Assessment of Annual Effective Dose from Measured Soil Radioactivity Levels Using HPGe Detector

Human beings are persistently exposed to ionizing radiation caused by terrestrial, extra-terrestrial and anthropogenic radionuclides. In order to assess the risks associated with exposure due to the natural radioactivity in soil, a radiological environmental monitoring survey was carried out in district Chakwal. In the present study, activities of Ra, Th and K are measured in soil samples using an HPGe based gamma spectrometry system. The measured mean specific radioactivity concentration of Ra, Th and K in the studied samples was 31.08 ± 1.2 Bqkg, 47.67 ± 2.30 Bqkg and 558.23 ± 17.52 Bqkg, respectively. From the measured activity concentration, radium equivalent activity, external and internal hazard indices, terrestrial absorbed dose and annual effective dose were calculated. Mean radium equivalent activity (Raeq), outdoor radiation hazard index (Hex), indoor radiation hazard index (Hin) and absorbed dose rate (D) for the area under study were determined as 142.18 Bqkg, 0.38, 0.47 and 66.13 nGyh respectively. The annual effective dose equivalent (AEDE) varied in the range from 0.10 mSv y to 0.16 mSv y. On the basis of measured activity and calculated values of hazards indices, it is concluded that the surveyed area does not pose any significant radiological risk to the population and environment.


Introduction
Naturally occurring radioactive materials are ubiquitous on earth and their radioactivity may become concentrated in certain region/area as a result of human activities. Natural radiation at the earth's surface consists of two components, namely cosmic and terrestrial radiation. Terrestrial radiation mainly originates from the primordial radioactive nuclides originated in the early stage of the formation of the solar system. Uranium, thorium and potassium are the main elements contributing to natural terrestrial radioactivity. It is an established fact that radioactivity in the soil adds to the background level of radiation and human beings are exposed. The level of contribution to the background radiation depends on the concentration of the radioactive materials in the soil but this amount may vary from area to area [1][2][3].
The naturally occurring radionuclides such as 238 U, 232 Th and 40 K are present in rocks and soil are one of the components of external gamma-ray exposure. These radionuclides are not uniformly distributed in the earth's crust and therefore, exposure of human being due the radiation emitted from radioactive materials also changes. The radionuclides released into the environment through several transfer processes and reach the human body. The concentration of these naturally occurring radionuclides in the surrounding environment and associated external exposure due to the gamma radiation depends primarily on the geological origin, environmental parameters and geographical conditions of the region [4][5][6][7].
It is now widely accepted that natural radiation accounts for the greatest part of public radiation exposure [8][9][10]. The exposure of human being to these naturally occurring radionuclides is an unavoidable consequence of their presence in the earth' s crust, soil, air, food and water. To ensure radiological safety of the general public, it is necessary to measure the radiation levels in the environment surrounding humans. The knowledge of naturally occurring radionuclides is useful in order to set the standards and national guidelines in the light of international recommendations. The aim of the present research work is to determine the radioactivity levels in soil samples of tehsil Talagang, district Chakwal.

Material and Methods
Fifteen samples were collected from various locations of tehsil Talagang of District Chakwal. The area under study is located between the longitudes of 32° 42′ and 33 0 42′ N and latitudes of 71̊ 54′ E and 72° 60′ E. Longitude and latitudes of sampling locations are shown in Table 1. The sampling was carried out following the systematic grid of 20x20 square kilometers. The studied area is shown in Figure 1.
The selected sampling sites were relatively flat, open and undisturbed. The soil samples were collected from the upper 4-6 cm layer with a coring tool. The collected samples were packed in polyethylene bags and labeled properly with date and place. In the laboratory, the roots and stones were removed from samples and oven dried at 150 0 C until the sample weight became constant. The treated soil was then ground and sieved. Soil samples of about 200 g were stored in air tight cylindrical plastic containers for 4 weeks before counting to attain secular equilibrium between 226 Ra and 232 Th and their short lived progeny. To determine the activities concentration of 238 U, 232 Th and 40 K, a high resolution gamma-ray spectrometer consisting of a HPGe detector (Model GC 3020 Canberra) coupled to PC based MCA card (Accuspec-A, Canberra) was used. The HPGe detector is available at HPD, PINSTECH, Nilore, and Islamabad. The relative efficiency of the detector was 30% and the resolution 2.23 keV at 1332 gamma-rays of 60 Co. The used detector was equipped with 8192-channels and it was shielded in an 8 cm lead chamber with an inner lining of 0.5 cm thick copper plate to reduce the background [11][12].
The results were analyzed by using Geni-2000 software (Canberra). Efficiency calibration of the detection system was evaluated with Soil-327 which was obtained from IAEA. The samples were counted for 65000 seconds. 40 K was analyzed by its single peak of 1460 keV. However, the analysis of 238 U and 232 Th was based upon the peaks of progeny in equilibrium with their parent radionuclides [11][12].

