Radiation Measurements 41 (2006) 598 – 601 www.elsevier.com/locate/radmeas
Assessment of dose rates around Manisa (Turkey) F.S. Eree¸s ∗ , S. Aközcan, Y. Parlak, S. Çam Faculty of Sciences and Arts, Physics Department, Celal Bayar University, Muradiye, Manisa, Turkey Received 24 June 2004; received in revised form 25 October 2004; accepted 29 November 2005
Abstract Natural radionuclide activity concentration of surface soils at 64 locations in central Manisa were measured using gamma spectrometry (ORTEC). The natural gamma radioactivity of the terrestrial radionuclides in soil samples and the gamma-absorbed dose rates of these radionuclides in air were calculated. In this study, track etch film (CR-39) was used to determine the distribution of radon levels in dwellings. The average annual effective dose equivalents from the calculated outdoor terrestrial gamma radiation for a person in Manisa is 66 Sv, whilst the annual effective dose equivalent from 222 Rn is calculated to be 4.83 mSv/yr. © 2006 Elsevier Ltd. All rights reserved. Keywords: Soil; Gamma ray spectrometry; Radon; Indoor; Track etch technique
1. Introduction People are exposed to ionizing radiation from naturally occurring radionuclides that are present in the soil. Radionuclides in soils, belonging to 232 Th and 238 U series as well as radioisotope of potassium (40 K) are the major contributors of outdoor terrestrial natural radiation. The city of Manisa is in the western part of Anatolia, Turkey and has a population of approximately 1,260,000 and has an area of approximately 13.830 km2 . The Köprüba¸sı Region (Salihli Basin) in Manisa City contain uranium deposits in fluvial sedimentary rocks, which are underlaid by high-grade metamorphic rocks of the Menderes Massif. The terrestrial component of the natural background is dependent on the composition of the soils. The specific activity levels of 232 Th, 238 U and 40 K in soils were used in an assessment of the associated terrestrial external gamma radiation dose rate prevalent in the monitored areas by using appropriate activity to dose rate conversion factors for the aforementioned radionuclides, given by UNSCEAR (2000). UNSCEAR (1993) gives a world average value of 2.4 mSv for annual effective
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dose equivalent from natural background radiation of which 1.4 mSv comes from radon, thoron and their daughter products. Indoor Rn-222 accounts for the approximately 60% of the total natural background radiation (Saravanan et al., 2003). Knowledge of the distribution pattern of both anthropogenic and natural radionuclides is essential in maintaining some sense of control of prevailing radiation levels. Measurement of natural and fallout radioactivities in soil gives information on natural sources, cumulative deposition from nuclear device testing and nuclear accidents (Baklanov et al., 2002). Measurements of uranium, radium and gamma activity within the Salihli area have been previously determined and the concentrations of eU, eTh and radioactive potassium in sediment and soil samples were found to vary between 0.53–4.46 ppm, 3.85–17.10 ppm, 0.37–2.15%, respectively (Bakaç, 2003). This work presents levels of 232 Th, 238 U, 40 K in soils, the associated external gamma dose rate estimates and the indoor radon concentrations for the central area of the city of Manisa, Turkey. The results will form a baseline data set, which will enable estimations of population exposure. Radioactivity levels in soil samples from other areas in Turkey have been documented in previously published papers (Kumru and Bakaç, 2003; Bakaç, 2003; Karahan and Bayulken, 2000; Karakelle et al., 2002) and will be used for comparison purposes.
F.S. Eree¸s et al. / Radiation Measurements 41 (2006) 598 – 601
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Fig. 1. Regional geological map showing the major structural elements of western Turkey, specifically the Gediz and Büyük Menderes Grabens and surrounding Neogene-Quatemary basins (Sözbilir, 2002). The study area is indicated.
