1. Introduction

Humans are primarily exposed to radiation from the sun, cosmic rays and naturally-occurring radioactive elements (NORM) found in the earth’s crust. Nearly in all nations, scientists probed since long time ago and are still probing the earth's crust and for a long time in the future to measure the radiation levels and quantify the hazards and doses affecting people, animals, plants and all kinds of life ^[1].

Human activities, such as a nuclear reactors and nuclear bomb testing are produced radionuclides and they called technical enhanced radioactive elements (TENORM). Some radionuclides, such as radium, uranium, thorium and potassium have existed since the formation of the earth. Natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions. They appear at different levels in soil of each region in the world ^{[2, 3, 4]}. Since the distribution of radioactive nuclei in the environment is random, the knowledge of their distribution in soil plays an important role in radiation protection and measurements ^[5]. The main contributor of radiation exposure is the emanation of Radon gas from soil, the progeny of ²²⁶Ra ^[6]. The sources of the radiations are terrestrial, extraterrestrial and anthropogenic. The radiation of terrestrial origin comes from various earthly materials which contain various amounts of 238U and 232Th and their decay products and ⁴⁰K. The level of radiation in any area depends on local geological conditions. Higher radiation levels are associated with igneous rocks, such as granite, granodiorites and syenites, and lower levels with sedimentary rocks with some exceptions where shales and phosphate rocks are a source of high radiation ^[7]. Assessment of radionuclides in soils and rocks in many parts of the world has been on the increase in the past two decades because of their hazard on the health of the populace . While this is the case, the research into natural radionuclide in the soils and rocks of Southern-Iraq is yet to receive the much needed attention compared to the rate of ordinary soil due to the increases of oil production in the area and processing being done in the region. As a contribution to focus on this area, a number of radiological indices were measured from on samples taken from oil field of Basrah.

The main objective of this study is to determine activity concentration of natural radionuclides in soil of selected area in Basrah Governorate. The radium equivalent, activity utilization indices and effective absorbed dose were calculated and compared with international available data.

2. Material and methods

The area of study, shown in Figure 1, represent southern part of Basrah Governorate, which is considered as contaminated area due to the fallout ash during the Allies war on Iraq.

Soil were collected at each sampling location from different depth. About 500-600 gm of each sample was crushed into fine powder, and fine quality of the sample was obtained using scientific sieve. Samples were heated in the oven at 110^oC for 24h to remove moisture, put inside Marinalli beakers and then stored for 30 days to allow the equilibrium between ²²⁶Ra and ²²²Rn. The activity concentration of ²²⁶Ra, ²²⁸Ra, ²³⁸U, ²³²Th and ⁴⁰K was estimated from the gamma spectrum using Na(Tl) detector 3x3 inch with a 1024 channel computer analyzer USX supplied by Spectrum Technique Company. The detector was employed with lead shielding, 4 cm thickness, which reduced the background. The detector was calibrated using standard sources of ⁵⁷Co (peak 122 keV), ¹³⁷Cs (peak 662 keV) and ⁶⁰Co (peaks 1173, 1333 keV). The detector resolution is about 8% at 662 keV of ¹³⁷Cs. The efficiency calibration was achieved using eight standard sources include the calibration sources. The system was running freely, for 12 h live time, to evaluate the background spectrum. The Marinalli beaker contains sample was placed over the detector for counting.

Activity concentration A_i of any gamma-rays line taken to represent this parameter for the environmental radionuclides has been calculated using the relation ^[8]

(1)

where ε is absolute gamma peak efficiency of the detector at this particular gamma-ray energy, decay intensity for the specific energy peak (including the decay branching ratio information), M the mass of the sample in kg and t is the counting time of the measurement in second.

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Figure 1. The studied area in Basrah governorate, southern part of Iraq

To evaluate activity concentrations of natural radionuclides, one has to recognized the belong city of each peak according to gamma decay of each isotope ^[9]. For ²²⁶Ra we are looking for the gamma ray lines 295 keV(19.2%), 352 keV (37.1%), 609 keV ( 46.1%), 1120 keV (15%) and 1760 keV (15.4%). The peak of 186 keV assumed to be from ²³⁵U since it has slight effect on the total concentration after subtracting the background, 42.8% for Ra and the rest for ²³⁵U. The determination of existence of ²³²Th was achieved by 338 keV (12%), 911 keV (29%), 964 keV (5.05%) and 969 keV (17%). The case of ²³⁸U is recognized by 1001 keV (83%), 766 keV (29%) and 2204 keV (5%). For ⁴⁰K, this directly determined using 1460 keV (10%) peak.

3. Calculation of Activities, Hazard Indices and Dose Parameters

The exposure due to gamma- radiation is usually defined in term of radium equivalent activity (Ra_eq). Radium equivalent activity is used to assess the hazards associated with materials that contain ²²⁶Ra, ²³²Th and ⁴⁰K in Bq kg^-1, which is, determined by assuming that 370 Bq kg^-1 of ²²⁶Ra or 260 Bq kg^-1 of ²³²Th or 4810 Bq kg^-1 of ⁴⁰K produce the same γ dose rate. The Ra_eq of a sample in (Bq kg^-1) can be achieved using the following relation ^[10];

(2)

The published maximal permissible Ra_eq is 370 Bq kg^{-1 [11]}.