Results and Discussion
The activity of 226 Ra, 232 Th and 40 K in the soil samples have been calculated by High Purity Germanium (HPGe) detector and the correspondence results are shown in Table 2. The large variations in the activity of these radionuclides are due to non-uniform distribution of the different primordial radionuclide in the soil of the study area. Table 2 represents the measured activity of 226 Ra, 232 Th and 40 K in all the sample of soil taken from different areas of tehsil Talagang of Chakwal district. The maximum and minimum activity of 226 Ra has been found 42.09 ± 1.29 Bqkg -1 in Gohal and 20.02 ± 1.13 Bqkg -1 in Hawapura, respectively. The mean radioactivity of 226 Ra in the study area is 31.08 ± 1.20 Bqkg -1 , which is less than the world average value of 50 Bqkg -1 [13].
Measured activity of 232 Th in all the samples of soil taken from study area are also shown in Table 2. The maximum and minimum activity of 232 Th has been found 65.30 ± 2.49 Bqkg -1 in Gohal and 33.67 ± 2.28 Bqkg -1 in Dhok Pathan respectively. The mean radioactivity of 232 Th in the study area is 47.67 ± 2.30 Bqkg -1 , which is less than the world average value, i.e. 50 Bqkg -1 . Measured activity of 40 K in all the soil sample of the study area, also shown in Table 2. According to the data, the maximum and minimum activity of 40 K has been found 635.80 ± 19.60 Bqkg -1 in Gohal and 371.77 ± 18.18 Bqkg -1 in Tamman, respectively. The mean radioactivity of 40 K in the study area is 558.23 ± 17.52 Bqkg -1 and it is higher than the mean value of the world 500 Bqkg -1 [13].
In present study, it has been observed that the specific activity of natural radionuclides in the soil is not uniform but varies from area to area depending upon the geological character and different minerals present in the soil.

Gamma Dose Rate
Gamma dose rates were calculated by using the formula given below: D (nGy h −1 ) = 0.427 + 0.662 ℎ + 0.043 where C K , C Ra and C Th are the specific activity concentrations of potassium, uranium and thorium, respectively. The variation in dose rates (D) which was determined by the formula for soil samples was found in the range of 52.76 nGyh -1 to 85.28 nGyh -1 . The highest absorbed dose was found in sample of Gohal while the lowest absorbed dose was found in sample of Hawapura. The mean absorbed dose rate of the study area is 66.13 nGyh -1 , which is 5.4% lower than world mean value i.e. 70 nGy h -1 . Table 3 shows calculated Gamma dose rate (D) of the studied area.

Radium Equivalent Activity
The radioactivity has been defined in terms of radium equivalent activity (Ra eq ) in Bqkg -1 to compare the specific activity of materials containing different amounts of 226 Ra, 232 Th and 40 K by using the given equation. Radium equivalent activity (Ra eq ) calculated for different soil samples investigated in the present study are given in Table 3. Radium equivalent activity (Ra eq ) is varying in the ranges 111.82 Bqkg -1 in Hawapura to 184.43 Bq kg -1 in Gohal. The mean of radium equivalent activity in the current area is 142.18 Bqkg -1 , less than the 370 Bqkg -1 which is in the safe limit.

External and Internal and Hazard Indices
The internal and external hazard indices are calculated by the following expressions The calculated external hazard (H ex ) values varied from 0.30 to 0.50, calculated from given equation. The current mean external hazard index is 0.38, which is 31.57% less than world mean value 0.5. Table 3 shows the external hazards of the sampling area.
The calculated internal hazard (H in ) values vary from 0.36 to 0.6. The lowest value found in soil sample of Hawapura and the highest value represent in soil sample of Gohal. The mean internal hazards index for the area is 0.47 which is less than world means value 0.5 [13].

Annual Effective Dose Equivalent
Annual effective dose equivalent (AEDE) was calculated from the formula given below.
Where "O" is the occupancy factor for outdoor and its value is 0.3 for Pakistan, "T" termed number of hours in one year equals 8760h, "Q" equals to 0.7 SvGy -1 is the quotient of effective dose rate "D" to absorb dose rate in air and 10 -6 is the conversion factor between nano and milli. Conversion factor of 0.7 SvGy -1 converts absorbed dose in air to human effective dose in adults. The variation in annual effective dose equivalent was found to be varied from 0.10 mSv y -1 to 0.16 mSv y -1 . The lowest value found in soil samples of Dhok Pathan and the highest value in Gohal. The mean annual effective dose for the study area is 0.12, which is less than the world mean value [13]. Table 3 shows the AEDE value of the sampling area.

Conclusions
The soil samples analyzed in the present study shows 226 Ra activity ranging from 20.02 ± 1.13 Bqkg -1 to 42.09 ± 1.29 Bqkg -1 and activity of 232 Th varies from 33.67 ± 2.28 Bqkg -1 to 65.30 ± 2.49 Bqkg -1 . The activity measured for 40 K in all soil samples is ranged from 371.77 ± 18.18 Bqkg -1 to 635.80 ± 19.60 Bqkg -1 . Therefore, the natural radionuclide 226 Ra and 232 Th are detected in less quantity, while 40 K, which is an essential constituent of all cellular material, was detected in higher amount. The mean radium equivalent activity for the measured soil samples was 142.18 Bq kg -1 . The calculated mean value of external radiation hazard index and internal radiation hazard was 0.38 and 0.47 respectively. The values of the radium equivalent activity and the external hazard index determined in the soil of the study area are less than the recommended safe levels. The mean absorbed dose rate and annual effective dose equivalent for the area under study was found to be 66.13 nGy h -1 and 0.12 mSv y -1 , respectively.