Effect of Radioactive Minerals Potentiality and Primordial Nuclei Distribution on Radiation Exposure Levels within Muscovite Granite , Wadi Nugrus , Southeastern Desert , Egypt

The studied area at Wadi Nugrus, Southeastern Desert, Egypt, is located between lat. 24 ̊41'00'' and 24 ̊41'35''N and long. 34 ̊36'47'' and 34 ̊37'09''E. The rock types are represented by layered metagabbros, biotite schists, gneisses, granodiorites, and Muscovite granites. The muscovite granite of Wadi Nugrus, is small exposure in size (~5.0 km2), emplaced along NW-SE trend, with about 0.5 4.5 Km in length and 100 250 m in width and intruding the biotite schists. The studied muscovite granite is composed mainly of plagioclases, potash feldspars, quartz, biotite and muscovite. The recorded minerals assemblage can be classified into secondary U-minerals (uranophane and meta-autunite), U-bearing minerals (uranothorite and columbite) and accessory minerals (zircon, flourite, allanite, zinnwaldite and hematite). The U/eU is more unity where, the measured chemical uranium is higher than the measured equivalent uranium in the most analyzed samples especially in trenches, which reflect disequilibrium state. The activity concentrations range from 251.72 to 1096.2 Bq∙kg−1 for 232Th, from 494 to 2593.5 Bq∙kg−1 for 226Ra, and from 1314.6 to 1846.7 Bq∙kg−1 for 40K. The obtained radiological data show that the average internal and external hazard indices are 9.11 and 5.78, respectively which are more than unity and highly exceeding the permissible limits (International Commission on Radiation Protection, ICRP). The internal and external hazards are mainly due to 226Ra nuclei while the absorbed dose rate (nGy/h) is related to 232Th nuclei. The contributions of the three nuclei in the total absorbed dose rates and internal and external hazardous, for 226Ra, it contributes by 72% in Hin, 57% in Hex and 55% of DR, for 232Th it contributes by 24% in Hin, 37% in Hex and 36.9% of DR, while for 40K it contributes by 4.1% in Hin, 6.4% in Hex and 8.1% of DR.


Introduction
Nugrus-Sikait area constitutes a part of the Arabian Nubian Shield bordering to the major shear zone known as Nugrus thrust fault [1], or Nugrus strike-slip fault [2] and Sha'it-Nugrus shear zone.This shear zone separates high-temperature metamorphic rocks of the Hafafit complex in the SW from mainly low-grade ophiolitic and arc volcanic assemblages known as Ghadir group [3].The Nugrus thrust runs along the upper part of Wadi Sikait in NW direction till the southern tip of Gable Ras Sha'it, then swings to a south westward direction along Wadi Sha'it west of Gable Migif.The Migif-Hafafit gneisses comprise the foot wall of the Nugrus thrust, while the Ghadir group comprises its hanging wall [4].
Peraluminous leucogranites are divided into two groups; the muscovite-bearing granites and the cordieritebearing granites.The majority of the two groups are formed by partial melting of the crustal rocks.The forming of peraluminous leucogranites does not depend only on the nature of the sources, but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced.The peraluminous leucogranites can probably be formed during the same geodynamic event and possibly from the source, but they are not different members of the same magmatic suite [8].
Radionuclides in our environment are of three general types: primordial, cosmogenic, and anthropogenic (manmade).Natural radionuclides are present in all rocks in varying amounts depending on their concentration levels in source rock materials.It is known that the radionuclides 238 U, 235 U and 232 Th may become incorporated in igneous materials when they are originally formed from the molten state [9].
Natural radionuclides in rocks and soil generate a significant component of the background radiation exposure to human beings, depend on the composition of the soils and rocks in which they are contained a significant contribution to total dose from natural sources which come from terrestrial radionuclides such as 238 U, 232 Th and 40 K present in soil [10]- [12].
Since these radionuclides are not uniformly distributed, the knowledge of their distribution in soil and rocks plays an important role in radiation protection and measurement [13].As radiation of natural origin is responsible for most of the total radiation exposure, knowledge of the dose received from natural sources is very important for not only of its effects on health but also of the incidence of other radiation from man-made sources [14].
There have been many surveys to determine the background levels of the radionuclides.All of these measurements indicate that gamma-emitting radionuclides in 226 Ra, 232 Th series and 40 K, make approximately equal contributions to the externally incident gamma radiation dose to individuals in typical situations both indoor and outdoor [15].
Measurement of natural radioactivity in rocks is crucial in implementing precautionary measures whenever the source is found to exceed the recommended limit; it determines the amount of change of the natural background activity with time as a result of any radioactive release.The levels due to the terrestrial background radiation are related to the types of rock from which the soils originate.Higher radiation levels are associated with igneous rocks such as granite and lower levels with sedimentary rocks.There are some exceptions however, since some shales and phosphate rocks have a relatively high content of radionuclides [16].

