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] .

Generally, the leucogranites are widely distributed all over the Egyptian Precambrian Shield, constituting approximately 10% of its plutonic assemblages [5] - [7] .

Peraluminous leucogranites are divided into two groups; the muscovite-bearing granites and the cordierite- bearing 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 ²³⁸U, ²³⁵U and ²³²Th 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 ²³⁸U, ²³²Th and ⁴⁰K 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 ²²⁶Ra, ²³²Th series and ⁴⁰K, 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] .

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.

The studied area is located between latitudes 24˚41'00'' and 24˚41'35''N and longititudes 34˚36'47'' and 34˚37'09''E., the lithology is arranged tectono-stratigraphically beginning with the oldest as follows; layerd metagabbro, biotite schist, gneisses rocks, granodiorite and muscovite granite (Figure 1).

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 km²) 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 uranophane- uranothorite 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.

Mineralogically, the studied muscovite granite are characterized by several mineral assemblages especially; secondary U-minerals (uranophane and meta-autunite) which occur as microfractures infilling or joint surface coating, U-bearing minerals (uranothorite) [17] - [19] and accessory minerals (columbite, zircon, flourite, zin- nwaldite and hematite).

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 (²²⁶Ra, ²³²Th and ⁴⁰K) 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 ²²⁶Ra was evaluated from gamma-ray lines of ²¹⁴Bi at 609.3, 1120.3 kev and ²¹⁴Pb at 351 kev, while the activity of ²³²Th was evaluated from gamma-ray lines of ²²⁸Ac at 338.4, 911.1and 968.9 kev. The activity of ⁴⁰K was determined directly from its 1460.8 kev gamma-ray line [20] - [22] . In general, the radioelement concentrations in rocks are expressed in mass concentrations of K%, ²³⁸U, ²³²Th and ²²⁶Ra. 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 ²³⁸U or ²²⁶Ra and 4.06 Bq/kg for 1ppm of ²³²Th.

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 cm² 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, ρ is the track density per cm² per day detector, K the calibration factors (T∙cm⁻²∙d⁻¹/pCi/l).

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 ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K as well as chemical analysis of U and Th.

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, uranothorite, columbite, metamict zircon, fluorite, zinnwaldite and hematite.

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.

The contributions of the three nuclei in the total absorbed dose rates and internal and external hazardous, for ²²⁶Ra, it contributes by 72% in H_in, 57% in H_ex and 55% of D_R, for ²³²Th it contributes by 24% in H_in, 37% in H_ex and 36.9% of D_R, while for ⁴⁰K it contributes by 4.1% in H_in, 6.4% in H_ex and 8.1% of D_R.

For the studied area, the most important nuclide is ²²⁶Ra which comes as a daughter nuclide of ²³⁸U and insures by the great concentrations of radon gas measured by cup technique in the study area. These results can also be treated as there is addition of uranium from outer sources as mentioned in Table 1. The above mentioned results are contradicts with that obtained by EL-Taher and AL-Zaharni [31] , which states that the contributions of ⁴⁰K is 65.6%, ²²⁶Ra 11.26% and ²³²Th 23.14% in soils of AL-Quassim region, Saudi Arabia. From the other side, increasing of contribution of radium reflects the more additions of recent uranium are took place which are supported by geological, mineralogical and geochemical studies.

For the measurements done by Refaat, [49] in metamorphosed sandstone, at Wadi Sikait area, he found that the contributions of the three nuclei in the total absorbed dose rates and internal and external hazardous, for ²²⁶Ra, it contributed by 53.76% in H_in, 38.66% in H_ex and 41.98 of D_R, for ²³²Th it contributed by 27.63% in H_in, 35.74% in H_ex and 29.74% of D_R, while for ⁴⁰K it contributed by 17.75% in H_in, 25.63% in H_ex and 28.27% of D_R. These results included that the contributions of ⁴⁰K were nearly three times greater than that obtained by this study that might be due to K-feldspar which were dominant. The contributions of ²²⁶Ra are in general less that obtained in this study; this means that the radioactivity in Wadi Sikait area is due to recent uranium.

A greet take care must be taken in account when dealing with the studied area, the precautions of radiation protection procedures should be taken in consideration. The samples collected from these areas are not safe and cannot used as a construction material because it possesses a great radiological threat to population and great care when handling is required.

The authors would like to express their wishes to deep thank for Prof. Dr. Gehad M. Saleh for helping and supporting this work. Deep thanks for Prof. Dr. Hamed A. Mira, for his help and reviewing the manuscript.

S. F.Hassan,M. A. M.Mahmoud,M. A. E.Abd El-Rahman, (2016) Effect of Radioactive Minerals Potentiality and Primordial Nuclei Distribution on Radiation Exposure Levels within Muscovite Granite, Wadi Nugrus, Southeastern Desert, Egypt. Journal of Geoscience and Environment Protection,04,62-78. doi: 10.4236/gep.2016.43006