Spatial Distributions and Risk Assessment of the Natural Radionuclides in the Granitic Rocks from the Eastern Desert, Egypt

: This paper investigates the distribution of four natural radioisotopes 238 U, 226 Ra, 232 Th and 40 K in one hundred twenty-ﬁve granitic samples covering sixteen mountainous areas situated at the northern, central and southern parts of the Eastern Desert of Egypt (EDE). The concentrations of the examined radioisotopes in the collected samples were recognized utilizing a HPGe detector based gamma spectrometry. The average concentrations of these radioisotopes were higher when compared with the worldwide reference values. The radiation risk indicators including the radium equivalent activity index (Ra eq ), external and internal hazard indicators (H ex and H in ), external and internal level indicators (I α and I γ ), absorbed dose rate (ADR), annual e ﬀ ective dose rate (AEDR), annual gonadal dosage equivalent (AGDE), and excess lifetime cancer risk (ELCR), associated with these radioisotopes have been calculated and compared with their recommended global values and safety limits. These indicators showed that the granites from most studied areas exceeded the universal standards pointing to the di ﬃ culty of using them as building materials. This study together with future investigations will serve to develop an essential database for future environmental monitoring surveys.


Introduction
All living organisms are exposed every day to natural radiation within the variety of particles and rays from cosmic sources and background radiation from Earth materials. Most of the natural radioactivity in rocks is caused by 238 U, 235 U, 232 Th, and to a lesser extent 40 K. Such radiation is a function of the geological conditions [1], such as rock type. The former has been estimated in numerous parts of the world using variety of techniques assessing the absorbed dosage rates.
Granitic rocks and their derived industrial products usually contain a particular amount of terrestrial radioisotopes at different concentrations based on their origin. Since granites are utilized in dwellings as decorative and building materials, it is important to measure the concentration levels of natural occurring radionuclides (e.g., 238 U, 232 Th and their progenies in addition to 40 K) in these rocks for evaluating the radiological risks to the population. Furthermore, the concentration level distribution measurements of radionuclides will be useful in establishing standards for management and usage of these rocks [2]. Some studies were performed to consider the natural radioactivity levels ( 238 U, 226 Ra, 232 Th, and 40 K) and the radiological risks parameters in the granitic rocks from the Eastern Desert of Egypt [3][4][5][6].
The natural radionuclide concentrations in the granitic rocks collected from Gebel G. Gattar (North Eastern Desert) were reported by El-Dine [7]. El-Taher [8] studied the granitic rocks used as

Geological Setting of the Northern Part of the Study Area (NPSA)
The NPSA is characterized by a granitic terrain, various gneissic rocks, a sequence of younger intracratonic rocks of the Dokhan-type volcanics with Hammamat-molasse sediments besides rare outcrops of ultramafics [14]. The NPSA involves five locations (G. El-Qattar, G. El-Risha, G. El-Urs, G. El-Dib and G. Um Mongul).

Geological Setting of the Northern Part of the Study Area (NPSA)
The NPSA is characterized by a granitic terrain, various gneissic rocks, a sequence of younger intracratonic rocks of the Dokhan-type volcanics with Hammamat-molasse sediments besides rare outcrops of ultramafics [14]. The NPSA involves five locations (G. El-Qattar, G. El-Risha, G. El-Urs, G. El-Dib and G. Um Mongul).
G. Khashm El-Risha (Risha) and El-Urs are situated at about 70 km west of Hurghada city. The Risha area is wrapped by granophyres comprising quartz and alkali feldspar in characteristic angular intergrowths. The granophyres outcrops exhibit moderate relief [12]. In contrast, the granites cover vast areas of El-Urs, forming moderate to high relief, presented by biotite and alkali-feldspar.
G. Um Mongul is composed of monzogranite, hornblende gabbro, and dacite. It is intruded by dykes of variable composition. The monzogranite is fresh containing xenoliths of the Dokhan Volcanics [25].

