Journal of Pure Applied and Industrial Physics, Vol.8(7), 82-89, July 16, 2018 ISSN 2229-7596 (Print) (An International Research Journal), IF = 4.715, www.physics-journal.org ISSN 2319-7617 (Online)

Radioactivity Distributions in Soils from Three Central Districts of and their Radiological Consequences

Khondokar Nazmus Sakib1, Md. Sujon Mian1, Md. Toriqul Islam1, Sopan Das2, Md. Mohiuddin Tasnim1 and Jobaidul Islam1

1Department of Physics, Mawlana Bhashani Science and Technology University, Santosh, Tangail-1902, BANGLADESH. 2Institute of Nuclear Science and Technology (INST), AERE, BAEC, Savar, Dhaka, BANGLADESH. email address of the corresponding author: [email protected].

(Received on : June 24, 2018 and Accepted on : June 28, 2018)

ABSTRACT

Radionuclides in soil creates a significant amount of background radiation and also responsible for food chain contamination. That’s why fifteen soil samples collected from three central have been evaluated using High Purity Germanium (HPGe) detector. Activity concentration were found in the range of 9.88 to 86.95 Bq kg-1 for 238U, 24.07 to 178.07 Bq kg-1 for 232Th and 68.01 to 792.68 Bq kg-1 for 40K. Artificial radionuclide 137Cs was not found in this study. Radium

equivalent activity (Raeq), gamma absorbed dose rate (D), external hazard index (Hex) -1 -1 and annual effective dose rate (Deff) were found 147.47 Bq kg , 68.064 nGyh , 0.404 and 83.46 µSv respectively, which shows that soils of the study area is radiologically safe.

Keywords: HPGe detector; Soil; Radioactivity; Radiological hazard.

INTRODUCTION

Radiation is common phenomenon in our life. Natural radioactivity is common in the rock and soil that makes up our planet. Among the radioactive elements in the environment, the most abundant are 40K, and the radioisotopes of the natural decay series of 238U and 232Th, which are present in the earth’s crust (Ferdous, J.,2015). Nuclear ore processing, fuel

82 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018) fabrication, reactor operation is mainly responsible for artificial radionuclides. The artificial radionuclide 137Cs absorbed by the soil and distributed in the different depth. Significant amount of background radiation is produced by artificial and natural radionuclides. Besides radionuclides present in the soil can enter into the food chain. That’s why radioactivity studies are important and have been carried out on soil samples in other parts of the world. Bangladesh is the neighboring country of India. As India, Pakistan, China has tested nuclear weapons in this region of Asia it is highly possible that radioactivity in Bangladesh will also increase. That’s why radioactivity monitoring is highly needed for Bangladesh for its global position. This study deals with natural and artificial radioactivity for soil samples collected from central region of Bangladesh. Besides, the radium equivalent activity, gamma absorbed dose rate, external hazard index and annual effective dose rate were evaluated and compared to the safe limits.

MATERIALS AND METHODS

Description of study area

In this study Tangail, and Jamalpur are the three central districts of Bangladesh have been considered for radioactivity monitoring. Tangail, Sirajganj and Jamalpur district comprises 12, 9 and 7 respectively. Soil samples were collected from Tangail Sadar and Ghatail of Tangail district, Sirajganj Sadar and Belkuchi upazila of and lastly Sarishabari upazila of Jamalpur district. This study intended to study the concentration of natural and artificial radionuclides in soils of Tangail, Sirajganj and Jamalpur districts to establish baseline radioactivity data for the area.

Preparation of samples and standards

Soil samples were collected during 2017 from 15 locations of Tangail, Sirajganj and Jamalpur district. Lands of slightly dry soil were chosen maintaining some criteria mentioned below:

1. The collection area should be undisturbed open terrain. 2. There should be very little or no runoff soil during heavy rain, particularly in rainy season.

The beakers were cleaned with deionized water and put into the oven to be dried. The identification number (ID) was given on each beaker according to the sample ID. Collected soil samples were put into the beakers corresponding to the given ID. The beakers containing the soil samples were then allowed to dry in the oven at 70C until having constant weight. The dried soil samples were then powdered by the use of an agate mortar and the beakers with soil were again put into the oven. plastic pots having the same geometry were cleaned by acetone for two times separately and were put to dry naturally. pot with the sample was weighed out and noted in the laboratory register. All the pots were remained un-disturb at least

83 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018) for 4 weeks before counting to attain parent’s daughter equilibrium in the 238U and 232Th decay series. For measuring the radioactivity two standards were used. One was IAEA Soil-6 and the 226 other was Ra(liquid) sprayed into Al2O3. Putting the IAEA Soil-6 and Al2O3 standard in the individual pots, the weight was measured and noted in the laboratory register.

