Received 14 March 2015; accepted 24 April 2015; published 29 April 2015

ABSTRACT

Ten soil samples from Jabal Al Qur, Wadi Baba, and Wadi Sieh in Sinai, Egypt, were analyzed by XRD spectroscopy. The XRD spectroscopy results indicate that the major, minor and trace constituents varied from one sample to another. Samples were also analyzed by HPGe gamma spectrometer to determine the activity concentration of U-238, Th-232 series and K-40. The concentrations for ²³⁸U ranged from 57.03 to 4220.41 Bq/kg with an average 1110.75 Bq/kg, for ²³²Th, ranged from 13.55 to130.46 Bq/kg with an average 71.85 Bq/Kg. The concentrations for ⁴⁰K were in the range from 12.18 to 948.93 Bq/kg with an average value 457.09 Bq/kg. The average activity concentration values of ²²⁶Ra, ²³²Th, and ⁴⁰K, in all the collected samples were higher than the world average. The radium equivalent (R_eq), absorbed dose rate (D_R), the effective dose rate (D_eff), and hazard indices resulted due to the natural radionuclides in soil are also calculated. The Results show that the study area is not safe for human and environments.

Keywords:

Sinai Soil, X-Ray Diffraction, Natural Radioactivity Concentration, Chemical Constituent, Diffract Meter

1. Introduction

Measurement of the concentrations of natural radionuclides in soil give information’s on the natural radionuclide sources. Beck suggested that 50% - 80% of the total gamma flux at the earth’s surface arises from ⁴⁰K and ²³²Th, ²³⁸U series [1] . Knowledge of the distribution of these radionuclides in the environment is essential in the sense of controlling radiation levels [2] . Further, data on natural radiation are important for designing rules and regulations for radiation protection purposes [3] . The variation in data for various locations is due to the differences in the geology of sampling sites [3] . So several authors have studied the levels of natural background radiation by analysis of radionuclide concentration in soil samples, see e.g.: [4] -[9] . These studies pointed out that the effective gamma radiation levels were generally in the range of 10 - 200 nGyh⁻¹ with a mean of 60 nGyh⁻¹ [10] .

X-Ray Diffraction (XRD) is a non-destructive analytical technique. When X-ray strikes on a crystal surface at an angle θ, a portion of the X-ray is scattered by a layer of atoms at the surface. Un-scattered X-ray penetrates to the second layer of atoms where again a fraction is scattered. The remaining X-ray passes to the third layer until the energy of the X-ray is dissipated completely. An X-ray detector measures the cumulative effect of the scattering beams. The X-ray appear to be reflected from the crystal only if the angle of incidence satisfies the condition of Bragg Equation, nλ = 2d_hklsinθ. The X-ray pattern shows the peaks at various angles of incidence [11] .

The objectives of the present study are to determine some metal concentrations and measure the natural radioactivity levels of ²²⁶Ra, ²³²Th, and ⁴⁰K in the soil samples taken from selected places in Sinai region, Egypt, also to assess the radiological Hazard indices in air and to compare the results with international levels. Radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K were measured with a well-type gamma-ray detector, and metal concentrations were determined by X-Ray diffraction, patterns were recorded using X’Pert PRO Powder X-Ray Diffraction with Cu Kα radiation (λ = 0.1542 nm), Ni-filter and general area detector. The data generated in this study provide baseline values of natural radioactivity in Sinai soils and may be useful for authorities in the implementation of radiation protection standards for the general population in the country as well as to plan and conduct further studies on this issue.

2. Analytical Technique

2.1. Study Area

Sinai can be divided into three areas: The northern region consists of sand dunes and fossil beaches formed by the changing levels of the Mediterranean Sea during the glacial periods two million years ago. The landscape is flat and uniform, interrupted by sand and limestone hills. The central area with limestone dating from the Tertiary Period is the highlands extend towards the south. The third area consists of granite and volcanic rocks. Limestone and sandstone sediments are replaced by granite and basalt. Both rocks are produced by volcanic activity on the bottom of the ocean. Ten soil samples were collected from different areas of Sinai; Jabal Al Qur (28˚49'46.06''N - 33˚17'45.41''E) is located inland south-east of Sinai and consists of granite and volcanic rocks, Limestone and sandstone sediments. Wadi Baba (28˚58'30.80''N - 33˚17'31.47''E) is a low altitude system located inland south-east of Wadi Gharandel. The system consists of several smaller interconnecting wadis with gravel beds and high rise sandstone cliffs either side. Wadi Sieh (29˚03'27.28''N - 33˚26'14.84''E) is located further inland north-east of Wadi Baba; the wadi has a sandy bed with sandstone canyons rising either side, as shown in Figure 1, (www.bedawi.com/Sinai_EN.html).