Geological Setting of the Central Part of the Study Area (CPSA)
The CPSA contains younger granites; amphibolite and older granite distributed along five various areas (G. El-Missikat, G. El-Gidamy, G. El-Aradiya, G. Rie El-Garra and G. Kab Amira). The detailed geological setting was considered by El-Gamal et al. [26].

Geological Setting of the Southern Part of the Study Area (SPSA)
The SPSA contains six locations (G. Mueilha, G. El-Sella, G. Sheikh Salem, G. Abu Dabbab, G. El Bakreya and G. El Shalul).
G. El Mueilha is medium to coarse grained granite. It is commonly pink, turns whitish towards the external parts and along fault planes [27][28][29].
G. Abu Dabbab is 0.4 km 2 , composed of albite granite intruding the ophiolitic mélange (exotic blocks of serpentinites, metavolcanics and metasediments) [36]. The area is cross-cut by faults and shear zones trending N-NE to S-SW, and dissected by quartz, amazonite veins and basic dykes. The granites under investigation have been collected from different surface outcrops, and across shear zones.
G. El Bakreya rises 553 m above sea level, flanked by low hills of older gabbroid rocks. The late orogenic plutonites at El Bakriya are represented by alkali granite, pegmatites, aplites, felsites, and quartz veins. The granite shows sharp intrusive contacts, frequently contains xenoliths and roof pendants of the country rocks. Mineralogically, the granites are characterized by the predominance of microcline-orthoclase over plagioclase and rare biotite. Accessory minerals are zircon and apatite.
G. El Shalul represents one of the most deformed plutons in the Eastern Desert forming a NW-SE trending antiform. The core is dominated by monzogranite, and granitic gneisses. The deformed granites show enclaves of monzogranite [37,38]. El Shalul granite consists of two large plutons (El Shalul and El Hassanawia plutons) contains quartz, perthitic K-feldspar, plagioclase, and biotite. The border and age relationship between the two plutons is vague [37,39].

Sample Preparation
A total of 125 granitic rock samples were sampled from the investigated areas (41 samples from NPSA, 44 samples from CPSA and 40 samples from SPSA). The samples were crushed by a jaw crusher to a downy powder, sifted through 200 µm mesh and dehydrated at 110 • C for at least 3h to expel moisture. Each sample was weighted and placed into a hermetically sealed cylindrical plastic canister of 82 mm height, 95.2 mm diameter, and 0.51mm thickness. Each canister was closed tightly with vinyl tape around its neck to prevent the escape of the radon gas. The samples were stored for at least 30 d to achieve secular equilibrium between 226 Ra, 222 Rn and the short-lived progeny.

