad, M. A. Amr, A. Ismail, A. I. Helal; J. Environ. Chem. and Ecotoxicol.; 2, 60 (2010). (3) WHO Depleted Uranium Sources, Exposure and Health Effects. World Health Organization, Vienna (2001). (4) R. Mehra, S. Singh, K. Singh, R. Sonkawade; Environ. Monit. Assess.; 134, 333 (1997). -208-
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