The external and internal hazard indices are an evaluation of the hazard of the natural gamma radiation. The prime objective of this index is to limit the radiation dose to the admissible permissible dose equivalent limit around 1mSvy⁻¹. In order to evaluate this index, one can use the following relations ^[10]

(3)

(4)

In order to estimate the annual effective dose rate in air, the conversion coefficient from absorbed dose in air to effective dose received by an adult must be considered. This value is published in UNSCEAR 2000 and UNSCEAR 1993, to be 0.7 SvGy^-1 for environmental exposure to gamma rays of moderate energy. The outdoor occupancy factor is about 0.2 ^[9]. The annual effective dose equivalent is given by the following equation ^[10];

(5)

where

(6)

and n=0.2 for outdoor and 0.8 for indoor.

The world average annual effective dose equivalent (E) from outdoor or indoor terrestrial gamma radiation only is 0.560 mSv/y [UNSCEAR].

4. Results and Discussions

The measured values of natural radioactivity concentration for ²²⁶Ra, ²³²Th, ²³⁸U and ⁴⁰K for different location of southern part of Basrah Governorate, Iraq are given in Table 1. The worldwide average activity concentration for ²²⁶Ra, ²³²Th and ⁴⁰K reported by UNSCEAR are 50 Bq kg^-1, 27 Bq kg^-1, 400 Bq kg^-1 respectively. From Table 1; the activity concentration of ²²⁶Ra ranges from 8.0±2.9 Bq kg^-1 to 197±16.8 Bq kg^-1 with average value of 84.0±7.7Bq kg^-1, which is higher than the recommended limit of UNSCEAR. The specific activity of ²³²Th ranged from 2.1±0.1Bq kg^-1to 12.0±0.5 Bq kg^-1, with average value of 8.2±1.0 Bq kg^-1. The range of activity for ⁴⁰K is from 57.0±2.9 Bq kg^-1 to 635.0±18.3 Bq kg^-1 and average value is 315±9 Bq kg^-1. Both average value for ²³²Th and ⁴⁰K are below the recommended limit of UNSCEAR.

Table 1. The values ²²⁶Ra, ²³²Th and ⁴⁰K specific activity concentrations using gamma ray spectroscopy of soil samples from Basrah Governorate. The letters a, b, c and d are closed locations

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From the table one can see that the maximum value of ²²⁶Ra is 197±16.8 Bq/kg and minimum value is 8.0±2.9 Bq/kg with average value 88.9 Bq/kg. The maximum value of ²³²Th is 12.0 Bq/kg and minimum is 1.6 Bq/kg with average value 8.4 Bq/kg. While in the case of ⁴⁰K the maximum value is 635.0±18.3 Bq/kg and minimum is 57.0±2.9 Bq/kg. All the results obtained were in the range of other workers in the same area, but in different locations ^{[12, 13, 14]}. No uniform trend in the variation of concentration of natural radionuclides found in the soil samples as can be seen from Figure 2. This figure shows no correlation between ²²⁶Ra and ⁴⁰K and positive weak correlation between ²²⁶Ra and ²³²Th.

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Figure 2. Correlation between ²²⁶Ra and ²³²Th and ⁴⁰K radionuclides

The radium equivalent activity (Ra_eq) of the soil samples tabulated in Table 2 ranges between 15 Bq kg^-1 to 235 Bq kg^-1 with average value 118 Bq kg^-1, which is lower than the recommended safe limit of ICRP, 370 Bq kg^-1. The air observed dose rate ranges between 15.3 nGy/h and 174.5 nGy/h with average value 93.0 nGy/h for outdoor. While for indoor; ranges between 29.0 nGy/h and 283.9 nGy/h with average value 167.4 nGy/h. The H_ex values are less than one in all samples, which is in agreement with equation (3). However, in the case of H_in ; there are four sample exceeded unity with small amount in disagreement with equation (4). The annual effective outdoor and indoor gamma ray dose rates calculated in the study area ranges from 0.093 to 0.861 mSv/y and 0.142 to 1.300 mSv/y with respective mean values of 0.524 mSv/y and 0.823 mSv/y. These results are higher than the average worldwide value 0.56 mSv/y but still less than the action level of 3 - 10 mSv/y according to ICRP ^[15].

Table 2. Values of ²²⁶Ra, ²³²Th and ⁴⁰K hazard indices for soil samples of selected locations in Basrah Governorate

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5. Conclusion

The measured activity concentrations of ²³²Th and ⁴⁰K in the soil samples of selected location in Basrah Governorate are lower than world level reported by UNSCEAR. However, the concentrations for ²²⁶Ra are found to be higher than the worldwide mean value in small amount. This is may be related the effect of dust which contains some radioactive material fallout during the previous wars in the area. The results obtained from external hazard are less than unity, while in the indoor hazard there are some locations has value higher than unity. The annual effective doses for outdoor and indoor are less than the action level of ICRP. Week correlation has been found between ²²⁶Ra and both ²³²Th and ⁴⁰K, which indicates that one can’t depends on the measurement of one parameter to determine the other.

Hence, the achievement of this work is that; soil taken from these locations in Basrah Governorate can be deal with and used as a construction material without posing any significant radiological threat to the occupiers.

Acknowledgments

The authors are very grateful for Prof. Laith A. Najam from Universityof Mosul for helping us in publishing the paper.

References

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