Radioactivity Measurement
All samples were investigated in the Nuclear Physics Laboratory of the Physics Department, Faculty of Sciences, Assiut University (Assiut, Egypt), using a gamma-ray spectrometer with an HPGe GR4020 model and a multichannel analyzer containing 16,384 channels. The identifier had closed coaxial gamma-ray detectors (type p) made out of high purity germanium (HPGe) in a vertical arrangement cooled by liquid nitrogen. The detector had a relative efficiency of 40%, and an energy resolution of 2 keV (FWHM) for the 1.332 MeV gamma-ray transition of 60 Co. The germanium detector was placed inside a lead shield (Model 747E, Canberra) to minimize the environmental background with the accompanying details: 9.5 mm (3/8 in) external jacket, thick, coarse carbon steel guard of 10 cm (4 in), low thickness background, slow coating of 1 mm (0.040 inches) of tin and 1.6 mm (0.062 inches) of copper.
The system was calibrated for both energy and efficiency. The energy calibration for the measuring system was performed by point sources of ( 133 Ba, 60 Co, 137 Cs, 54 Mn, 22 Na, and 65 Zn) using LabSOCS (Laboratory Sourceless Calibration Software) mathematical calibration software based on Monte Carlo simulation. Primary calibration measurements were carried out at the factory where results were utilized to build the detector's characterization file. For the calibration of the efficiency, calibration files were created by Geometry Composer Tool in the software for each sample. For the latter, all variables linked to the measurements such as dimensions of the counting geometry, container properties, the density of each sample, physical and chemical compositions; in addition to the distance between the radioactive source and detector end-cap, attenuation and absorption were taken into consideration.
The efficiency data obtained by LabSOCS was validated and verified through the measurements performed in our laboratory using a set of calibrated point sources ( 133 Ba, 60 Co and 137 Cs) put at a distance that ranged from 0 to 15 cm from the detector end-cap. The computed results gave a great agreement between mathematical and empirical peak efficiencies with differences of less than 10%, (Figures 2 and 3).
measuring system was performed by point sources of ( 133 Ba, 60 Co, 137 Cs, 54 Mn, 22 Na, and 65 Zn) using LabSOCS (Laboratory Sourceless Calibration Software) mathematical calibration software based on Monte Carlo simulation. Primary calibration measurements were carried out at the factory where results were utilized to build the detector's characterization file. For the calibration of the efficiency, calibration files were created by Geometry Composer Tool in the software for each sample. For the latter, all variables linked to the measurements such as dimensions of the counting geometry, container properties, the density of each sample, physical and chemical compositions; in addition to the distance between the radioactive source and detector end-cap, attenuation and absorption were taken into consideration.
The efficiency data obtained by LabSOCS was validated and verified through the measurements performed in our laboratory using a set of calibrated point sources ( 133 Ba, 60 Co and 137 Cs) put at a distance that ranged from 0 to 15 cm from the detector end-cap. The computed results gave a great agreement between mathematical and empirical peak efficiencies with differences of less than 10%, (Figures 2 and 3).  The concentration of any radioisotope of interest in samples was determined in Bq kg −1 using the count spectrum obtained for each sample. This count spectrum was obtained using the PC program Canberra's Genie 2000 [40] utilized for the acquisition of the detector signals. The measuring time was between 8 to 24 h for samples and 48 h for background.
The 238 U activity of the samples was resolved via its daughter 234m Pa using the γ-ray line (photopeak) with an energy of 1001.03 keV. The radioactivity concentration of 226   The concentration of any radioisotope of interest in samples was determined in Bq kg −1 using the count spectrum obtained for each sample. This count spectrum was obtained using the PC program Canberra's Genie 2000 [40] utilized for the acquisition of the detector signals. The measuring time was between 8 to 24 h for samples and 48 h for background. The 238 U activity of the samples was resolved via its daughter 234m Pa using the γ-ray line (photopeak) with an energy of 1001.03 keV. The radioactivity concentration of 226  The activity concentrations, computed from the intensity of gamma rays resulting from the radioactive nuclides of the samples, were calculated to give an average activity per radioisotope.
The activity concentration (AC) is the activity in Bq kg −1 for the granitic samples and was obtained as follows: where N cps is the net count per second, equal to (N cps ) Sample minus (N cps ) background , I is the gamma decay transition probability in a radioisotope, ε is the absolute efficiency for each gamma-line and m s is the sample mass in kg. The uncertainty of activity, U AC , was computed in terms of component uncertainties in N cps , I, ε and m s using the following Equation: where U Ncps , U ε , U I and U ms are respective uncertainties for the net count per second, the detector efficiency, the gamma decay transition probability and the sample mass.

