The Kingdom of Saudi Arabia occupies an area of more than two million square kilometers and its geological features allowed the formation of mineral deposits over the last billion years. The Arabian Shield is one of the Kingdom’s geology. It is in the west running parallel to the Red Sea, covering one third of the Kingdom and is a major area for precious and basic minerals in addition to some industrial minerals [6] . Harrats cover an area over about 90,000 km², and extending over parts of the Proterozoic Arabian shield and adjacent Phanerozoic rocks of the Arabian Platform and Red Sea basin. Most of industrial raw materials which used in building have been found in these harrats where Basalt samples were collected. The Arabian Shield has 4000 probable sites where Gold could be mined, but Mahd Al Thahab and Al Najadi are the most prominent (Gold samples were collected from these two mines). Al Quryyah in the province of Tabuk is characterized by the presence of high concentrations of uranium, thorium, rare elements (Ta, Nb, Zr and Y) and raw material (Granite were collected from this area). Andesite and Marble samples were collected from Bahrahand Madrekah (White Mountain) [7] .

Sixteen rock samples (7 Basalt, 4 Granite, 3 Gold, 1 Andesite, and 1 Marble) from nine different sites were collected at a randomly chosen point within the site area. Sampling points at each site were selected at a distance of 1 - 2 km from each other in order to cover a large area and to observe variation in radioactivity. The sites under investigation are shown in Figure 1 and their Sites, names, and locations are given in Table 1.

These samples were grounded, sieved by 1 mm × 1 mm, then dried for 24 h to remove moisture to 80˚C not to lose the volatile ¹³⁷Cs or the natural polonium. Ten gm of the dried samples were analyzed by XRDmodelBurkerXR-DD8 Advance for the chemical and mineral composition; also ten gm of them were used for the analysis by atomic absorption model OPTIMA 4000 DV Series Perkin Elmer for the Al, Bi, Pb, U, Th, and K concentrations. For radiometric analysis, each dried sample was weighed and transferred to 640 cc polyethylene Marinelli beakers then sealed and stored for 2 - 4 month to prevent the escape of Radon gas and to allow the attainment of radioactive secular equilibrium between ²³⁸U, ²³²Th and their progenies. The samples were analyzed non-de- structively, 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 this time period in order to collect the background count rates. Then, each sample was measured during a same accumulating time

The concentration of ²²⁶Ra was determined from the average concentrations of gamma-ray lines of energies 295.09 and 351.87 keV of ²¹⁴Pb and 609.31, 1120.27, and 1764.49 keV of ²¹⁴Bi (since there is secular radioactivity equilibrium in ²²⁶Ra series). 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 lines at 338.32, 911.16, and 968.97 kev) and of ²¹²Bi at the line 727 keV, also 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, Determination of activity concentrations in Bq/kg dry weight was calculated using Equation (1) [8] .

where: C is the net peak area of specific gamma ray energy (count per second), m is the mass of the samples in (kg), is the transition probability of gamma-decay, is the detector absolute efficiency at the specific gamma-ray energy. Exposure to radiation has been defined in terms of the radium equivalent Ra_eq Bq/kg which is calculated from Equation (2) [9] .

where: C_Ra, C_Th and C_K are the concentrations in Bq/kg dry weight for radium, thorium and potassium respectively. The total air absorbed dose rate (nGy/h) in the outdoor air at 1 m above the ground due to the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K (Bq/kg) dry weight was calculated using Equation (3) [10] [11] .

where: C_Ra, C_Th, and C_K are the specific activities (concentrations) of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg dry weight respectively. The annual effective dose equivalent D_eff (mSv/y) in air was calculated using the values of the absorbed dose rate by applying the dose conversion factor of 0.7 Sv/Gy and the outdoor occupancy factor of 0.2 (people spend about 20% of their life outdoor) the Annual Effective Dose (in mSv/y) received by population can be calculated using equation:

where: D (nG/h) is the total air absorbed dose rate in the outdoor. 8,766 h is the number of hours in 1 year. 10⁻⁶ is conversion factor of nano and milli. To limit the annual external gamma-ray dose to 1.5 Gy for the samples under investigation, the external hazard index (H_ex) is given by Equation (5) [12] .

The internal exposure to ²²²Rn and its radioactive progeny is controlled by the internal hazard index (H_in), which is given by Equation (6) [13] .

