1. Introduction

The natural radioactivity in the environment is the main source of radiation exposure for human 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 ²²⁶Ra, ²³²Th and ⁴⁰K. Since these radionuclides in soils are not uniformly distributed and vary from region to region ^(1,2).. Therefore 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 radioactivity 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 Sylhet district is a traditional tea growing area in Bangladesh. It has about 150 tea gardens, and nearly 300,000 workers are employed in the tea gardens. Bangladesh is earning a lot of foreign currencies by exporting tea in abroad.

Tea is a one kind of popular soft drinks. About 70% of the people in Bangladesh drink tea for refreshment. The Sylhet area is very near to Assam and Tripura of India. Some Uranium 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 Thorium 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 (²³⁸U, ²³²Th and ⁴⁰K) in soil samples of Malnichera tea garden in Sylhet. 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.

2. Methods and Materials

2.1. Study Area

Sylhet Division is the northeastern part of Bangladesh, named after its main city, Sylhet. 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 traditional tea growing area. The Malnichera tea garden is situded in Sylhet sadar district which is nearly 3 Km northern side from the main Sylhet town. A map of Sylhet sadar and the location of this tea garden is shown in Fig. 1.

Figure 1. Location map of Malnichera tea garden in Sylhet district of Bangladesh

2.2. Sampling Locations

A total number of 7 soil samples were collected from the Malnichera tea garden in Sylhet. It is one of the largest tea garden in Bangladesh .All the soil samples were collected during the whole day of 8 March in 2010. The sampling stations are located by the geographical position of Latitude ranged over 2455.43′-2503.56′N and Longitude 9151.41′ - 9220.10′E.

2.3. Sample Collection and Preparation

Locations of 7 soil samples were chosen randomly from Malnichara tea garden in Sylhet. Each soil sample was collected from the surface and was marked by the number A1, A2, A3 …..etc.. After collection of samples, they were cleaned and dried into the sun and crushed into fine powder by using a mortar.The homogenized soil samples 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 equilibrium between ²²⁶Ra and its daughters, the packed PVC containers were stored for a period of 4 weeks.

2.4. Gamma-ray Detection System

In γ-ray spectrometry, the full energy peak efficiency of a high purity Germanium (HpGe) detector is the number of γ- rays detected by the detector to the number of photons emitted by the source for a specific energy, which is defined as ⁽⁴⁾

(1)

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 which were calculated by using the well known decay law: A = A₀ e-λ^t. 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 (IAEA) reference samples were used for the calibration of detector efficiencies. The IAEA reference samples are: RGU-1, Uranium is in silica matrix, RGTh-1: Thorium 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 ²³⁸U, ²³²Th and ⁴⁰K radionuclides supplied by the Canada Centre for Mineral and Energy Technology (CAMET) under a contract with the IAEA

2.5. Activity Concentrations of Soil Samples

The radioactivity of each sample was measured using the calibrated high purity Germanium (HpGe) Detector of energy resolution of 2 .0 KeV at 1.33 MeV of ⁶⁰Co. 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:

(2)

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 = Sample weight in gm.

For the analysis of peak areas of gamma spectra, a Computer software programming (GENE 2000) was used.

2.6. Absorbed Dose Rates

The external outdoor absorbed gamma dose rates due to terrestrial γ- rays from the nuclides ²²⁶Ra, ²³²Th and ⁴⁰K at 1 m above the ground level was calculated as (UNSCEAR 2000) ⁽⁹⁾

(3)

Where, C_K, C_Th, C_Ra, are the average activity concentrations of ⁴⁰K, ²³²Th and ²²⁶Ra. About 98% of the external γ dose rate from ²³⁸U series is delivered by the ²²⁶Ra sub series. So disequilibrium, between ²²⁶Ra and ²³⁸U will not affect the results of dose calculations from the measurement of ²²⁶Ra.

2.7. Annual Effective Dose Rates

The absorbed dose rate was converted into annual effective dose equivalent by using a conversion factor of 0.7 SvGy^-1 recommended by the UNSCEAR⁽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:

(4)

2.8. Radium Equivalent Activity

Radionuclides of ²²⁶Ra, ²³²Th and ⁴⁰K are not homogeneously distributed in soil. The inhomogeneous distribution from these naturally occurring radionuclides is due to disequilibrium between ²²⁶Ra and its decay products. For uniformity in exposure estimates, the radionuclide concentrations have been defined in terms of radium equivalent activity (Ra_eq) in Bq kg^-1. This allows comparison of the specific activity of materials containing different amounts of ²²⁶Ra, ²³²Th and ⁴⁰K according to Beretka and Mathew ⁽¹⁰⁾ as follows.

(5)

Where, C_Ra, C_Th and C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq ^kg-1, respectively.

