Natural Radioactivity and Associated Dose Rates in Soil Samples of Malnichera Tea Garden in Sylhet District of Bangladesh

The activ ity concentrations of naturally occurring radionuclides Ra, Th and K, were measured by using a Broad Energy Ge (BEGe) detector for soil samples collected from the Malnichara tea garden in Sylhet district of Bangladesh. The average activity concentrations of Ra, Th and K in soil samples were found to be 55.284±4.68, 125.270±5.81 and 497.91±43.83 Bqkg, respectively. The results obtained for the corresponding nuclides are slightly higher than the worldwide average values of 35, 30, and 400 Bqkg, respectively. The average outdoor absorbed dose and the annual effective dose rates due to Ra, Th and K were observed to be 124.12 ± 7.59 nGyh and 152.22±9.31 μSvy, respectively; which are above the world average. Whereas, the radium equivalent activity and external hazard indices are less than the world average values.


Introduction
The natural radioactivity in the environ ment is the main source of radiation exposure for hu man body. Natural radionuclide in soil contributes a significant amount of background radiation exposure to the population through inhalation and ingestion. The main contributors of radionuclides are 226 Ra, 232 Th and 40 K. Since these radionuclides in soils are not uniformly d istributed and vary fro m region to region (1,2). . Therefo re the knowledge of their distribution in soil and rock play an important role in radiation protection and measurement (3,4). The radioactivity concentration of these nuclides above permissible level is very harmful to the human body. Therefore, measurements of natural radioactiv ity in soils and the radiation doses are of great interest to the researchers which have led the nationwide surveys throughout the world (5,6). . There have been many surveys to determine the background levels of radionuclides in soils which have been turned out to calculate the absorbed dose in air (4).
The Sy lhet d istrict is a trad itional tea gro wing area in Bangladesh. It has about 150 tea gardens, and nearly 300,000 workers are emp loyed in the tea gardens. Bang ladesh is earning a lot o f foreign currencies by exporting tea in abroad.
Tea is a one kind of popular soft drin ks. About 70% of the people in Bangladesh drink tea for refreshment. The Sylhet area is very near to Assam and Tripura of India. So me Uraniu m deposits were found in those States of India. The aim of the present work is to search the possibility of uranium deposition as ore and health risk assessment associated with Thoriu m and Potassium in the study area. This work was undertaken to measure the activity concentrations and γ-ray absorbed doses of the naturally occurring radionuclides ( 238 U, 232 Th and 40 K) in soil samples of Malnichera tea garden in Sy lhet. Moreover, another aim of the present work is to create the public awareness about the radiation hazards and the workers those who are working in this tea garden .This study will also be helpful to establish a research base line in this area.

Study Area
Sylhet Div ision is the northeastern part of Bangladesh, named after its main city, Sy lhet. It is bounded by Meghalaya State of India on the north, Tripura State on the south, Assam State of India on the east and Dhaka division on the west and Chittagong division on the southwest . The area around Sylhet is a t raditional tea gro wing area. The Malnichera tea garden is situded in Sylhet sadar district which is nearly 3 Km northern side fro m the main Sy lhet town. A map of Sylhet sadar and the location of this tea garden is shown in Fig. 1.

Sampling Locati ons
A total number of 7 soil samples were collected fro m the Malnichera tea garden in Sylhet. It is one of the largest tea garden in Bangladesh .All the soil samples were co llected during the whole day of 8 March in 2010. The sampling stations are located by the geographical position of Lat itude ranged over 24°55.43′-25°03.56′N and Longitude 91°51.41′ -92°20.10′ E.

Sample Collection and Preparation
Locations of 7 soil samp les were chosen randomly fro m Malnichara tea garden in Sy lhet. Each soil samp le was collected fro m the surface and was marked by the number A1, A2, A3 …..etc.. After collection of samp les, they were cleaned and dried into the sun and crushed into fine powder by using a mortar.The homogenized soil samp les were then dried in an oven at 60 -80℃ for about 24 hours and then weighted by an electrical balance. All the samples were packed into containers and then sealed tightly with an insulating tape around the opening of the containers for impeding the possibility of moisture contamination of air. In order to maintain radioactive equilibriu m between 226 Ra and its daughters, the packed PVC containers were stored for a period of 4 weeks.