Radiological Hazard Indices and Dose Parameters
Many indicators were utilized to evaluate the gamma radiation hazards emitted from the natural radioisotopes of the granitic rocks. These include: the radium equivalent activity indicator (Ra eq ), proposed for evaluating the radiation risks emitted from the examined radionuclides and estimating their actual activity levels in the granite products; external and internal hazard indicators (H ex and H in ) (it is essential that H ex should not go beyond unity which is equivalent to 370 Bq kg −1 (the upper limit of Ra eq criterion)), while H in was used to quantify the internal exposure due to radon and its progeny); alpha and gamma indicators (Iα and I γ ) are used only as screening mechanisms to identify building materials of concern; absorbed dose and annual effective dosage rates (ADR and AEDR); annual gonadal dosage equivalent (AGDE); and excess lifetime cancer risk (ELCR). These parameters can be determined using the following Equations: ADR nGyh −1 = 0.462AC Ra + 0.604AC Th + 0.042AC K AGDE µSv y −1 = 3.09AC Ra + 4.18AC Th + 0.314AC K where, AC Ra , AC Th and AC K are the specific activity concentrations of 226 Ra, 232 Th and 40 K in Bq kg −1 , respectively. Finally, the excess lifetime cancer risk can be calculated using the Equation presented by Agbalagba et al. [1].
where (DL) is life expectancy, considered to be 70 years, and (RF) is the risk factor referred to as cancer risk per sievert (for stochastic effects, RF value is 0.05 for the public (ICRP [41])). All of these parameters were discussed in detail by El-Gamal et al. [26,27]. Table 1 summarizes ranges and mean values of the specific activity concentrations of 238 U, 226 Ra, 232 Th, and 40 K in the studied rocks. Figure 4 compares the average estimates for radionuclide concentrations in the studied areas with the corresponding worldwide average. It is clear from Table 1 and Figure 4 that the average activity concentrations of 238 U, 226 Ra, 232 Th and 40 K in all areas were higher than the overall typical values for these radioisotopes in standard soil, (35,35,30 and 400 Bq kg −1 for 238 U, 226 Ra, 232 Th and 40 K, respectively [42]). They were above the allowable values in building materials, (50, 50, 50 and 500 Bq kg −1 for 238 U, 226 Ra, 232 Th and 40 K, respectively [43]). Furthermore, it can be seen from Table 1 and Figure 4 that the highest average values for 238 U, 226 Ra and 232 Th were in granites of the CPSA, whereas 40 K recorded the highest average values in the NPSA.

Activity Concentration
Hierarchical cluster analysis illustrates the similarity between the locations from the three regions in relation to their radionuclide concentrations. The results of the hierarchical cluster analysis are illustrated as a dendrogram in Figure 5; 16 locations from the three regions can be divided into six, five, four, three, and two groups (clusters) depending on the mean concentrations of 238 U, 226 Ra, 232 Th and 40 K. In this analysis, we see that the locations having the same mean values of radionuclide concentrations are within a homogeneous group. As shown from the dendrogram (Figure 5), the number of clusters is eventually reduced from six to two statistically significant different clusters (Cluster I and Cluster II). Cluster I contains 14 locations (Rie El Garra, EL Bakreya, Um Mongul, Qattar, Mueilha, Kab Amira, Sheikh Salem, Risha, El Sella, Abu Dabbab, Shalul, Gidamy, Aradiya and ElDib areas) with high similarity. Cluster II contains the two remaining locations (El Urs and Missikat areas) with less similarity.  It is clear from Table 1 and Figure 4 that the average activity concentrations of 238 U, 226 Ra, 232 Th and 40 K in all areas were higher than the overall typical values for these radioisotopes in standard soil, (35,35,30 and 400 Bq kg −1 for 238 U, 226 Ra, 232 Th and 40 K, respectively [42]). They were above the allowable values in building materials, (50, 50, 50 and 500 Bq kg −1 for 238 U, 226 Ra, 232 Th and 40 K, respectively [43]). Furthermore, it can be seen from Table 1 and Figure 4 that the highest average values for 238 U, 226 Ra and 232 Th were in granites of the CPSA, whereas 40 K recorded the highest average values in the NPSA. Hierarchical cluster analysis illustrates the similarity between the locations from the three regions in relation to their radionuclide concentrations. The results of the hierarchical cluster analysis are illustrated as a dendrogram in Figure 5; 16 locations from the three regions can be divided into six, five, four, three, and two groups (clusters) depending on the mean concentrations of 238 U, 226 Ra, 232 Th and 40 K. In this analysis, we see that the locations having the same mean values of radionuclide concentrations are within a homogeneous group. As shown from the dendrogram (Figure 5), the number of clusters is eventually reduced from six to two statistically significant different clusters (Cluster I and Cluster II). Cluster I contains 14 locations (Rie El Garra, EL Bakreya, Um Mongul, Qattar, Mueilha, Kab Amira, Sheikh Salem, Risha, El Sella, Abu Dabbab, Shalul, Gidamy, Aradiya and ElDib areas) with high similarity. Cluster II contains the two remaining locations (El Urs and Missikat areas) with less similarity.