Experimental setup

In counting system, a High Purity Germanium (HPGe) detector (Canberra, 40% Relative efficiency) was used to measure the radioactivity of the samples and standards of interest. The detector was coupled with a personal computer (PC) based Digital Spectrum Analyzer DSA-1000 coupled with Genie-2000 MCA software. For counting the samples and standards each was placed individually on the surface of the detector and a count was taken for 20000 second for each. Energy range of the spectra was restricted by adjusting the Multichannel Analyzer (MCA) to ‘0 to 3210’ KeV. The dead time of the detector was kept below 1% for all over the counting of the samples and standards. The spectra in all cases were saved in the PC. A background count for the time 20000 seconds was taken to get the background contribution in the samples and standards.

Calculation of the radiological effects

Radium equivalent activity (Raeq) is the most widely used radiation hazard index. Radium equivalent activity (Raeq) calculated using the following relation (Beretka, J.,1985): Raeq = CRa + 1.43CTh +0.07CK (1) 226 232 40 where CRa, CTh and CK are the activity concentrations of Ra, Th and K in Bq/kg, respectively. The external gamma absorbed dose rate in the air at lm above ground level was calculated from the measured activities of 226Ra, 232Th and 40K in soil assuming that the other radionuclides, such as 137Cs, 90Sr and the 235U series can be neglected as they contribute very little to the total dose from environmental background. The calculations were performed according to the following equation (UNSCEAR, 1993): D = 0.462CRa + 0.604CTh + 0.042CK (2) where D is the dose rate in nGy/h and CRa, CTh and CK are the specific activities (Bq/kg) of 226Ra, 232Th and 40K, respectively. The external hazard index, Hex, is defined as (UNSCEAR, 2000): Hex = CRa/370 + CTh/259 + CK / 4810 (3) 226 232 40 where CRa, CTh and CK are the specific activities (Bq/kg) of Ra, Th and K, respectively. The value of this index must be less than unity in order to keep the radiation hazard insignificant.

To estimate annual effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7SvGy−1) and an outdoor occupancy factor (0.2) proposed by UNSCEAR, 2000 are used. Therefore, the annual effective dose rate was calculated by the formula (UNSCAER, 2000): Deff(Sv) = D(nGy/h) × (24 × 365) (h) × 0.7 × 0.2 (4)

84 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018) RESULTS AND DISCUSSION

It was found that, the activity concentration ranges from 9.88 (Ghatail, Tangail) – 86.95 (Sirajganj Sadar, Sirajganj) Bq/kg for Uranium-238, 24.07 (Ghatail, Tangail) – 178.07 (Sirajganj Sadar, Sirajganj) Bq/kg for Thorium-232 and 68.01(Ghatail, Tangail) – 792.68 (Sarishabari, Jamalpur) Bq/kg for Potassium-40, with mean values of 31.39 Bq/kg, 63.33 Bq/kg and 364.46 Bq/kg respectively (Table-1). We did not get artificial radionuclide (137Cs) in any kind of samples. Results revealed that Potassium-40 was the most abundant radioactive element under consideration.

Table-1: Radioactivities of 238U, 232Th and 40K in soil samples at different locations.

Sample no Sample location 238U (Bq kg-1) 232Th (Bq kg-1) 40K (Bq kg-1) S-1A Tangail Sadar, Tangail 39.84±2.21 76.71±3.85 485.79±19.49

S-1B Tangail Sadar, Tangail 25.65±1.57 47.48±2.68 187.17±9.47

S-1C Tangail Sadar, Tangail 26.35±1.83 57.70±3.36 497.37±20.57

S-2A Ghatail, Tangail 9.88±0.86 28.56±1.86 68.01±4.80

S-2B Ghatail, Tangail 24.71±1.59 55.25±3.03 283.00±12.81

S-2C Ghatail, Tangail 11.28±0.91 24.07±1.69 143.07±7.69

S-3A Belkuchi, Sirajganj 25.7±1.78 65.58±3.63 581.73±23.08

S-3B Belkuchi, Sirajganj 23.66±1.49 29.68±1.98 196.81±9.71

S-3C Belkuchi, Sirajganj 35.86±1.20 48.34±2.77 496.22±19.52

S-4A Sirajganj Sadar, Sirajganj 22.81±1.42 48.83±2.62 189.56±9.21

S-4B Sirajganj Sadar, Sirajganj 36.51±2.03 60.69±3.18 406.75±16.62

S-4C Sirajganj Sadar, Sirajganj 86.95±3.47 178.07±6.57 110.64±6.20

S-5A Sarishabari, Jamalpur 34.32±1.99 80.72±3.93 447.89±18.11

S-5B Sarishabari, Jamalpur 33.30±2.13 74.38±4.02 792.68±29.80

S-5C Sarishabari, Jamalpur 34.08±2.08 74.03±3.87 580.35±22.79

Average 31.39±1.77 63.33±3.26 364.46±15.32

85 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018) The world average activity concentration and ranges for 238U, 232Th and 40K are 35 Bq kg-1 with a range of 17 - 60 Bq kg-1, 30 Bq kg-1 with a range of 11 - 64 Bq kg-1 and 400 Bq kg- 1 with a range of 140 – 850 Bq kg-1 respectively (UNSCEAR, 2000). The average values obtained in this study fells within the range of world values and other published values mentioned in Table-2.