2.2. Sample Collection, Preparation and Measuring Methods

Ten soil samples were collected from different areas of Sinai; Jabal Al Qur, Wadi Baba and Wadi Sieh. These collected samples have different Descriptions as shown in Table 1. Samples were grounded and passed through a 1 mm sieve and dried to 95˚C for 24 hours in order not to lose the volatile polonium or cesium. Ten gm of the dried samples were analyzed by XRD model X’Pert PRO powder diffract meter equipped with Cu anode, for the chemical and mineral composition. The X-ray source used in this research has a wavelength of 1.540562 Å. The diffract grams were recorded in the 2θ range of 0.5˚ - 70˚ with step size of 0.02 Å and a step time of 0.60 s. For radiometric analysis, the dried fine grained samples were packed in polyethylene Marinelli beaker for gamma spectroscopy and then stored for four weeks to reach secular equilibrium between radium-226 and thorium-232 and their progenies. The samples were analyzed non-destructively, using gamma-ray spectrometry with Canberra high purity germanium (HPGe) coaxial detector with relative efficiency of 25% and FWHM 2.0 keV at 1332 keV of ⁶⁰Co. Genie 2000 basic spectroscopic software was installed in the computer for data acquisition and analysis. The system was calibrated for energy and absolute efficiency. The measurements were done for a time period of 82,800 sec. An empty polyethylene Marinelli beaker was placed in the detection system for the same time period of measurement, in order to collect the background count rates. Then, each sample was measured during the same accumulating time.

3. Calculations

3.1. Activity Concentrations

The concentration of ²²⁶Ra was determined from the average concentration of gamma-ray lines of energies 351.87 keV of ²¹⁴Pb and 609.31 keV of ²¹⁴Bi (since there is secular radioactivity equilibrium in²²⁶Ra series). Also, the concentration of ²³²Th, which it is in secular radioactivity equilibrium with its short half-life daughters, was determined from the average concentrations of ²²⁸Ac (with gamma-ray line 911.16 kev) and of ²⁰⁸Tl (with gamma-ray line 583.10 keV). The analysis of ⁴⁰K concentrations was based on its single peak in the spectrum at energy 1460.80 kev. The activity concentrations “As” of the natural radionuclides in the measured samples were computed using the following relation [12] :

(1)

where: Ca is the net gamma counting rate (counts per second), ε the detector efficiency of the specific γ-ray, Pr the absolute transition probability of Gamma-decay and Ms the mass of the sample (kg).

3.2. Radiological Hazard Indices Radium Equivalent Activity Ra_eq (Bq/kg)

To assess the real activity level of ²²⁶Ra, ²³²Th and ⁴⁰K in soil, a common radiological index has been defined in terms of radium equivalent activity (Ra_eq) in Bq/kg can be used, provides a very useful guideline in regulating the safety standards in radiation protection for a human population. The index was calculated through the following formula is based on the assumption that 370 Bq/kg of ²²⁶Ra, 259 Bq/kg of ²³²Th and 4810 Bq/kg of ⁴⁰K produce the same gamma-ray dose rate [10] :

(2)

where: C_Ra, C_Th and C_K are the specific activities (Bq/kg dry weight) of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

3.3. Absorbed Dose Rate and Annual Effective Dose

The measured activity of ²²⁶Ra, ²³²Th and ⁴⁰K were converted into doses by applying the factors 0.4551, 0.5835 and 0.0429(nGyh⁻¹/Bqkg⁻¹) for radium, thorium and potassium, respectively. These factors were used to calculate the total absorbed gamma dose rate in air at 1 m above the ground level using the following equation [10] :

(3)

where: C_Ra, C_Th and C_K are the activity concentrations (Bq/kg) of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

Annual estimated average effective dose equivalent received by a member is calculated using a conversion factor of 0.7 Sv/Gy, which is used to convert the absorbed dose rate to annual effective dose with an outdoor occupancy of 20% [10] :

(4)

3.4. External Hazard Index and Internal Hazard Index

In the literature a number of criterion formulae have been derived over the years to assess the radiation dose rate due to exposure to gamma radiation from the natural radionuclides contained in soil. To limit the annual external gamma-ray dose to 1.5 Gy for the samples under investigation [10] , the external hazard index (H_ex) is given by the equation:

(5)

where: C_Ra, C_Th and C_K are the activity concentrations in Bq/kg of ²²⁶Ra,²³²Th and ⁴⁰K Respectively. The value of this index must be less than unity for the radiation hazard to be negligible.