Gamma-ray Detection System
In γ-ray spectrometry, the full energy peak efficiency of a high purity Germaniu m (Hp Ge ) detector is the number o f γrays detected by the detector to the number of photons emitted by the source for a specific energy, which is defined as (4) where, n(E) is the net count rate of the photo peak for the corresponding energy E , A is the present activities of the standard reference source wh ich were calcu lated by using the well known decay law: A = A 0 e-λt . and I γ is the Intensity of the gamma energy. For the determination of efficiency of the detector (HpGe), the contributions for the coincidence summing effect and the angular correlations due to the cascading gamma-rays were taken into account followed by the literatures (7,8). In the present study, the International Atomic Energy Agency (IA EA) reference samp les were used for the calibration of detector efficiencies. The IAEA reference samp les are: RGU-1, Uran iu m is in silica matrix, RGTh-1: Thoriu m is in silica matrix and RGK-1: potassium sulphate. The standard reference source has the same diameter as the soil samples of known concentrations of 238 U, 232 Th and 40 K radionuclides supplied by the Canada Centre

Malnichera tea garden
for Mineral and Energy Technology (CAM ET) under a contract with the IAEA

Acti vity Concentrations of Soil Samples
The radioactiv ity of each samp le was measured using the calibrated h igh purity Germaniu m (Hp Ge) Detector of energy resolution of 2 .0 KeV at 1.33 MeV o f 60 Co. for a period of 10,000 s. Keeping the samples one by one on the top of the detector and counted for a period of 10,000 s. The activity concentration (A) of each radionuclide in the sample was determined by using the net count rates (N c ) after subtracting the background counts from the gross counts for the same counting time under the selected photo peaks, weight of the sample, the photopeak efficiency and the gamma intensity at a specific energy as: Where, A = Activity concentration of the sample in Bq/.Kg Net count rate, N c = Gross counts per second from the samples -background counts per second ε = Efficiency of the detector for the specific energy. I γ = Intensity of the gamma ray. W = Samp le weight in g m. For the analysis of peak areas of gamma spectra, a Co mputer software programming (GENE 2000) was used.

Abs orbed Dose Rates
The external outdoor absorbed gamma dose rates due to terrestrial γ-rays fro m the nuclides 226 Ra, 232 Th and 40 K at 1 m above the ground level was calculated as (UNSCEAR 2000) (9) D = (0.0414 C K + 0.623 C Th + 0.461 C Ra ) n Gy h -1 (3) Where, C K, C Th, C Ra, are the average activity concentrations of 40 K, 232 Th and 226 Ra. About 98% of the e xte rnal γ dose rate fro m 238 U series is delivered by the 226 Ra sub series. So disequilibriu m, between 226 Ra and 238 U will not affect the results of dose calculations from the measurement of 226 Ra.

Annual Effecti ve Dose Rates
The absorbed dose rate was converted into annual effective dose equivalent by using a conversion factor of 0.7 Sv Gy -1 recommended by the UNSCEA R (9) and 0.2 for the outdoor occupancy factor by considering that the people on the average, spent ∼ 20% of their time in outdoors (8) . The Effective dose due to natural activity in the soil was calculated by: Effective dose, E = Dose rate, D (n Gyh -1 ) × 8760(hy -1 ) × 0.2 × 0.7Sv Gy -1 ×10 3 (4)

Radium Equi valent Acti vity
Radionuclides of 226 Ra, 232 Th and 40 K are not homogeneously distributed in soil. The inhomogeneous distribution fro m these naturally occurring rad ionuclides is due to disequilibriu m between 226 Ra and its decay products. For uniformity in exposure estimates, the rad ionuclide concentrations have been defined in terms of radiu m equivalent activity (Ra eq ) in Bq kg -1 . This allows comparison of the specific activ ity of materials containing different amounts of 226 Ra, 232 Th and 40 K according to Beretka and Mathew (10) as follows.
Ra eq = C Ra + 1.43 C Th + 0.077 C K Where, C Ra , C Th and C K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq kg-1 , respectively.

External Hazard Index
The external hazard index (H ex ) is the indoor radiat ion dose rate due to the external exposure to gamma radiation in construction materials of dwellings which was calculated by (11): Where, C Ra , C Th and C k are the specific activ ities of 226 Ra, 232 Th and 40 K in Bq.kg-1, respectively. Th is index value must be less than unity to the radiation hazard insignificant i.e; the area is safe to the human for living.

The Ratio of 226 R/ 228 R
The ratio of 226 R / 228 R concentration activities is less than 1 because the concentration activity of 226 Ra in soil is less than the concentration of 228 Ra. In other word, the concentration of uraniu m in soil is less than thorium, since uranium and thorium are the parents of 226 Ra and 228 Ra, respectively. The range of this ratio is obtained 0.356-0.586.

