THE ANNUAL EFFECTIVE DOSE FROM NATURAL RADIONUCLIDES SOIL SURFACES OF UZHGOROD AREA I. Pataki, O. Parlag, V. Maslyuk, A. Lengyel, Z. Torich Institute of Electron Physics Ukrainian National Academy of Sciences, Uzhgorod, Ukraine E-mail: nuclear@email.uz.ua VII. Hungarian Radon Forum and Radon in Environment Satellite Workshop 16-17th May 2013 Veszprém, Hungary
It is well known that average annual exposure is equals to 2,8 msv worldwide and and 85% of these equaled the natural radioactivity. The 232Th and 238U decay chains, as well as radionuclide 40K plays the main role in the external irradiation of the city inhabitants [1]. [1] H. Orabi, A. Al-Shareaif, M. El Galefi Gamma-ray measurements of naturally occurring radioactive sample from Alkharje City // Journal of Radioanalytical and Nuclear Chemistry, Vol. 269. 2006. p. 99 102. The aim of the present work is to determinate the absorbed and annual effective doses of the Uzhgorod city population from natural radionuclides series.
The samples from the soil surface had been collected from the 37 check points (Fig. 1) at the Uzhgorod area. Samples were collected in March, 2012. A GPS (global positioning and navigation) system was used to obtain information about the geographical positions of the sampling locations (Table 1). Each sample had been obtained from the 3 subsamples that was collected the 1 m2 (square meter) area and the depth of 10 cm and were homogenized in situ. All samples were dried at the 60 C temperature during 48 h, sieved through the 2 mm strainer, weighted and placed (stored) in cylindrical polyethylene containers (0,5 dm3 (cubic decimetre) volume) for the following measurements.
Fig. 1 Geographic location of Uzhgorod and sampling points.
Table1. GPS coordinates samplings location:
The measurements were carried out using high resolution gamma-spectrometry technique with the Ge(Li)- detector (100 cm3). The detector was placed inside a massive 15 cm lead shield with an inner sheet of aluminium (3 mm), copper (8 mm), cadmium (1 mm) in order to reduce background radiation. Figures 2a show the scheme of the shield of Ge(Li)- detector The Ge(Li)- detector and the below electronics were arranged according to the Fig. 2b. The detector has an average gamma-ray photo peak resolution of 3.5 kev for 1332.5 kev (60Co). Bulk standard γ-sources (40K, 137Cs, 152Eu, 232Th) had been used for the detector efficiency calibration.
Fig. 2a The scheme of the shield of Ge(Li)- detector
Fig. 2b Electronic block diagram gamma-ray spectrometry system Fig. 2c Standard measurement geometry
The absolute efficiency of the detector was related to energy by the expression below [2]: lnε(eγ) = -0.1909 (lneγ /E0) - 0.2195(lnEγ 2/E0) + + 4421.445( lneγ 3/E0) 0.0161/Ea (1) where ε(eγ) - is the efficiency, Eγ - is the gamma energy (kev), E0 = 1 kev, and Ea = 1.844209 kev. [2] Potoki І.S. Parametric description of the absolute efficiency of semiconductor detectors for measuring of bulk sample activity // Uzhgorod University Scientific Herald. Series Physics. Issue 31. 2012. P. 196-201. A graphic presentation of the efficiency calibration is shown in Fig. 3.
Fig. 3. Efficiency as a function of photon energy circle experiment line parametric description
The minimum detectable activity (MDA) of the system has been estimated with 95% confidence level using the following formula [3,4]: MDA = 4.66 x B1/2 / (t x γ x ε x w) (2) where B is the background counts, t is the counting time (s), γ is the gamma emission probability, ε is the absolute efficiency of the detector at particular gamma energy and w is the sample weight (kg). [3] L.A. Currie Limits for qualitative detection and quantitative determination // Analitical Chemistry 1968. V 40, No 3. P. 586-593. [4] S. Chinnaesakki et al Performance of HPGe gamma spectrometry system for the measurement of low level radioactivity // J Radioanal Nucl Chem 2012. V. 294, Is. 1. - P. 143-147. The minimum detectable activities (MDA) with 95% confidence level (for 650 g soil sample and 20,000 s counting time) for important radionuclides such as 238U(226Ra), 232Th, and 40K are 20.98, 8.59, 18.36 Bq kg-1, respectively.
