1. Introduction

Natural radionuclides have been the components of the earths^, since its existence. It is widely spread in earth environment and exists in soil, sediment, water, plants and air. There are many naturally occurring radionuclides in environment, containing uranium and thorium series radioisotopes and natural ⁴⁰K ^{[1, 2]}. The natural radioactivity in soil comes from U and Th series and natural K. Natural environmental radioactivity and associated external exposure due to gamma radiation depend primarily on the geological conditions of soil and sediment formations of each region in the world ^[3]. The study of natural radioactivity in marine and coastal environments is of significant importance for better understanding of oceanographic and sediment logical processes. The distribution of natural radionuclides in the seabed can be used as a tracer for both sediment sand dredged soil dispersal and accumulation mechanisms ^[4]. The radionuclides are associated with heavy minerals, whereas ⁴⁰K is concentrated within clay minerals ^[5]. Radioisotopes that are present in soil significantly affect terrestrial gamma radiation levels. In the last decade, several studies were carried out to assess the average outdoor terrestrial gamma dose rate in air at 1 m from the ground. The effective gamma radiation levels were generally in the range of 10–200 nGy /h with a mean of 60 nGy/h ^{[6, 7]}. Human beings are exposed to background radiation that stems both from natural and man-made sources. Natural background radiation, which is equivalent to 2.4 mSv per person, makes up approximately 80% of the total radiation dose a person is exposed in a year ^[8]. In addition to the natural sources, soil radioactivity is also affected from man-made activities. Caesium-137 (¹³⁷Cs) is a fission product which is formed through nuclear tests and accidents. The deposition of ¹³⁷Cs in soil is important since its half-life is 30.2 years and it has a gamma emission of 661 keV ^[9].

The aim of the present work is determine the activity utilization index (AUI), alpha index (I_α) and radiation exposure rate (I) for soil samples in different areas of Nineveh province.

2. Materials and Methods

This study was carried out at Nineveh province, ten districts has been selected from the center of Mosul city (i.e. Al-Medan, Camp of Gazlany and College of Agriculture at University of Mosul) and other locations from some small towns outside the city center of Mosul like (Sinjar, Rabeaa, Telfar, Al-Koosh, Telkaif, Al-Hamdanea and Bacheeka) as shown in Table 1.

Table 1. Names and codes of collected soil samples from Nineveh province

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The specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K and were calculated using the following relation ^{[10, 11]}:

(1)

where is the specific activities in (Bq/kg), I is line intensity of - line in a radionuclide, ε is the measured efficiency of each - line observed, m is the weight of each sample and t is the sample counting time in (sec.).

Ten soil samples were collected from all of the districts in the selected study area, every one of these samples weigh about 1.5 Kg , and the stones organic matter were removed from these samples, then the samples dried by placing it in the oven of 110°C for period of 24 hours, then crushed to pass through 2 mm sieve to be homogenized in size. The homogenized soil samples were sealed in plastic containers and left for at least one month to attain secular equilibrium, before gamma spectrometer consisting of vertical HPGe setup and multichannel analyzer (8192) channel, with the following specifications: resolution (FEHM) at 1.33 MeV 60Co is 2.0 keV, relative efficiency at 1.33 MeV 60Co is 40%. The detector shield are with activity adequate to accommodate large samples. Shield has walls 10 cm lead, thick lined inside with graded absorber of Cd ~ 1.6 mm, Cu~ 0.4mm. This shield serves to reduce different background radioactivity. To minimize the effect of the scattered radiation from the shield, the detector is located in the center of the chamber.

3. Results and Discussion

The Radium Equivalent Activity (R_aeq), Absorbed Gamma Dose rate (D), External and Internal Hazard Indices (H_ex and H_in) and Gamma index(I_γ) were calculated and mentioned in ^[10].

In order to facilitate the calculation of dose rates in air from different combinations of the three radionuclides in sediments and by applying the appropriate conversion factors, an activity utilization index (AUI) is constructed that is given by the following expression ^[12]:

(2)

where , and are activity concentrations in (Bq /Kg) of ²²⁶Ra, ²³²Th, and ⁴⁰K taken from ^[7], , and (0.462) are the fractional contributions to the total dose rate in air due to gamma radiation from the actual concentrations of these radionuclides. In the NEA-OECD Report, typical activities per unit mass of ²²⁶Ra, ²³²Th, and ⁴⁰K in sediments , and are referred to be 50, 50 and 500 Bq/Kg, respectively ^[13]. The activity utilization index of the soil samples is calculated by using eq.(2). The calculated values from Table 2, vary from 0.275 in SS9 (Al-Medan) location to 0.735 in SS2 (Rebeaa) location with an average value of 0.593. This value shows that AUI is less than 2, which corresponds to an annual effective dose of < 0.3 mSv/y ^[14]. The values of AUI are shown graphically in Figure 1 and listed in Table 2.

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Figure 1. The values of AUI for soil samples in different areas of Nineveh province

Table 2. The value of activity utilization index (AUI) of soil samples of different areas in Nineveh Province

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The excess alpha radiation due to radon inhalation originating from building materials, such as soil, was estimated using the following relation ^[15]

(3)

where:

: is the alpha index

: is the specific activity concentrations of ²²⁶Ra assumed in equilibrium with ²³⁸U.

The recommended upper limit of specific activity concentration is (200 Bq/Kg), for which alpha index (I_α) is equal to 1. The values of alpha index shown graphically in Figure 2 and listed in Table 3.

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Figure 2. The values of Iα for soil samples in different areas of Nineveh province

An index, I, in µR/hr, was selected to estimate the radiation exposure rate potential due to the combined radionuclide concentrations, in pCi/g, of the ²³⁸U progeny, ²³²Th progeny and ⁴⁰K in a material. This index was taken to be the exposure rate measured 1 m above an infinite hemisphere or slab with respect to the y-rays emitted by the radionuclides. The following relation, with the first coefficient modified on the basis of more recent decay information, is given by ^{[16, 17, 18]}.

(4)

Concentrations of ²²⁶Ra and ²²⁸Ra can be substituted for those of ²³⁸Uand ²³²Th, respectively, because X- and y-rays in the chains before radium contribute little to the total exposure rate.

The values of radiation exposure rate (I) ranged from 107.20 µR/h in SS9 to 260.37 µR/h in SS2 with an average value of 186.88 µR/h. The values of I is shown graphically in Figure 3, and also listed in Table 3.

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Figure 3. The values of radiation exposure rate for soil samples in different areas of Nineveh province

Table 3. The values of alpha index(Iα) and radiation exposure rate of soil samples of different areas in Nineveh Province for ²²⁶Ra, ²³²Th and ⁴⁰K

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4. Conclusions

It is important to determine background radiation level in order to evaluate the health hazards. This study determined the values of activity utilization index, alpha index and the radiation exposure rate, have been found to be lower than their corresponding allowed limits, hence they will pose relatively no serious health risk.

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