Aim of Work
This work aims to study the effect of radioactive minerals and distributions and natural radioactive nuclei distributions on the radiation exposure levels in muscovite granite at Wadi Nugrus area.
Wadi Nugrus was studied by many authors, e.g., [17]- [19], which they are concluded that the leucogranites are peraluminous in nature and contain some spots of radioactive anomalies.
The muscovite granite exposure at Wadi Nugrus is small in size (~4.0 km 2 ) and emplaced along NW-SE trend (Figure 2(a)), reached about 4 Km length and 100 -250 m width.It is dissected by the major sets of joints and faults trending N-S and NE-SW with steep angle of dip.The muscovite granite has white colour, but at some outer contacts, it is highly weathered, reddish color due to the hematitization or whitish due to kaolinization.The studied muscovite granite is intruding the biotite schists with the foliation of the host rocks at high angle (Figure 2(b)).It is posseses some xenoliths from the host rocks.NE-SW and N-S strike slip faults dislocate the studied granite with common displacement.
Petrographically, the studied muscovite granite revealed the presence of varying amounts of quartz, feldspar, muscovite and biotite, as well as accessory minerals assemblage including zircon, allanite, garnet and fluorite (Figure 2(c) and Figure 2(d)).The main hosts for U and Th are zircon, allanite, and members of the uranophaneuranothorite solid solution series that are associated with biotite and muscovite.The muscovite granite also shows evidence of post-crystallization hydrothermal alteration in thin section.The hydrothermal mineral assemblage includes hematite, chlorite, fluorite, clay minerals, quartz, carbonate, and sausserite.

Methodology
Twenty eight stations (included 14 trenches), were chosen and covered the studied muscovite granite at Wadi Nugrus (Figure 1), to spectrometrical and radioactivity levels investigations.A mass of 200 g of each sample was packed in special plastic cup then closed and tightly sealed using cellotape.The containers were labeled appropriately and then kept for 30 days.
The concentrations of radionuclides ( 226 Ra, 232 Th and 40 K) in each prepared samples were determined using NaI gamma ray spectrometer by counting each sample for 1000sec, the values of eU, eTh, eRa, in ppm, and K, in%.The activity of 226 Ra was evaluated from gamma-ray lines of 214 Bi at 609.3, 1120.3 kev and 214 Pb at 351 kev, while the activity of 232 Th was evaluated from gamma-ray lines of 228 Ac at 338.4, 911.1and 968.9 kev.The activity of 40 K was determined directly from its 1460.8kev gamma-ray line [20]- [22].In general, the radioelement concentrations in rocks are expressed in mass concentrations of K%, 238 U, 232 Th and 226 Ra.According to IAEA [23], the conversion of radioelement concentrations in rocks to specific activity are given as 313 Bq/kg for of 1% K, 12.35 Bq/kg for 1ppm of 238 U or 226 Ra and 4.06 Bq/kg for 1ppm of 232 Th.
The radon gas concentration were measured in the field by using cup techniques employing SSNTDs detectors, in which a plastic cup of about 12 cm height, 7 cm diameter at the open mouth and 5.4 cm diameter at the bottom, is fitted with a 1.5 × 1.5 cm 2 CR-39 type detector attached to its inside bottom.The open mouth of the cup is covered with filter, which permits radon gas only to enter the cup and prevents the entrance of radon daughters.The CR-39 plastic used in this work is 500 μm in thickness, manufactured by Pershore Molding Limited, England.The cups was then immersed in the ground for 30 day as an exposure time, then collected, etched under optimum conditions and counted using an optical microscope with a total magnification of 400 X [24].The track density (T•cm −2 •d −1 ) and the calibration factor (CF) of the used CR-39 detector were obtained as value, CF = 0.2153 ± 0.035 (T•cm −2 •d −1 /pCi/l) [25].
The calculated radon gas concentration (pCi/l) at each monitoring station is obtained as follows: ( ) where, C Rn the calculated radon in ( ) pCi l , ρ is the track density per cm 2 per day detector, K the calibration factors The studied samples were taken for chemical and gamma spectrometric analysis in the Nuclear Materials Authority (NMA) and Military Technical College laboratories to determine the equivalent 238 U, 226 Ra, 232 Th and 40 K as well as chemical analysis of U and Th.

