Natural Radioactivity in Soil Samples in Nineveh Province and the Associated Radiation Hazards

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2.2. Collection and Preparation of Samples

A total of ten soil samples have been collected from ten districts of Nineveh area. These samples were crushed and dried in an oven at110 C^◦ for 24 h to ensure than any significant moisture was removed from the samples. Later, they were grinded into fine powder to pass through a 2mm mesh and the homogenized sample was sealed in marinelli beaker. The samples were then stored and kept sealed for about a four- week period to allow radioactive equilibrium among the daughter products of radon (²²²Ra), thoron (²²⁰Ra) and their short lived decay products.

2.3. Gamma Ray Spectrometry

Each Sample was measured with gamma –ray spectrometry 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. Then the sample was placed over the detector for at least 2h. The spectrum was evaluated with the computer software program Maestro(EG&G ORTEC). The energy regions selected for the corresponding radionuclides were 609 keV, 11120 keV for ²¹⁴Bi for ²²⁶Ra, 911keV and 969 keV of ²²⁸Ac for ²³²Th and 1460 keV for ⁴⁰K.

The activity concentration of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were calculated using the following relation ^[12]:

(1)

Where A is the activity concentration in Bq/kg (CPS) net is the (count per second) and equal [(CPS) sample –(CPS) background], I is the intensity of γ-line in a radionuclide, is the measured efficiency of each γ- line observed, m is the weight of each sample and t is the sample counting time 7200 sec.

3. Results and Discussion

3.1. Activity Concentration in Soil Samples

From Table 2, it can be noticed that the highest value of specific activity concentration of ²²⁶Ra was found in (S1)(Sinjar) district which was equal to (38.83 Bq/kg), while the lowest value of specific activity concentration of ²²⁶Ra was found in (S9) (Al-Medan) district which was equal to (16.21 Bq/kg), with an average value of (32.52± 6.48 Bq/kg).The present results have shown that values of specific activity concentrations in the studied districts in Nineveh province were lower than value of specific activity concentration of ²²⁶Ra global limit which equal to (370 Bq/kg) ^[4]. The highest value of specific activity concentration of ²³²Th was found in (S2)(Rabeaa) district which was equal to (28.37 Bq/kg), while the lowest value of specific activity concentration of ²³²Th was found in (S9) (Al-Medan) district which was equal to (8.53 Bq/kg), with an average value of (20.30± 5.3 Bq/kg).The present results have shown that values of specific activity concentrations in the studied districts in Nineveh province were lower than value of specific activity concentration of ²³²Th global limit which equal to (45 Bq/kg) ^[4]. The highest value of specific activity concentration of ⁴⁰K was found in (S2)(Rabeaa) district which was equal to (613.11 Bq/kg), while the lowest value of specific activity concentration of⁴⁰K was found in (S10) (College of Agriculture) district which was equal to (261.81 Bq/kg), with an average value of (378.93± 123.29 Bq/kg).The present results have shown that values of specific activity concentrations in the studied districts in Nineveh province were lower than value of specific activity concentration of ⁴⁰K global limit which equal to (420 Bq/kg) ^[4] except Rabeaa and Camp of Gazlany districts (S2 and S8). The highest value of specific activity concentration of ¹³⁷Cs was found in (S8)(Camp of Gazlany) district which was equal to (17.92 Bq/kg), while the lowest value of specific activity concentration of ¹³⁷Cs found in (S5) (Telkaif) district which was equal to (2.18 Bq/kg), with an average value of (8.17± 5.55 Bq/kg).The present results have shown that values of specific activity concentrations in the studied districts in Nineveh province were lower than value of specific activity concentration of ¹³⁷Cs global limit which equal to (14.8 Bq/kg) ^[4] except Al-Hamdanea and Backeeka districts (S6 and S7).The results of the activity concentrations for ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were shown graphically in Figure 1, Figure 2, Figure 3 and Figure 4.

Table 2. Specific activity concentrations of ²²⁶Ra,²³²Th,⁴⁰K and ¹³⁷Cs for soil samples in Nineveh province

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Figure 1. Activity concentration for ²²⁶Ra in soil samples

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Figure 2. Activity concentration for ²³²Th in soil samples

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Figure 3. Activity concentration for ⁴⁰K in soil samples.

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Figure 4. Activity concentration for ¹³⁷Cs in soil samples

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Figure 5. Bar graph showing the Radium equivalent activity R_aeq (Bq/kg) of soil samples in different districts

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3.2. Gamma Radiation Parameters
3.2.1. Radium Equivalent Activity (Ra_eq)

The distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in soil is not uniform. Uniformity with respect to exposure to radiation has been defined in terms of radium equivalent activity (R_aeq) in Bq/kg to compare the specific activity of the material containing different amounts of ²²⁶Ra, ²³²Th and ⁴⁰K.It is calculated through the following relation ^[13]:

(2)

Where A_Ra, A_Th and A_Kare the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg, respectively. While defining Raeq activity, it has been assumed that 370 Bq/kg of ²²⁶Ra or 259 Bq/kg of ²³²Th or 4810 Bq/kg of ⁴⁰K produce the same gamma dose rate. The results of radium equivalent activity shown graphically in fig.5.

3.2.2. Absorbed Gamma Dose Rate(D)

The external outdoor absorbed gamma dose rate due to terrestrial gamma rays from the nuclides ²²⁶Ra, ²³²Th and ⁴⁰K at 1m above the ground level were calculated as follows outlined by ^[14]:

(3)

Where A_Ra, A_Th and A_Kare the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg. Fig.6 shows graphically the results of absorbed gamma dose rate.

Figure 6. Bar graph showing the Absorbed gamma dose rate D(nGy/h) of soil samples in different districts

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3.2.3. External and Internal Hazard Indices (H_exand H_i_n)

Local soil of the area is used for the construction of houses. These soils will contribute to the external gamma dose rate in these houses. The external hazard index (H_ex) can be defined as follows ^[15]:

(4)

Where A_Ra, A_Th and A_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg, respectively. When the value of this index is less than unity then the radiation received by occupants will be < 1.5 mGy/y. The internal exposure to ²²²Rn and its radioactive progeny is controlled by the internal hazard index(H_in), which is calculated by the relation ^[16]:

(5)

H_in should be <1 for safe use of soil.

The results of H_in and H_ex shown graphically in Figure 7.

Figure 7. Bar graph showing the External and Internal hazard indices (H_ex& H_in) of soil Samples in different districts

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3.2.4. Gamma Index (I_γ)

Another radiation hazard called the representative level index I_γ, is defend as follows ^{[17, 18]}:

(6)

Where A_Ra, A_Th and A_Kare the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg, respectively.

I_γ varied from 0.388 in S9 (Al-Medan district) to 0.940 in S2 (Rabeea district), with an average value of 0.65± 0.16.

Figure 8. Bar graph showing the Gamma index ( I_γ) of soil samples in different districts

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This index value must be less than unity in order to keep the radiation hazard to be in significant. Fig.8 shows graphically the results of gamma index.

The results of radium equivalent activity( R_aeq), Absorbed dose rate(D), External and Internal hazard indices (H_ex,H_in) and Gamma index (I_γ) are listed in Table 3.

Table 3. Radium equivalent activity(R_aeq), Absorbed dose rate (D), External and Internal hazard indices(H_ex,H_in) and Gamma index(I_γ) for soil samples in selected districts in Nineveh province

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