2.9. External Hazard Index

The external hazard index (H_ex) is the indoor radiation dose rate due to the external exposure to gamma radiation in construction materials of dwellings which was calculated by^(11):

(6)

Where, C_Ra, C_Th and C_k are the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq.kg-1, respectively. This index value must be less than unity to the radiation hazard insignificant i.e; the area is safe to the human for living.

2.10. The Ratio of ²²⁶R/²²⁸R

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

3. Results and Discussion

In the present study of soil samples at Malnichara tea garden in Sylhet district of Bangladesh, the results can be summarized as:

i). Activity concentrations, and ii).Radiological indices.

3.1. Activity Concentrations in Soil Samples

Activity concentrations for nuclides ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs 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 from place to place 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 fertilizers and pesticides. All the associated errors were added in quadrature in order to obtain the total uncertainty. The results for the nuclides ( ²²⁶Ra, ²³²Th and ⁴⁰K) are also shown independently through Fig. 2, 3 & 4, respectively.

3.2. ²²⁶Ra Activity

In soil samples, the activity concentrations of ²²⁶Ra 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. This result is higher than the worldwide average value of 35 Bq/kg (shown by the dotted line) for the same radionuclide in soils reported by UNSCEAR⁽⁹⁾

3.3. ²³²Th Activity

The activity concentration of ²³²T 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 .This result is also higher than the world average of 30 Bq/kg ⁽⁹⁾ (shown by the dotted line).

Table 1. Activity concentrations (in Bq/kg) in soil samples

Figure 2. Distribution of 226Ra in soil samples

Figure 3. Distribution of 232Th in soil samples

Figure 4. Distribution of 40K in soil samples

Table 2. The ratio 226Ra/228Ra, Radium equivalent activity, Absorbed dose rate, Effective dose rate and External hazard index Sample ID 226Ra/228Ra Raeq activity (Bq/kg) Absorved dose rate, D (nGy/h) Effective dose rate, E (µSv/y) External hazard index (Hex) 1A 0.586 241.10±14.19 112.47±6.72 137.93±7.72 0.661 2A 0.406 149.45±14.79 72.18±7.11 88.52±8.72 0.415 3A 0.356 259.06±16.39 119.15±7.72 146.12±9.46 0.708 4A 0.549 145.74±14.68 70.52±7.03 86.48±8.63 0.404 5A 0.485 215.71±15.91 100.34±7.53 121.69±9.23 0.589 6A 0.547 167.18±10.75 78.61±5.15 96.41±6.31 0.460 7A 0.388 704.53±25.68 315.61±11.88 387.06±14.57 0.968 Average 0.441 269.24±16.06 124.12±7.59 152.23±9.31 0.601 Range 0.356- 0.586 145.74±14.68-704.53±25.68 70.52±7.03-315.60±11.88 86.48±8.62-387.06±14.57 0.404- 0.968

3.4. ⁴⁰ K Activity

The activity of ⁴⁰K 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 higher than the worldwide average of 400Bq/kg (dotted line) for the same kind of nuclide ⁽⁵⁾

3.5. ¹³⁷Cs Activity

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

3.6. Radiological Indices

In order to assess the health effects, the radiation hazards such as absorbed dose rate (D), effective dose rates (E), radium equivalent activity (Ra_eq), and external hazard index (H_ex) have been calculated from the activity of nuclides ²²⁶Ra, ²³²Th, ⁴⁰K using the equations (3), (4), (5) and (6), respectively and the values have shown in Table 2.

From Table 2, it shows that the radium 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 recommended by the organization for Economic 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 ²²⁶Ra, ³³²Th and ⁴⁰K and the range is 70.52±7.03 – 315.60±11.88 nGyh^{-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 SvGy^-1 to convert the absorbed dose rate to the effective dose equivalent and 0.2 for the outdoor occupancy factor. The annual effective 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 ²²⁶R / ²²⁸R concentration activities is less than 1 because the concentration activity of ²²⁶Ra in soil is less than the concentration of ²²⁸Ra. In other words, the concentration of uranium in soil is less than thorium, since uranium and thorium are the parents of ²²⁶Ra and ²²⁸Ra, respectively. The range of this ratio is obtained 0.356- 0.586.

4. Conclusions

In the present study, the results indicate that the natural radioactivity concentration of ²²⁶Ra is relatively lower than that of ²³²Th and ⁴⁰K at the vicinity of Malnichera tea garden in Sylhet district of Bangladesh. The values of mean absorbed dose rate, annual effective dose rate and the radium equivalent activity were higher than the global average values; whereas the external hazard index is less than unity which indicates that the area is no threat of exposure for the population..

Previously, no works on the tea garden for radioactivity measurement of the soil samples in Sylhet division was carried out. Therefore, this work can be used as a baseline data for further research work.