The internal exposure to ²²²Rn and its radioactive progeny is controlled by the internal hazard index (H_in) [10] , which is given by the equation:

(6)

where: C denotes the respective specific activity in Bq/kg, for the safety, H_in should be less than unity.

3.5. Representative Level Index (I_γr)

Representative level index (I_γr) is used to estimate the level of γ-radiation hazard associated with the natural radionuclides in specific building materials, is defined as [4] [10] :

(7)

where: C_Ra, C_Th and C_k are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/k, respectively.

4. Results and Discussion

4.1. XRD Analysis

X-ray diffraction is a non-destructive analytical technique, which provides detailed information about the atomic structure of crystalline substances, chemical composition, and physical properties of materials. In the present study, the XRD results indicate that the main major, minor and trace constituents varied from one sample to another, as shown in Figure 2(a), Figure 2(b), and in Table 2. The major element defines the samples and has the highest concentrations. In sample soil1, Calcium Thorium Phosphate is the major in XRD spectrum Figure 2(a), Figure 2(b). While, trace elements occur in small concentrations (usually measured in ppm). They do not change the essence of what a material is and minor elements are in between major and trace elements. XRD spectrum Figure 2(a), Figure 2(b), shows that Thorium (V) sulfide and Thorium are minor and trace elements in sample 1 respectively.

Table 3 is a review of the chemical composition of each mineral and its description which reflects the con- dition of its formation [13] .

^*[14] .

4.2. Gamma Analysis

4.2.1. Radionuclide Activity Concentration

Table 4 represents the concentrations in Bq/kg of the different radionuclides in the samples. The radioactivity of the samples, as shown in Table 4, for Ra-226 ranged from 57.03 Bq/kg (sample 3) to 4220.41 Bq/kg (sample 2) with average value 1104.14 Bq/kg. Meanwhile; for Th-232 series the lowest value was13.55 Bq/kg (sample 5) and the highest value was 130.46 Bq/kg (sample 10) with an average value 74.21 Bq/kg, for K-40 activities ranged from 12.18 Bq/kg (sample 3) to 948.93 Bq/kg (sample 1) with average value 455.80 Bq/kg. In general, all the existed results were higher than the given values by UNSCEAR 2000 as: (35 Bq/Kg for ²²⁶Ra), (30 Bq/kg for ²³²Th) and (400 Bq/kg for ⁴⁰K, except samples 3, 5, and 6 of ²³²Th and samples 2, 3, 5, and 6 of ⁴⁰K. The Ra- 226 activity concentrations of the samples are higher than those of Th-232 and K-40. The high concentrations results of ²³⁸U in these areas of Sinai are due to the presence of phosphate and granite rocks with highly enriched with this radioactive nuclide and the weathering effects [15] .

4.2.2. Hazard Indices

As shown in Table 5, the average values of radium equivalent (Ra_eq), Hazard indices (H_ex and H_in), Gamma index (I_γr), the total dose rate (D_R), And Annual effective Dose (D_eff ) were 1245.37 Bq/Kg, 3.37, 6.35, 8.41, 537.30 (nGy/h), and 0.6589 (mSv/y) respectively. These average values were very high compared to the world standard values as given by UNSCEAR 2000. Based on these results of Hazard, one can deduce that the use of soil samples for construction of the dwellings is considered to be not safe for human habitation.

5. Conclusions

The results of the present work indicate that:

・ XRD measurements show that the major, minor, and some of trace elements concentrations for the samples are uranium compounds and thorium compounds.

・ The study area shows very high values of ²³⁸U and ²³²Th concentrations except sample 3. Also, ⁴⁰K has high concentrations except sample 3 and 5.

・ These average values for the soil samples from these regions (Jabal Al Qur, Wadi Baba, and Wadi Sieh) are considered to be very high levels of radioactivity compared to the world standard.

・ The average of absorbed dose rate, annual effective dose, radium equivalent, and the radioactivity hazard in-

dices values are much higher than the corresponding world average.

・ Precautions and recommendations should be taken into consideration for high levels of radioactivity concentrations in these samples especially when people may inhabit in these areas. I recommend that this study to be taken as a base line for any future studies in this area.

References