Radiological Hazard Indicators
The mean values of radiation hazards parameters are plotted on Figures 6 and 7. Table 2 lists the average values of Raeq, ADR, AEDR, Hex, Hin, AGDE, ELCR, Iα and Iγ for the granites of the NPSA, CPSA and SPSA. It is quite evident (Table 2 and Figure 6) that the highest average values for these parameters are in the granites of CPSA, whereas the lowest e values in the SPSA. This is a reflection for the highest average values of 238 U, 226 Ra and 232 Th concentrations for the granites from the two parts.

Radiological Hazard Indicators
The mean values of radiation hazards parameters are plotted on Figures 6 and 7. Table 2 lists the average values of Ra eq , ADR, AEDR, H ex , H in , AGDE, ELCR, I α and I γ for the granites of the NPSA, CPSA and SPSA. It is quite evident (Table 2 and Figure 6) that the highest average values for these parameters are in the granites of CPSA, whereas the lowest e values in the SPSA. This is a reflection for the highest average values of 238 U, 226 Ra and 232 Th concentrations for the granites from the two parts.
The  Figures 6 and 7). In contrast, the granites from the SPSA did not exceed the recommended referenced values for most of these parameters ( Table 2, Figures 6 and 7). These variations demonstrate the difficulty of utilizing the majority of these granitic rocks as decorative stone in bulk amount. Nonetheless, they point to the probability of utilizing these rocks as superficial building materials or building materials with limited use.

Radiological Hazard Indicators
The mean values of radiation hazards parameters are plotted on Figures 6 and 7. Table 2 lists the average values of Raeq, ADR, AEDR, Hex, Hin, AGDE, ELCR, Iα and Iγ for the granites of the NPSA, CPSA and SPSA. It is quite evident (Table 2 and Figure 6) that the highest average values for these parameters are in the granites of CPSA, whereas the lowest e values in the SPSA. This is a reflection for the highest average values of 238 U, 226 Ra and 232 Th concentrations for the granites from the two parts.

Conclusions
The natural radioactivity of the granitic rocks in a vast mountainous region from the northern, central and southern parts of the Eastern Desert of Egypt (EDE) has been measured. The measured activity concentrations of the examined radioisotopes ( 238 U, 226 Ra, 232 Th and 40 K) in the granitic samples are highly variable and depend upon their radioactive mineral content. The average activity concentrations for 238 U, 226 Ra, 232 Th, and 40 K in all studied localities are above the worldwide reference for standard soil and building materials except for G. El-Shalul and G. Abu Dabbab, which are slightly less than the worldwide average in 238 U and 232 Th content.
The radiological risk parameters (radium equivalent (Ra eq ), external hazard index (H ex ), internal hazard index (H in ), absorbed dose rate outdoors (ADR), annual effective dose rate outdoors (AEDR), annual gonadal dosage equivalent (AGDE), excess lifetime cancer risks (ELCR), alpha index (Iα) and gamma index (Iγ) all indicate that these granitic rocks can be used as solely superficial building materials or as ornamental materials with limited use. Most of the granites from the northern and central parts have radiological parameters exceeding the recommended reference values.