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. It is a powerful tool in the identification of minerals in rocks and soils [14] . In the present study, the XRD results indicate that the main constituents (major) of these samples are Albite (NaAlSi₃O₈), Anorthite (CaAl₂Si₂O₈), Augite ((Ca, Na) (Mg, Fe, Al, Ti) (Si, Al)₂O₆)) in Basalt, Albite (NaAlSi₃O₈), Microcline (KAlSi₃O₈), Quartz (SiO₂) in Granite, Quartz (SiO₂) in Gold, Albite (NaAlSi₃O₈), Clinochlore (MgFe²⁺)₅Si₃Al₂O₁₀(OH)₈, Quartz (SiO₂) in Andesite, and Dolomite CaMg (CO₃)₂ in Marble. Small amount (Minor) and (Trace) minerals are presented as well. The mineral constituents of 16 samples analyzed by XRD spectrometer are shown in Table 2. Table 3 represents the mineral chemical composition and its description.

Table 4 gives concentrations of Al%, Bi, Pb, Th, U, and K in ppm by atomic absorption analysis. For Basalt

samples, the concentrations of aluminum range from 8.17% to 9.42%, the stable Lead (²⁰⁸Pb, ²⁰⁶Pb, and ²⁰⁷Pb) vary from LDL (<7.5) to 9.75 ppm, thorium vary from LDL (<1) to 3.75 ppm (from <4.07 to 15.24 Bq/kg), potassium range from 3240 to 10440 ppm (from 100.31 to 323.22). For Granite, the concentrations of Aluminum range from 6.23% to 6.95%, lead vary from 20.23 to 40.53 ppm, thorium vary from 9.01 to 23.42 ppm (from 36.63 to 95.20 Bq/kg), potassium range from 31,960 to 40,600 ppm (from 989.47 to 1256.66 Bq/kg). For Gold, the concentrations of aluminum range from 1.29% to 2.71%, lead range from <7.5 to 42.86 ppm, thorium vary from <1 to 1.49 ppm (from <4.07 to 6.06), potassium range from 1100 to 10,700 ppm (from 34.06 to 331.27 Bq/kg) For Andesite, values of the concentrations are of aluminum 9.58, lead 11.08, thorium <1 ppm (<4.07 Bq/kg) and 11,800 ppm (365.33 Bq/kg). For Marble, the concentrations are of aluminum 0.04%, Lead <7.5 ppm, thorium <1 ppm (<4.07 Bq/kg) and potassium 140 ppm (4.33 Bq/kg). Both bismuth (²⁰⁹Bi) and uranium are LDL (<10 and <5 ppm respectively) for all samples except sample Gold-12 with value 13.76 ppm and sample Gold-13 with value 14.31 ppm. Conversion factors are 1 ppm = 4.06 Bq/kg for ²³²Th and 32.3 ppm = 1 Bq/kg for ⁴⁰K [17] . Table 4 and Table 5 show that atomic absorption analysis results are in a good agreement with those obtained by gamma spectrometry measurements for thorium and for potassium. So, the atomic absorption analysis method is important for geochemical analysis to determine the concentrations of elements appearing in the examined minerals.

The measured concentrations in (Bq/kg) dry weight of natural radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K in sixteen studied samples are listed in Table 5 and shown in Figure 2. The concentrations in Bq/kg dry weight of ²²⁶Ra range from 03.04 to 10.91 with an average value of 07.90, ²³²Th concentrations in Bq/kg dry weight range from 03.19 to 13.31with an average value of 09.14, whereas ⁴⁰K concentrations in Bq/kg dry weight exist in the range from 95.59 to 361.52 with an average value of 234.81 in Basalt samples. For Granite samples, the concentrations in Bq/kg dry weight of ²²⁶Ra, ²³²Th and ⁴⁰K range from 28.42 to 112.77, from 33.81to 89.13, from 1260.13 to 1629.21 respectively and the average activity of ²²⁶Ra, ²³²Th and ⁴⁰K are 80.85, 68.50 and 1376.69 respectively. The concentrations in Bq/kg dry weight of ²²⁶Ra, ²³²Th and ⁴⁰K in gold samples range from 00.78 to 11.84, from 01.48 to 4.69, and from 13.76 to 445.09 respectively. The activity concentrations in Bq/kg dry weight of ²²⁶Ra, ²³²Th as well as ⁴⁰K for Andesite are 05.72, 03.73, 471.93 and for Marble are 01.56, 01.38, 10.15 respectively. The results showed that, the Granite samples have the highest radiation comparison with other samples, while the Marble sample has the lowest. Also the results showed that, Harrat As Sirat sample (Basalt-3) has the lowest activity concentrations of other Harrat and Al Najadi Gold mine sample (Gold-14) has the highest activity concentrations of MahdAlThahab mine samples (Gold 12 and 13). The results of the activity of ²²⁶Ra/²³²Th ratios