The Distribution and Relation of U ch /eU
The U ch /eU is equal to the ratio of the chemically determined uranium (U ch )/radiometrically measured uranium (eU).If the U ch /eU is approximately unity or more, it indicates addition or removal of uranium, respectively [26].From Table 1, and the bar-diagram (Figure 3), the studied Muscovite granite of Wadi Nugrus shows that U ch /eU is more unity and the measured U ch is higher than the measured eU in the most analyzed samples specially taken from trenches, which reflects disequilibrium due to the addition of uranium minerals.

Radioelement Concentrations
The concentration of the measured radioelements eU, eRa eTh, in (ppm) and K% beside the specific activity of these radionuclide calculated using the previously mentioned conversion factor are listed in Table 2. From this table, the measurements of the collected samples were ranged from 40 -210., 21 -120., 62 -270 ppm and 4.2% -5.9%, for eRa, eU, eTh, (ppm) and K% respectively, with average values of 99.86 ± 46.20 (ppm), 51.07 ± 22.31 (ppm), 135.61 ± 52.63 (ppm) and 4.96 ± 0.54 (%) respectively.It is noted that the measured values are greater than the overall average world values for theses nuclei.A small range and small value for the standard deviation that measured for 40 K nuclei indicates that there is some uniform distribution for this nuclide inside the studied area.The Presence of such high radioactivity may be attributed to the presence of relatively increased amount of secondary uranium minerals and accessory minerals such as zircon, iron oxides, fluorite and other radioactive related minerals.These minerals play an important role in controlling the distribution of uranium and thorium.

Activity Concentrations
The correlation between the measured three nuclei were plotted in Figure 4, which shows that a very good correlations between 226 Ra and 232 Th with correlation coefficient of 0.822, whereas a poor correlations between 232 Th with 40 K and 226 Ra with 40 K, with correlation coefficients of 0.063 and 0.089 respectively.   2Th, (b) 232 Th vs. 40 K, and (c) 226 Ra vs. 40 K, in the studied muscovite granite, Wadi Nugrus.
Figure 5 shows the graphical representations for the average activity distributions of the radionuclides 238 U, 226 Ra, 232 Th and 40 K, in the studied muscovite granite.The graphical representations demonstrated that the variation of natural radioactivity levels at the studied area due to the variation of concentrations of radionuclides in the geological rocks.Also indicated that there is some uniformity in the distribution of 226 Ra, 2 38 U, and 232 Th nuclei along the studied muscovite granite, but the observed difference was shown for 40 K nuclei.

