Measurement of Natural and Artificial Radioactivity in Soil at Some Selected Thanas around the TRIGA Mark-II Research Reactor at AERE , Savar , Dhaka

The activity concentration of natural and fallout radionuclides in the soil at some selected Thanas around the TRIGA Mark-II Research Reactor at Atomic Energy Research Establishment (AERE), Savar, Dhaka were measured by using a high purity germanium detector (HPGe). The study revealed that only natural radionuclides were present in the samples and no trace of any artificial radionuclide was found. The average activity concentration of U, Th and K were found to be 37.8 ± 5.6 Bq·kg, 58.2 ± 11.0 Bq·kg and 790.8 ± 153.4 Bq·kg respectively. The radium equivalent activity (Req), absorbed dose rate (D), external radiation hazard index (Hex) and internal radiation hazard index (Hin) were also calculated to find out the probable radiological hazard of the natural radioactivity.


Introduction
Radioactivity may be natural and artificial.Natural radioactivity occurs due to extraterrestrial sources as well as from radioactive elements in the earth's crust.Significant amount of artificial (man-made) radioactivity is emitted by nuclear power plant, industrial plant and research facilities.A large amount of radiation releases due to accident of nuclear reactor.
Most widely spread natural radionuclides are from the family of Uranium ( 238 U), Thorium ( 232 Th), Actinium ( 235 Ac) and Kalium ( 40 K).Significant amount of manmade radionuclides 137 Cs and 90 Sr may also present in soil as a result of nuclear weapon testing in the atmosphere, accidents (such as the Chernobyl power plant accident) and the routine discharge of radionuclides from nuclear installations [1]. 137Cs dominates among durable artificial gamma radiators.It takes 300 year for it to fragment completely.Due to that it migrates in various geospheres and biological links.The biggest part of 137 Cs is accumulated in the upper layer of the soil and forest floor [2].
The TRIGA Mark-II Research Reactor of AERE is located at Savar of Dhaka District is a light-water-cooled graphite reflected reactor designed for continuous operation at a steady-state power level of 3 MW.A significant amount of low-level of solid, liquid and gaseous radioactive wastes are being generated from operation and maintenance of 3 MW TRIGA Mark-II Research Reactor.Radiation may release from these waste.These radioactive substance precipitated on the earth surface are either lifted again by the wind or penetrate into the ground.The radioactive substances are absorbed by plant through their roots and finally reach human body [2].
The aim of this study is to detect the natural ( 238 U, 232 Th and 40 K) and probable artificial radionuclide ( 137 Cs) and to determine their activity level in the soil at Some Selected Thanas around the TRIGA Mark-II Research Reactor at AERE, Savar, Dhaka. the TRIGA Mark-II Research Reactor at AERE, Savar, Dhaka

Sample Collections and Preparation
Eighteen soil samples were collected from the four Thanas (Dhamrai, Ashulia, Savar and Singair) around the TRIGA Mark -II Research Reactor, AERE, Savar, Dhaka.Each sample was collected maintaining a distance of about 1 km to 5 km from each other.The soil samples were collected at the depth of 5 cm with respect to the surface.About 1 kg of sample was collected from each location and each of the samples was placed in plastic packet and transported to the laboratory.All the samples were collected during the period of 25 October to 16 December in 2008.Each sample was segregated for stone and grass and then dried at about 110˚C in an oven for 24 hours.The samples were then ground into fine powder with a grinder and collected after passing through a 10mesh screen.Thus, homogenized sample was transferred to sealable cylindrical plastic container of 7 cm height and 5.5 cm in diameter, marked individually with identification parameters.The net weights of all the samples were noted.All the sample containers were then sealed tightly with cap and wrapped with Teflon and thick vinyl tapes around their screw necks and finally air tightened with polythene pack and stored for minimum four weeks prior to counting, allowing establishment of secular equilibrium between the long lived 238 U, 232 Th and their decay products.tions.

