Abstract
The natural radioactivity existed since creation of the universe due to the long life time of some radionuclides. This natural radioactivity is caused by γ-radiation originating from the uranium and thorium series and 40K. In this study, the gamma radiation has been measured to determine natural radioactivity of 238U, 232Th, and 40K in collected sedimentary rock samples in different places of Turkey. The measurements have been performed using γ-ray spectrometer containing NaI(Tl) detector and multichannel analyser (MCA). Absorbed dose rate (D), annual effective dose (AED), radium equivalent activities (), external hazard index (), and internal hazard index () associated with the natural radionuclide were calculated to assess the radiation hazard of the natural radioactivity in the sedimentary rock samples. The average values of absorbed dose rate in air (D), annual effective dose (AED), radium equivalent activity (), external hazard index (), and internal hazard index () were calculated and these were 45.425 nGy/h, 0.056 mSv/y, 99.014 Bq/kg, 0.267, and 0.361, respectively.
1. Introduction
The natural radioactivity existed since creation of the universe due to the long life time of radionuclide. This natural radioactivity is caused by γ-radiation originating from the uranium and thorium series and 40 K. Natural radioactivity arising from natural sources is the main contribution to the annual dose received by the world’s population, exposure resulting from radionuclides inherent in the earth’s crust and from cosmic rays. The terrestrial radionuclides are ubiquitous, belonging to the 238U and 232Th series and their decay products as well as single decay radionuclides, particularly 40 K. Gamma radiation emitted from such naturally occurring radionuclides in all ground formations represents the main external exposure to human body [1]. The knowledge of concentrations and distributions of the radionuclides are of interest since it provides useful information in the monitoring of environmental radioactivity. Natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions and appear at different levels in the soils and rocks of each region in the world [2–4].
In this study, the natural radioactivity concentrations of 40 K, 238U (226Ra), and 232Th in some sedimentary rock samples collected in different regions of Turkey have been investigated. The results were used to assess the radiological hazard associated with the absorbed gamma dose rate in air (D), the annual effective dose rate, radium equivalent activities (), external hazard index (), and internal hazard index () from gamma radiation.
2. Material and Methods
In this study 19 different types of sedimentary rocks have been collected in different regions of Turkey. In Figure 1 the locations of Turkey in which samples were collected have been shown. After collection of samples, they were crushed and grinded to get proper grain size 100 mesh sieve. Then, samples were dried until 100°C in an oven for about 24 h. The dried samples have been filled in a cup which is sealed tightly with a thick tape around its neck to limit any gas escape from it, and it was stored for four weeks to get secular equilibrium to be achieved between 238U and its progeny [5].
The radioactivity of 226Ra, 232Th, and 40 K in the sedimentary rock samples was determined using a gamma-ray spectrometry [6] consisting of a NaI(Tl) detector connected to a 16384-channel multichannel analyser (MCA). Before measurement the system should be calibrated. This is done using 137Cs and 60Co radioactive sources, which produce γ-ray energy of 662, 1173, and 1332 keV, respectively. The γ-ray spectrum obtained from the mentioned source and related fit has been displayed in Figure 2.
(a)
(b)
The spectrum is analyzed using the MAESTRO32 obtained from ORTEC. The measurement was based on recording natural radioactivity quantities of three natural long-live elements: 226Ra, 232Th, and 40 K which are considered the photopeaks at 1760, 2610, and 1461 keV, respectively, in the natural γ-ray spectrum [5]. The efficiency calibration of the detector system was measured [7] and the results have been used in this work.
The activities for the natural radionuclides were calculated using the following relation [8]: where is the activity of the radionuclide in Bq/kg and is the net peak area under the most prominent photo peaks calculated by subtracting the respective count rate from the background spectrum obtained for the same counting time. The net count rate in the measurement is calculated from the background subtracted area of prominent gamma-ray peaks. is the detector efficiency of the specific gamma-ray, the absolute transition probability of gamma decay, the counting time (s) which is 72000 s, and the mass of the sample (kg).
3. Results and Discussion
The activity concentrations of natural radionuclides (40 K, 238U (226Ra), and 232Th) in the sedimentary rocks collected in Turkey are presented in Table 1. As shown in Table 1, the activity concentrations of sedimentary rock samples have ranged from 143.97 to 452.34 Bq/kg for 40 K, from 12.01 to 48.95 Bq/kg for 226Ra, and from 8.2 to 53.27 Bq/kg for 232Th. The obtained results have been displayed in Figure 3.