Radon Gas Concentration Using Cup Techniques Employing CR-39 SSNTDs Detectors
Table 2 shows the measured values for the radon gas concentrations calculated using Equations (1) as well as the track density (T•cm 2 /d) counted for each detector.For track density (T•cm 2 /d) it ranges from 17.30 ± 1.68 to 95.58 ± 7.43 with an average value of 50.50 ± 4.65, and standard deviation of 23.94 ± 1.87, the wide range indicates a large spread in the radon gas concentrations this also was assured by the large values for the standard deviation which is nearly 0.47 from the original results.For radon gas concentration (pci/l), the same behavior can be obtained for the radon concentration which is ranged from 80.34 ± 7.15 to 443.94 ± 31.14 pci/l with an average value of 234.58 ± 11.20 pci/l and standard deviation of 111.20 ± 8.70.
The relation between the measured Rn-gas concentration measured by using cup techniques and the measured concentrations for uranium and radium in (ppm) were shown in Figure 6(a) & Figure 6(b) respectively.From this figure, it is shown that there is a positive correlation namely, 0.618 and 0.797 for uranium and radium, where a great value of radium is expected because the radium is the main cause of radon concentration.The relationship between uranium and radium concentration values in (ppm) was estimated as in Figure 6(c).This relation shows that the obtained correlation factor of is 0.7585.Hence we can predict a state of some secular equilibrium in these studied samples.

Radiological Parameters
The main and common pathways by which some environmental impacts may be caused are direct external exposure to radiation, inhalation and ingestion of radioactive materials.This depends on several factors such as radiation type and energy, their total dose, size of irradiation section, the radio sensitivity of the individual and age [27].Several methods proposed to estimate and discuss the environmental impacts of radiation [14] [28] [29].

Absorbed Dose Rate
The absorbed gamma dose rate D R (nGy/hr), in the air at 1m above the ground level for the uniform distribution of radionuclides ( 226 Ra, 232 Th and 40 K) can be estimated by using the specific activity concentration in (Bq/kg) from the following equation which is provided by UNSCEAR [14] [30] as; D R (nGy/h) = 0.427A Ra + 0.623A Th + 0.043A K (2) where, A Ra , A Th and A K are the activity concentration, (Bq/kg) for 226 Ra, 232 Th and 40 K respectively.The values 0.427, 0.623 and 0.043 are the conversion factor (nGy/h) per Bq/kg for 238 U, 232 Th and 40 K nuclei respectively.For normal background areas, the average dose rate value is 59 nGy/h [14].
The absorbed dose rate (nGy/h) calculated using Equation ( 2), are ranged from 507.43 to 1846.88 nGy/h with an average value of 936.36 ± 383.96 nGy/h, (Table 3) which is about fifteen times higher than the world average value of 59 nGy•h −1 [14].The fractional contributions of the radionuclides ( 226 Ra, 232 Th and 40 K) to the total absorbed dose rate were listed in Table 3 for the 226 Ra it ranges from 0.40 to 0.62 with average value of 0.55 for 232 Th it ranges from 0.30 to 0.48 with average value of 0.37 while for 40 K it ranges from 0.031 to 0.15 with average value of 0.081 from these data we can conclude that the relative contributions due to 226 Ra, was 55% followed by contributions due to 232 Th and 40 K as 36.9% and 8.1% respectively this means that the potassium contributes by a very small values in the total absorbed dose rate.These data are contradicts with that obtained by EL-Taher and Al Zahrani [31], for the contributions of 226 Ra, 232 Th and 40 K in Saudi Arabia soils.