Data Collection and Analysis
Each of the prepared samples and standard (Al 2 O 3 based 226 Ra) were placed on the top of the HPGe detector within the shielding arrangement and counted for 10,000 seconds.The software of the HPGe detecting system provides the corresponding gamma spectra collected for both samples and standards.Gamma ray spectrometry can be used to identify gamma ray energies and consequently the radioactive species which are producing them.The area under the peak in a gamma ray spectrum represents the number of counts collected for only that gamma ray energy.These peak areas were used for determination of radioactivity concentration of the radionuclides present in the sample.The net count of the sample is obtained by subtracting a linear background distribution of the pulse height spectra from the corresponding peak energy area.
From the net counts of the samples activity concentration of the radionuclides were calculated using the formula where, A is the activity concentration in Bq•kg -1 , CPS is the net peak counts per second of the samples, W is the weight of the sample in gm, ε(abs) is the absolute gamma eak detection efficiency, is the absolute gamp where N s is the counts measured in time T s and N b is the background counts measured in time T b .The standard deviation 2  in CPS was converted into activity concentration in Bq•kg -1 according to Equation (1).
To determine the activity concentration, the γ-ray line The radium equivalent activity (R eq ) in Bq•kg -1 was calculated to compare the specific activity of the material containing different amount 238 U, 232 Th and 40 K using the following relation given in [4]: where C Ra , C Th and C K are the activity concentration of 238 U, 232 Th and 40 K respectively.The γ-ray absorbed dose rate (D) in nGyh -1 in air due to the natural radionuclides 238 U, 232 Th and 40 K was calculated using the formula as reported in [1]: where C Ra , C Th and C K have the same meaning as Equation (3).The soil is used for making earthen huts and bricks have the external radiation hazard index (H ex ) and internal radiation hazard index (H in ).The H ex and H in were calculated using the formula as given in [1].

Results and Discussion
The mean were measured by using a HPG 238 concentration of U ranged from 25.5 ± 5.4 to 64.4 ± 6 with an average value of 37. The results of the present study showed that the activity concentration of thorium is 1.5 times higher than that of uranium.It is also observed that the activity concentration of 40 K is 13.5 times higher than that of thorium and 20.9 times higher than that of uranium.The excessive usage of Potassium containing fertilizers (NPKS, MOP etc.) in the area adjacent to the sampling sites may contribute to the higher value of 40 K activity [1].
A comparative study was also performed for the activity concentrations in the present work with the other studies performed in home and aboard and is shown in the Table 3.
The activity concentrations of the radionuclides in the soil samples of four Thanas around the TRIGA Mark-II research reactor in Bangladesh are within the range of values reported in the other work performed in home and abroad.
Since no 137 Cs radionuclide was detected in any of the soil samples, it indicates that there is no fission product Copyright © 2011 SciRes.JEP

Conclusions
The Savar region is one of the most populated regions in area.il sam f the in other countries, y le gher than the pe leve a and 40 K respectively [11].Further, the results of the present study would be useful as a base line data of the regions under study and also help as a guideline for the competent authority (Bangladesh Atomic Energy Commission) to go forward to fix up the dose limit for the radiation protection activities of the country and in the academic activities of the health physics, geophysics and environmental science.

Figures 1 , 2 Figure 1 .Figure 2 .
Figure 1.Map showing the different locations of sample collection in Savar and Ashulia Thana.

Figure 3 .
Figure 3. Map showing the different locations of sample collection in Singair Thana.ma intensity of the corresponding gamma ray energy.Gamma rays intensities were taken from the literature [3].The peak detection efficiencies were calculated from the full energy peak detection efficiency curve plotted using Al 2 O 3 based 226 Ra standard as shown Figure 4.The error in the measurement have been expressed in terms of standard deviation ( 2  ), where  is expressed as, 1 2

Figure 5 .
Figure 5. Mean activity concentration of 238 U in Bq•kg -1 in the collected soil samples.

Figure 6 .
Figure 6.Mean activity concentration of 232 Th in Bq•kg -1 in the collected soil samples.

Figure 7 .
Figure 7. Mean activity concentration of 40 K in Bq•kg -1 in the collected soil samples.spread due to the operation of the TRIGA Mark-II research reactor or d where.

Table 1 and
Table 2, respectively.The mean activity concentrations of 238 U, 232 Th and 40 K are

Table 1 . Mean activity concentration of 238 U, 232 Th, 40 K and dose
rate (D), external hazard index (H ex ) and internal hazard ishown graphically in the Figures5-7, respectively.

Activity concentration in Bq•kg -1 Sam. ID 238 U 232 Th 40 K 137 Cs Req in Bq•kg -1 Dose rate in nGy•h -1 H ex H in Soil of Dhamrai Thana
ND: Not Detected.