indicate that in Basalt samples, with two exceptions 2 and 3, ²³²Th is in excess to ²²⁶Ra. This is due to weather-

ing processes and water interaction with samples. For Granite samples, ²²⁶Ra is in excess to ²³²Th except sample 10. Gold has low activity of ²²⁶Ra/²³²Th ratios in two samples 12 and 13. Sample 14 (from AlNajadi) is enriched in ²²⁶Ra (²³⁸U). For Andesite and Marble, activities of ²²⁶Ra/²³²Th ratios are high for ²²⁶Ra. In general, all results existed (except granite samples 8, 9 and 11, for ²²⁶Ra and ²³²Th and all Granite samples for ⁴⁰K) within the given values in building materials by UNSCEAR 1993: (50 Bq/kg for ²²⁶Ra), (50 Bq/kg for ²³²Th) and (500 Bq/kg for ⁴⁰K) [18] .

It is important to evaluate the gamma radiation hazards by calculating the different radiation hazard indices. The radium equivalent concept allows describing the gamma radiation from radium, thorium and ⁴⁰K in samples. Table 6 presents Ra_eq (Bq/kg), D (nGy/h), D_eff (mSv/y), H_ex and H_in for studied samples. The radium equivalent activities Ra_eq (Bq/kg) of samples under investigation ranged from 14.96 to 57.42 with an average value of 36.58 for Basalt, 173.80 to 348.98 with an average value of 284.81 for Granite, 3.96 to 52.82 with an average value of 23.79 for Gold, 47.39 for Andesite and 4.32 for Marble. These calculated values of radium equivalent activities for all samples were below the recommended maximum value of 370 Bq/kg where the highest value is 348.98 Bq/kg from Alquryyah (Granite 9). For Basalt, the absorbed gamma dose rates D (nGy/h) ranged from 07.25 to 27.74 with an average value of 17.61 and for Granite, it ranged between 85.22 and 165.02 with an average vale 135.17. These calculated values were higher than the estimate of average global terrestrial radiation of 55 (nGy/h) UNSCEER. Gold samples had values of the absorbed gamma dose rates D (nGy/h) between 1.82 and 26.55 with an average value of 11.91. Andesite and Marble dose rates D (nGy/h) were 24.25 and 1.97. The annual effective dose rate D_eff (mSv/y) ranged from 0.009 to 0.034 for Basalt and from 0.11 to 0.20 for Granite and from 0.002 to 0.033 Gold and for Andesite and for Marble were 0.029756, 0.002418 respectively. The average values were 0.022, 0.17, and 0.015 for Basalt, Granite and Gold. The External hazard index H_ex and the internal hazard index H_in average values were 0.099 and 0.118 for Basalt and 0.77 and 0.99 for Granite and 0.065 and 0.077 for Gold and 0.128 and 0.143 for Andesite and 0.012 and 0.016 for Marble. The average values of H_ex and of all samples studied in this work were less than unity which means it is safe for human.

The calculated radium equivalent activities of the samples are shown in Figure 3. Maximum Ra_eq activity was calculated for sample (Granite-9) with 348.98 Bq/kg. The minimum Ra_eq activity value was calculated for sample (Gold-12) with 3.96 Bq/kg.

Comparison of the activity concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and Ra_eq (Bq/kg) in the present work and other countries are shown in Table 7. It was observed that activity concentrations of Basalt in Yemen showed the greatest concentrations, while Basalt in Iraq, Austria and Saudi Arabia (present work) contained only less of radionuclides. Granite in Iran, Greece and present work showed high concentrations of radionuclides, while Granite in Saudi Arabia (Ryadh) and Egypt contained less of radionuclides. For Marble, the maximum activity value 56.78 (Bq/kg) of ²²⁶Ra was in Egypt and the maximum activity values in (Bq/kg) 18.00 and 310.00 of ²³²Th and ⁴⁰K were found in Algeria. Marble in Saudi Arabia contained only traces of radionuclides.