Annual Effective Dose Equivalent
To estimate annual effective dose rate, account must be taken of (a) the conversion coefficient from absorbed dose in air to effective dose and (b) the outdoor occupancy factor.The average numerical values of those parameters vary with the age of the population and the climate at the location considered.According to UNSCEAR [14] [30], the committee used 0.7 Sv•Gy/y for the conversion coefficient from absorbed dose rate in air to effective dose received by adults and 0.2 for the outdoors occupancy factor, 0.8 for indoor, the components of the annual effective dose rate (AEDE) are determined as follows (qoseem).where, mSv•y −1 is the effective dose rate, (nGy/h) is the dose rate and 0.7 Sv•Gy −1 ., is the conversion factor., the average numerical values are vary with age of the population and the climate at the location considered.The resulting world average of the annual effective dose is 0.48 mSv [14].
The obtained values for the outdoor annual effective dose rates (mSv•y −1 ), which calculated using Equation (3) are ranged from 1.609 to 6.731 mSv•y −1 with an average value 3.331 ± 1.201 mSv•y −1 , The corresponding world average value is 0.41 mSv•y −1 of which 0.07 mSv•y −1 is from outdoor and 0.34 mSv•y −1 from indoor exposure [32].There for the study area is out of normal values and has a great threat to the environment as well as human health.
The obtained values for the indoor annual effective dose equivalent (mSv•y −1 ), which calculated using Equation (4) are ranged from 0.402 to 1.682 mSv•y −1 with an average value 0.833 ± 0.300 mSv•y −1 (Table 3), where the (1 mSv•y −1 ) is the annual effective dose rate limit for the public exposure (ICRP, 2000).The resulting worldwide average of the annual dose is 0.48 mSv, with the results for individual countries being generally within the 0.3 -0.6 mSv, range [14].

Radium Equivalent Activity (R eq )
R eq is the weighted sum of activities of the three radionuclides 226 Ra, 232 Th and 40 K based on the estimation that 370 Bq•kg −1 of 226 Ra, 259 Bq•kg −1 of 232 Th and 4810 Bq•kg −1 of 40 K produce the same gamma-ray dose rate [33], its mathematically defined by UNSCEAR [14] as: Ra eq (Bq/kg) = A Ra + 1.43A Th + 0.077A K ≤ 370 (5) where, A U , A Th and A K are the specific activities of U, Th, and K respectively.By assuming that 10 Bq/kg of 238 Ra equal 7 Bq/kg of 232 Th and 130 Bq/kg of 40 K produced equal gamma dose rate [34] [35].The maximum value of Ra eq must be less than 370 Bq/kg, which equivalent to the annual dose rate of 1 mSv/y, and meeting the maximum permissible dose to human from their exposure to natural radiation from soil in one year.The obtained values of the R a that calculated using Equation ( 5) are ranged from 1048.95 to 4262.29 Bq•kg −1 with an average value 2140.1 ± 861.16 Bq•kg −1 (Table 3).The calculated values are very greater than the recommended value.

External Hazard Index (Hex)
Radiation exposure due to 226 Ra, 232 Th and 40 K may be external or internal.The external hazard index or outdoor radiation hazard index (H ex ) was used to measure the external hazard due to the emitted gamma radiation and it can be calculated using the following equation [36].
where, H ex is the external hazard index, A Ra , A Th and A K are activity concentrations of 226 Ra, 232 Th and 40 K, respectively in Bq•kg −1 .
The values of the hazard index must be less than the unity in order to keep the radiation (external) hazard (which is caused by gamma rays) to be insignificant unity corresponds to the upper limit of radiation (radium) equivalent activity (370 Bq/kg).

Internal Hazard Index (H in )
In addition to external hazard index, radon and its short-lived products (isotopes) are also hazardous to the respiratory organs.So, the internal to radon and its daughter products is quantified by the internal hazard index (H in ) which controls the internal exposure to 222 Rn and its radioactive progeny [37] [38] as: where H in is internal hazard index and A Ra , A Th and A K are activity concentrations of 226 Ra, 232 Th and 40 K, respectively, in Bq•kg −1 .
The value of this index should be less than unity in order for the radiation hazard to have negligible hazardous effects to the respiratory organs of the public [35].The value of the external and internal hazard index calculated using Equations (6 & 7) was represented in Table 3, which shows that, for the external hazard index (H ex ) the averages from all the studied samples were ranged from 2.83 to 11.52 with an average value of 5.78 ± 2.33, this average value is greater than the acceptable average value of unity [39], which means that the external hazard due to gamma emission is very serious, while, for the internal hazard index (H in ) the averages values from all studied samples were ranged from 4.17 to 18.52 with an average value of 9.11 ± 3.86, which is a very great values compared with the acceptable values, unity, this means that radon an its radioactive progeny plays a significant role in the expected internal hazard due to radiation in the studied area.