2. Methods
Table 1 Experimental factors
2.1. Natural radioactivity measurements
Concentrations equations
Surface soil samples (0–15 cm depths) were collected from Manisa in the spring of 2001 (Fig. 1). Soil samples were crushed in the laboratury, oven dried at a temperature of 105 ◦ C for 8 h, and sieved through a 270 mesh, 100 g of the homogenous soil samples and were then packed in a polyethylene beaker, weighed and carefully sealed and stored for at least 4 weeks before counting to allow time for 238 U and 232 Th to reach equilibrium with their respective radionuclide daughters. There are several direct and indirect methods for measuring 238 U in geological samples. Among these, the most widely used is scintillation gamma spectrometry, based on the detection of high energy gamma rays of 214 Bi. However, there is always an important problem due to the 238 U and 226 Ra disequilibrium in geological materials. That is why the concentration determined through product activities relies on the assumption that the 238 U decay series is equilibrium and is called the equivalent uranium (eU) concentration (Kumru and Bakaç, 2003). This method was used for the determination of eU, eTh (equivalent thorium) and K (radioactive potassium) in the present study. The natural radioactivity in soil of region has been measured by gamma ray spectrometry using 3 × 3 NaI ( Tl ) dedector (ORTEC-905-4). The best resolution achievable is typically < 7.5% for the 662 keV gamma ray from Cs-137. In this study, the 1.76 MeV peak of Bi-214, the 2.62 MeV peak of Tl-208 and the 1.46 MeV peak of radioactive potassium were used for quantitative determination of uranium, thorium and potassium,
eTh(ppm) = C(Th)/K1 eU(ppm) = (C(U) − C(Th))/K2 K(%) = (C(K)-(C(U)-C(Th))-C(Th))/K3 Stripping rations
Sensitivity factors
= 0.72 = 0.84 = 1.41
K1 = 18.11 K2 = 53.12 K3 = 519.3
C(U), C(Th), C(K): count rates to the each channel of U, Th, and K in the samples.
respectively. The samples were counted for 7200 s with background measurements made under the same conditions. In order to relate obtained count rates to the concentrations of U, Th, and K in the samples, concentration equations given in Table 1 were applied. In these equations , , are known as stripping rations and they indicate the interaction among the K, U, Th channels during counting. Determination of the stripping rations was undertaken by accurately measuring count rates in all channels from pure series-equilibrium uranium and thorium sources. K1 , K2 and K3 are sensitivity factors for each channel and were determined by the measurement of standard samples (625 ppm eU, 150 ppm eTh and 52% K) under appropriate conditions. The sensitivity factors and stripping rations of the detector were determined using the indicated standards. Experimental values of stripping rations and sensitive factors are given in Table 1.
600
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to naturally occurring radionuclides is found to be 54 nGyh−1 , whilst the average dose rate for the world is approximately 57 nGy/h. The activity concentrations of 238 U, 232 Th, 40 K were found to be within the world average. The world average annual effective dose equivalent from outdoor terrestrial gamma radiation is 70 Sv according to UNSCEAR (Karahan and Bayulken, 2000), whilst that of Manisa has been determined to be 66 Sv. Outdoor gamma survey measurements taken 100 cm above the ground at 64 different locations in central Manisa were made using a portable radiometric instrument (TAEK-NEB.211L), which utilises a Geiger–Muller tube as the radiation detector. Readings are represented in terms of R/h. Minimum and maximum of average exposure rates were 9 R/h (78.30 nGy/h) and 15.6 R/h (135.72 nGy/h). These values are comparable to the world average value of 30–70 nGy/h (Ramli, 1997). The concentrations of 222 Rn measured indoor were in the range of 47–146 Bq/m3 . UNSCEAR (1993) gives a global average of indoor radon concentration level of 27.2 Bq/m3 (Sroor et al., 2001). From the calculated annual effective dose equivalent for given amounts of the radon concentrations it is known that 1 Bq/m3 results in 0.05 mSv/yr (Green et al., 1993). Annual effective dose equivalent from 222 Rn is calculated as 2.35 mSv/yr and 7.3 mSv/yr for radon concentrations of 47 and 146 Bq/m3 (Table 4). The mean indoor radon concentration has been determined to be 97 Bq/m3 . Indoor gamma survey measurements taken 100 cm above the ground using a portable radiometric instrument (TAEK-NEB.211L) revealed minimum and