Table-2: Comparison of radioactivity levels of the soil samples of different countries with that of this study.

Countries Average Specific Average Specific Average Specific References radioactivity of 238U radioactivity of radioactivity of (Bq kg-1) 232Th (Bq kg-1) 40K (Bq kg-1) USA 40 35 370 (UNSCEAR, 2000) China 32 41 440 (UNSCEAR, 2000) Japan 33 28 310 (UNSCEAR, 2000) India 29 64 400 (UNSCEAR, 2000) Iran 28 22 640 (UNSCEAR, 2000) Poland 26 21 410 (UNSCEAR, 2000) Spain 32 33 470 (UNSCEAR, 2000) Saudi Arabia 15 11 225 (Alaamer, A. S., 2008) Nigeria 14 19 896 (Okeyode, I., 2010) Turkey 21 25 298 (Bozkurt, A., 2007) Pakistan 30 56 642 (Akhtar, N., 2004) Bangladesh 48 53 481 (Kabir, K. A., 2009) Worldwide average 35 30 400 (UNSCEAR, 2000) Central region of 31.39 63.33 364.46 Bangladesh (Present study)

Results also revealed that the activity concentration of 238U, 232Th and 40K in this study were comparable with other published results in Bangladesh (Table-3).

Table-3: Average activity concentration of 238U, 232Th & 40K in soil samples for different regions within Bangladesh.

Sample location Average Specific Average Specific Average Specific References radioactivity radioactivity of radioactivity of of 238U 232Th (Bq kg-1) 40K (Bq kg-1) (Bq kg-1) Chittagong 35 60 438 (Chowdhury et al., 1999) Pabna 33 47 449 (Roy et al., 2001) Dhaka 33 55 574 (Miah et al., 1998) Jessore 48 53 481 (Miah et al., 1985) Sitakunda 31 62 467 (Rahman et al., 2012) Sylhet 55 125 491 (Chowdhury et al., 1999) Central region of 31.39 63.33 364.46 Bangladesh (Present study)

86 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018)

Radium equivalent activity (Raeq), gamma absorbed dose rate (D), external hazard index (Hex), annual effective dose rate (Deff) for different soil samples were 147.47Bq/kg, 68.064 nGy/h, 0.404, 83.46 µSv respectively (Table 4).

Table-4: Radium equivalent activity, dose rate, annual effective dose and External Hazard Index for soil samples at different locations.

Sample Sample location Radium Equivalent Dose Annual Effective External Hazard no Activity, Raeq (Bq/kg) Rate, D Dose, Deff Index, Hex (nGy/h) (10-6 Sv) S-1A Tangail Sadar, 183.54 85.14 0.504 104.41 Tangail S-1B Tangail Sadar, 106.64 48.38 0.291 59.33 Tangail S-1C Tangail Sadar, 143.67 67.91 0.397 83.28 Tangail S-2A Ghatail, Tangail 55.48 24.67 0.151 30.25 S-2B Ghatail, Tangail 123.52 56.67 0.338 69.50 S-2C Ghatail, Tangail 55.71 25.75 0.153 31.57 S-3A Belkuchi, 160.20 75.91 0.443 93.09 Sirajganj S-3B Belkuchi, 79.87 37.12 0.219 45.52 Sirajganj S-3C Belkuchi, 139.72 66.60 0.386 81.67 Sirajganj S-4A Sirajganj Sadar, 105.90 47.99 0.289 58.85 Sirajganj S-4B Sirajganj Sadar, 151.76 70.60 0.417 86.58 Sirajganj S-4C Sirajganj Sadar, 349.33 152.37 0.945 186.86 Sirajganj S-5A Sarishabari, 181.10 83.42 0.497 102.30 Jamalpur S-5B Sarishabari, 195.15 93.60 0.541 114.79 Jamalpur S-5C Sarishabari, 180.56 84.83 0.498 104.03 Jamalpur Average 147.47 68.064 0.404 83.46

CONCLUSION

Fifteen soil samples collected from central region of Bangladesh have been studied and evaluated due to revealed radiological information. The average activity concentrations for 238U, and 40K in this study were lower than the worldwide average whereas activity concentration for 232Th were higher than the worldwide average in soils. The average value of gamma dose rate obtained in this study is higher than to the world average 57 nGy/h. All values obtained for radium equivalent activity are less than 370 Bq/kg, which are acceptable for safe

87 Khondokar Nazmus Sakib, et al., J. Pure Appl. & Ind. Phys. Vol.8 (7), 82-89 (2018) use. The average calculated result for annual effective dose of soil samples was less than the average world recommended value of 1.0 mSv (ICRP, 1990). The calculated values of Hex for the soil samples is less than unity. The values of the radiation hazard parameters from this study were not extremely high compared to either the world averages or the recommended limits, therefore soils of the studied area were safe for normal use.

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