Representative Level Index (Iγ r )
The radioactivity level index is used to estimate the level of radiation risk, especially γ-rays, associated with natural radionuclides in specific materials.It is defined by NEA-OECD and Alam [40] where, A Ra , A Th and A K are the Activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg respectively.The safety value for this index is ≤ 1. Table 3 shows the obtained results for (I-γ r ) that calculated using Equation (8).The obtained values are ranged from 7.38 to 29.13 with an average value of 14 ± 5.84, which is a very far from the acceptable values indicating a very high radiation risk in the study area.
The fractional contributions of the radionuclides ( 226 Ra, 232 Th and 40 K) to the internal and external hazard indices factor were listed in Table 4.In case of internal hazard index (H in ): For the 226 Ra it ranges from 0.59 to 0.78 with average value of 0.72 for 232 Th it ranges from 0.19 to 0.35 with average value of 0.24 while for 40 K it ranges from 0.015 to 0.081 with average value of 0.041 from these data we can conclude that the relative contributions due to 226 Ra, was 72% followed by contributions due to 232 Th and 40 K as 24% and 4.1%.
In case of external hazard index (H ex ): For the 226 Ra, it ranges from 0.42 to 0.63 with average value of 0.57 for 232 Th it ranges from 0.30 to 0.49 with average value of 0.37 while for 40 K it ranges from 0.024 to 0.119 with average value of 0.064 from these data we can conclude that the relative contributions due to 226 Ra, was 57% followed by contributions due to 232 Th and 40 K as 37% and 6% respectively.

Discussion and Conclusion
The muscovite granite of Wadi Nugrus has an irregular shape being emplaced into the biotite schists which undergo high temperature-low pressure metamorphism [42] [43].In addition to quartz, perthitic K-feldspar, and albite, the muscovite granite consists of variable contents of muscovite, garnet, biotite, and accessory as allanite, zircon, and fluorite.
Melting of crustal material of variable compositions to produce granitic melts of orogenic and of anorogenic characters has been documented for a number of granitic complexes [44]- [48].The highly evolved nature of the studied muscovite granite, it's exclusively peraluminous character, with mineral phases containing A/CNK > 1 [18] [19].Mineralogically, the studied muscovite granite has important minerals as: uranophane, autunite, ura-

Figure 3 .
Figure 3. Bar-diagram showing the distribution of eU, eTh, U ch and Th ch in the studied muscovite granite of Wadi Nugrus.and 1552.70 ± 167.87 Bq•kg −1 respectively.The general observation indicates that the all studied samples, has the highest activity concentration of 40 K.All the average values were higher than the world wide average activity concentrations of 40 K, 226 Ra and 232 Th which are 370, 30 and 25 Bq•kg −1 , respectively [14].The 40 K activity varied widely due to heterogeneous rocks characteristics.The Presence of such high radioactivity may be attributed to the presence of relatively increased amount of secondary uranium minerals and accessory minerals such as zircon, iron oxides, fluorite and other radioactive related minerals.These minerals play an important role in controlling the distribution of uranium and thorium.The correlation between the measured three nuclei were plotted in Figure4, which shows that a very good correlations between226 Ra and232 Th with correlation coefficient of 0.822, whereas a poor correlations between232 Th with 40 K and 226 Ra with40 K, with correlation coefficients of 0.063 and 0.089 respectively.

Table 1 .
Results of U-spectrometric (eU), eTh and U ch chemical analysis, U/eU ratios in the studied muscovite granite of Wadi Nugrus. S.

Table 3 .
Calculated values for the radiological parameters at the studied area. [41]as;

Table 4 .
The fractional contributions of the natural radioactive nuclides in the total absorbed dose rate as well as internal and external radiation indices at the studied area.