2.2. Radon measurements For the measurement of indoor radon levels in Manisa, CR-39 films (1 × 1) were used. Films were fixed at the bottom of plastic cups measuring 70 mm in diameter and 45 mm in height. The top of the cup was covered with a semi-permeable membrane. These films were placed in 22 different buildings in Manisa. Buildings constructed with bricks and cemented flooring were chosen with detectors being exposed at each location for a period of one month. After exposure, films were etched in 2.5 N NaOH solution at 70 ◦ C for 8 h. Following etching films were washed with distilled water and dried, with alpha tracks subsequently counted under a microscope. 3. Results and discussion A total of 64 measurements were made covering the sampling stations in Manisa and its surroundings (Table 2). The activity concentrations of 238 U, 232 Th, 40 K were found to be within the average values as reported in the literature (Table 3). The contribution of terrestrial gamma radiation to absorbed doses in air can be calculated using the Beck formula (Karahan and Bayulken, 2000). The dose rate of 238 U varies from 9.3 to 14.9 nGy/h with a mean value of 12.1. The 232 Th dose rate varies from 11.9 to 23.8 nGy/h with a mean value of 17.85 nGy/h and that of K-40 varies from 9.03 to 20.21 nGy/h with a mean value of 14.62 nGy/h. Therefore the largest contribution from natural radionuclides in Manisa soil samples to the absorbed doses in air is due to 232 Th. The mean absorbed dose rate in air due
Table 2 Concentrations of natural radionuclides in soils of central Manisa compared with world-wide averages
238 U 232 Th 40 K
Average (and the range) of concentrations of natural radionuclides in soils of central Manisa (Bq/kg)
World-wide average concentrations of natural radionuclides—UNSCEAR (Karahan and Bayulken, 2000) (Bq/kg)
World-wide average concentrations of natural radionuclides ( Karakelle et al., 2002) (Bq/kg)
29 (22 to 35) 27 (18 to 36) 340 (210 to 470)
25 25 370
40 40 400
Table 3 The avarege concentrations of the natural radionuclides in soil samples (in Bq/kg) from different parts of the world Sample no.
Location
238 U
232 Th
40 K
Gamma dose rate (nGy/h)
References
1 2 3 4 5 6 7 8
Egyptian India Taiwan Japan Ireland Kocaeli–Turkey ˙Istanbul–Turkey Manisa
18.7 — 30.0 32.4 37.0 30.0 21.0 28.5
24.7 29.8 44.0 54 26.0 38.0 37.0 27.0
331.0 117.5 431.0 794.0 350.0 562.5 342 340
— 41.5 54.0 83 — — 65.0 54.0
Ebaid et al., 2000 Narayana et al., 2001 Selvasekarapandian et al., 2000 Selvasekarapandian et al., 2000 Selvasekarapandian et al., 2000 Karakelle et al., 2002 Karahan and Bayulken, 2000 Present study
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Table 4 Indoor radon concentrations from different parts of Turkey Sample no.
Location
222 Rn (Bq/m 3 )
mSv/yr
References
1 2 3 4 5 6
˙Izmir–Turkey ˙Izmir–Turkey Yata˘gan–Turkey Köprüba¸sı–Turkey ˙Istanbul–Turkey Manisa
53–86 35–80 10–120 20–340 10–260 47–146
2.69–4.3 — 0.6–7.3 — — 2.35–7.3
Eree¸s and Yener, 1999 Yaprak and Kınacı, 1993 Yaprak, 1999 Yaprak and Kınacı, 1996 Köksal et al., 1993 Present study
maximum gamma exposure rates of 113.1 nGy/h (13 R/h) and 243.6 nGy/h (28 R/h). 4. Conclusions Natural radionuclide activity concentrations (238 U, 232 Th, of surface soils at 64 locations in central Manisa were measured using gamma spectrometry. Considering the Worldwide average concentrations of natural radionuclides (Karahan and Bayulken, 2000) the 238 U and 232 Th concentrations within the study area are found to be slightly high. On the other hand, according to the value reported by UNSCEAR for the same radionuclides (Karakelle et al., 2002), the average concentrations of natural radionuclides in soils of central Manisa (Bq/kg) indicates normal level of World-wide average concentrations of natural radionuclides (Bq/kg). The average exposure rate for outdoor gamma survey measurements is higher than the reported value (Ramli, 1997). The average exposure rate for indoor gamma survey measurements was ranged from 113.1 nGy/h (13 R/h) to 243.6 nGy/h (28 R/h). Radon measurements were made at 22 buildings in Manisa by using solid state nuclear track detector. According to reported global average of indoor radon concentration level (Sroor et al., 2001), the radon concentration shows high concentration. But, intervention limit given by the EPA (US Environmental Protection Agency) for the average indoor radon concentrations is 150 Bq/m3 (Farid, 1997). The average annual effective dose equivalents from the calculated outdoor terrestrial gamma radiation for a person in Manisa is 66 Sv, whilst the annual effective dose equivalent from 222 Rn is calculated to be 4.83 mSv/yr. The average exposure rate for outdoor gamma survey measurements, the average exposure for indoor gamma survey measurements and the indoor radon concentration are found to be higher than the global average. 40 K)
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