The highest concentrations for all natural radionuclides were detected in the sample 13 (S13) which was collected from Afyon region. The worldwide average concentrations have been reported as 50 Bq/kg for 226Ra and 232Th concentrations and 500 Bq/kg for 40 K in the UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation [9–12]. It was found that obtained results for 40 K and 226Ra concentrations are lower than the worldwide average values. For 232Th concentration, S13 is higher than the worldwide average values.
The 40 K, 226Ra, and 232Th activity concentrations are used for estimation of the absorbed dose rate (D), the annual effective dose rates (AED), the radium equivalent activity (Raeq), and the index of external and internal radiation hazard ( and ).
The absorbed dose rate (D) in air at 1 m above the ground is calculated to provide a characteristic of the external terrestrial γ-ray [5, 11, 13]: where , , and are activity concentrations of 226Ra, 232Th, and 40 K in Bq/kg, respectively. The absorbed dose rate ranged from 30.13 to 73.65 nGy·h−1. The absorbed dose rate is displayed in Figure 4. The global average value of absorbed dose rate is 55 nGy·h−1 [11].
The annual effective dose rates (AED) should be obtained to test the health effect of those absorbed dose rates. In order to estimate the annual effective doses, one has to take into account to conversion coefficient from absorbed dose in air to effective and the outdoor occupancy factor. In the UNSCEAR [11] reports, a value of 0.7 Sv/Gy was used for the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.2 for the outdoor occupancy factor. The annual effective dose equivalent was calculated from the following equation: The annual effective dose rate values varied from 0.037 to 0.09 mSv·y−1. The AED results have been displayed in Figure 5. The average AED from the terrestrial radionuclides is 0.46 mSv·y−1 in areas with the normal background radiation [11].
Distribution of 40 K, 226Ra, and 232Th was not uniform in sedimentary rock samples. Uniformity with respect to exposure to radiation was defined in terms of radium equivalent activity (Raeq) in Bq/kg to compare the specific activity of materials containing different amounts of 40 K, 226Ra, and 232Th. The radium equivalent activity was calculated as follows [14]: where , , and are the activity concentration of 226Ra, 232Th, and 40 K in Bq/kg, respectively. is estimated for the collected samples and is displayed in Figure 6. The values of varied from 62.44 to 159.15 Bq/kg. The recommended maximum value for is 370 Bq/kg [14].
The external hazard index () and internal hazard index () are calculated from the equations [14] where , , and are activity concentrations of 226Ra, 232Th, and 40 K in Bq/kg, respectively. The values of internal and external radiation hazard index must be less than unity for radiation hazard to be negligible.
The value of must be less than unity, which corresponds to the upper limit of (370 Bq/kg), in order to keep the radiation hazard under limit. The results have been displayed in Figure 7.
Also, all index results have been displayed in Table 2. It can be seen that the obtained values of absorbed dose rate for sedimentary rock samples except those collected from Afyon (S13), Kastamonu (S17), and Amasya (S19) are lower than the global average value of absorbed dose rate, which is 55 nGy·h−1. For the annual effective dose, it is clear that none of the results are higher than the UNSCEAR limit, where the average AED from the terrestrial radionuclides is 0.46 mSv·y−1 in areas with the normal background radiation. The estimated values of in the present work are lower than the recommended maximum value of 370 Bq/kg. For internal and external radiation hazard index, it can be seen that all measured results are lower than the upper limit of unity.
4. Conclusion
The level of natural radioactivity in sedimentary rock samples collected from different regions of Turkey was evaluated using gamma-ray spectrometry. The mean activity concentrations of 40 K, 226Ra, and 232Th were 243.08, 34.66, and 31.92 Bq/kg, respectively. The mean activity concentrations are lower than the world mean values identified by UNSCEAR [11]. From the measured values, the average values of absorbed dose rate in air (D), annual effective dose (AED), radium equivalent activity (), external hazard index (), and internal hazard index () were calculated and these were 45.43 nGy/h, 0.056 mSv/y, 99.0 Bq/kg, 0.27, and 0.36, respectively. All the calculated values are lower than the recommended maximum values in the UNSCEAR [11] reports. As a result of these, it can be concluded that for people who are using and mining sedimentary rock in these regions of Turkey are safe in terms of radiation hazards.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the Suleyman Demirel University Foundation Unit (BAP) with the Project no. of 2226-D-10. The authors wish to thank to Professor Dr. R. Altındag for helping in providing rocks samples.