Results and Discussion
In the present study of soil samp les at Malnichara tea garden in Sylhet district of Bangladesh, the results can be summarized as: i). Activity concentrations, and ii).Radio logical indices.

Ac ti vity Concentrations in Soil Samples
Activity concentrations for nuclides 226 Ra, 232 Th, 40 K and 137 Cs in Malnichara tea garden soil samples were determined by equation (2) and the results for the same have been shown in Table 1 with the uncertainty of 1σ level. Here, the table shows that the highest value is found for sample 7A. The highest value of the nuclide may vary fro m p lace to p lace and this variation may be due to the chemical changes of the constituent elements of soils. Moreover, it is also possible due to the excess use of agricultural fert ilizers and pesticides. All the associated errors were added in quadrature in order to obtain the total uncertainty. The results for the nuclides ( 226 Ra, 232 Th and 40 K) are also shown independently through Fig. 2, 3 & 4, respectively.

226 Ra Acti vi ty
In soil samp les, the activity concentrations of 226 Ra were found in the range of 23.91±4.19 -143.70±7.38 Bq/kg, with an average value of 55.25±4.68 Bq/kg. Th is result is higher than the worldwide average value of 35 Bq/kg (shown by the dotted line) fo r the same radionuclide in soils reported by UNSCEAR (9)

232 Th Acti vi ty
The activity concentration of 232 T is found in the range of 53.25±4.80 -369.60±10.52 Bq/kg with mean value 125.27± 5.81 Bq/kg .Th is result is also higher than the world average of 30 Bq/kg (9) (shown by the dotted line).

40 K Activity
The activity of 40 K is found in the range 369.35±43.56 -593.02±51.52 Bq/kg with an average value of 497.91±43.83 Bq/kg. This result is also slightly h igher than the worldwide average of 400Bq/kg (dotted line) for the same kind of nuclide (5)

137 Cs Acti vity
The concentration of 137 Cs is found to be 17.32±1.17 Bq/Kg in the soil sample 2A.In other soil samp les, the concentrations of 137 Cs are not detectable.

Radiol ogical Indices
In order to assess the health effects, the radiation hazards such as absorbed dose rate (D), effective dose rates (E), radiu m equivalent activ ity (Ra eq ), and external hazard index (H ex ) have been calculated from the activity of nuclides 226 Ra, 232 Th, 40 K using the equations (3), (4), (5) and (6), respectively and the values have shown in Table 2.
Fro m Table 2, it shows that the radiu m equivalents activity (Ra eq ) is found in the range of 145.74± 14.68 -704.53 ± 25.68 Bq / kg, an average value of 269.24 ± 16.06. The average value of the radium equivalent is less than the safe limits as reco mmended by the organization for Econo mic cooperation and development (ECD) (6). Any R aeq Concentration value that exceeds 370 Bq / kg may pose radiation hazards The outdoor air absorbed dose rate due to terrestrial gamma rays at 1m above the ground were calculated for 226 Ra, 332 Th and 40 K and the range is 70.52±7.03 -315.60±11.88 n Gyh -1 . with an average of 124.12±7.59 nGyh -1 which is higher than the world average value of 60 nGyh-1 (5) .
The annual effective dose rate equivalent is calculated using a conversion factor of 07 Sv Gy -1 to convert the absorbed dose rate to the effective dose equivalent and 0.2 for the outdoor occupancy factor. The annual effect ive dose rates are found in the range of 86.48±8.62 -387.06±14.57 µSvy -1 with an average value is 152.23±9.31 µSvy -1 , which is also higher than the world average of 80 µSvy -1 . Indoor dose rates were not evaluated because the essential data on the average build-up of radon gas in the indoor atmosphere were not available.
The mean value of external radiation hazard index is 0.60 which is less than 1, and confirms that it is safe to carry out the activities for the human in that area. The ratio of 226 R / 228 R concentration activities is less than 1 because the

Conclusions
In the present study, the results indicate that the natural radioactivity concentration of 226 Ra is relatively lo wer than that of 232 Th and 40 K at the vicinity of Malnichera tea garden in Sy lhet district of Bangladesh. The values of mean absorbed dose rate, annual effective dose rate and the radium equivalent activity were h igher than the global average values; whereas the external hazard index is less than unity which indicates that the area is no threat of exposure fo r the population..
Previously, no works on the tea garden for radioactiv ity measurement of the soil samples in Sylhet div ision was carried out. Therefore, this work can be used as a baseline data for further research work.