All samples were measured in the identical geometry. The measurement time was equal to 20000 s for every sample. Typical gamma spectra of the sample and the background are shown in Fig. 4. During the samples measuring the spectrometer was controlled by the following parameters: channel drift, energy resolution and efficiency. Changing these parameters did not exceed 1% during the measurements. The activity concentrations of 238U(226Ra) had been calculated using the weighted means of gammaray lines of 214Bi (Eγ = 609.3 kev). In the case of 232Th series, the activities have been calculated by using the gamma-ray lines of 228Ac (Eγ = 911.1) and 208Tl (Eγ = 2614.5 kev). The 40K activity concentration was determined by measuring the (Eγ = 1460.8 kev) gamma-ray line.
Fig. 4. Typical gamma spectra of the sample and the background
Specific activity (in Bq kg-1) of samples was calculated using the following formula: A = C / (t x γ x ε x w) (3) where C is the net peak area of the peak, t is the counting time (s), γ is the gamma emission probability, ε is the absolute efficiency of the detector at particular gamma energy and w is the dry weight of the sample (kg). The statistic error of the measured specific activity was in the range of 8 12 %. The values of the specific activity for 40K, 232Th, and 238U from the soil surface samples are shown in Fig. 5 and Table 2.
Table 2. The values of the specific activity for 40K, 232Th, and 238U from the soil surface samples
Fig. 5. Values of specific activity 40K, 232Th, and 238U of measuring samples.
Table 3. The activity concentration of 238U, 232Th, 40K in samples (Bq kg-1) with surface layer soil of Uzhgorod
The values of the specific activity of the natural radionuclide 40K and the members of the 232Th and 238U series were used in order to determinate the absorbed and annual effective doses. The total air absorbed dose rate (ngy h-1) in 1 m above ground level due to the specific activities of natural radionuclides was calculated using the equation [6] D = 0,462 AU + 0,604 ATh + 0,042 AK (4) where AU, ATh and AK are specific activities [Bq kg-1] of the 238U, 232Th, and 40K radioisotope, respectively. [6] UNSCEAR 2000, Annex B. Exposure from natural radiation sources // United Nations. 2000. P. 84-156. The values of absorbed dose from natural radionuclides (40K, 232Th, 238U) shown in Fig. 6.
Fig. 6. Values of absorbed dose from natural radionuclides (40K, 232Th, 238U) measured in Uzhgorod
Fig. 7. Relative contribution to total adsorbed dose rate (ngy.h 1) due to thorium (232Th), uranium (238U) and potassium (40K) for all surface soil samples in Uzhgorod city
The values of the external hazard index of natural radionuclides were determined by the formula: Hex = AU/370 + ATh/259 + AK/4810 (5) The values of external hazard index are usually less than 1, but in the case of artificial (technogenic) pollution is more than 1 [6]. The level of the annual effective dose of natural radionuclides was determined by ratio [6]: Sef = D x 24 x 365 x 0.7 x 0.2 (4) where D is the absorbed dose [ngy h-1]. The values of the annual effective doses from natural radionuclides are presented in Fig. 8.
Fig. 8. Values of annual effective dose from natural radionuclides measured in Uzhgorod
CONCLUSION The following values of the average specific activity had been detected for different radioisotopes from the soil surface of the Uzhgorod area: 40K ~ 310,4±12,.9 Bq kg-1, 232Th (228Ac) ~ 24,8±2,8 Bq kg-1, 238U (214Bi) ~ 23,0±1,3 Bq kg-1. The average value of the absorbed dose was equal to 38,6±2,4 ngy h-1. The average value of the external hazard index of the natural radionuclides was equal to 0.22, that testify the absence of technogenic (artificially) pollution. The average annual effective dose of the natural radionuclides is equal to 4,7 ± 0,3 Sv 10-5. This value is equal to 7,48 Sv 10-5 in Hungary.
Table 4. [1] Niki I. Radiat. Prot. Dosim. 1996;24:387-9 [2] Mamont-Cieśla K. IRPA Regional Congress, 1995 [3] UNSCEAR (2000) [4] Iacob O. J Prev Med. 1996;4:73-82
Fig. 9. Values of annual effective dose from natural radionuclides
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