Sixteen sandstone samples were collected from two regions of Saudi Arabia (Al Wajh on Red Sea coast and Al Ula north-East of Medina) (SS: 1 to 7), (North of Al Ula and Az Zabirah, North-West of Hail) (SS: 8 to 16) Table 1, Figure 1.

The samples were dried in oven of one hundred ˚C to remove the moisture completely, crushed in an agate mortar, sieved in 2 mm to be homogenized in size. Each weighted sample (550 gm) was transferred to cylindrical plastic-container (Marinelli Beaker) then labelled and taped up tightly. The samples were stored for two months before counting to reach secular equilibrium between ²³⁸U and ²³²Th with their daughter nuclides.

The samples were analysed non-destructively, using gamma-ray spectrometry with Canberra (Model number GC2520) high purity coaxial germanium (HPGe) detector with efficiency of 25% and energy resolution of 2 keV FWHM for the 1332 keV line of ⁶⁰Co, and (16 k) MCA card with software Gamma (Gennie 2000) was used for Gamma acquisition and data analysis in our nuclear lab Jeddah university. The detector is boarded inside a thick lead shield (100 mm) with a fixed bottom and movable cover to reduce gamma ray background. The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) to absorb X-ray generated in the lead. In order to determine the background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples. Each sample was counted for 28,800 s. The background was measured many times at the same conditions of the measurement. The system was calibrated for energy and efficiency (IAEA, 2018). The lowest detection limits (DL) of HPGe detector system were 0.33, 0.27, and 2.31 for ²²⁶Ra, ²³²Th, and ⁴⁰K respectively for a counting time of 82,800 seconds. ²²⁶Ra activity concentrations were evaluated using gamma-ray lines of its related isotopes, ²¹⁴Pb (352 keV) and ²¹⁴Bi (609.31, 1120.27, 1764.49 keV). For ²³²Th, gamma ray lines of ²¹²Pb (238.6), Tl (583) KeV and ²²⁸Ac (338.42, 911.16, 964.6, 968.97 KeV) were used to measure the activity concentrations. The activity concentrations of ⁴⁰K were determined by using 1460.8 keV gamma ray line.

(AAS) is a method often used in environmental studies (Haswell, 1991). The application of (AAS) to soil analysis has been discussed by (Ure, 1991). For this study, the samples were analysed by Atomic Absorption spectrometer model OPTIMA 4000 DV Series Perkin Elmer for Al, Ca, Fe, K, Mg and Bi % for the concentrations.

Determination of activity concentrations in Bq/kg dry weight was calculated from the following equation (Amrani & Tahtat, 2001).

A = C m β ε (1)

where: C is the net peak area of specific gamma ray energy (count per second), m is the ass of the samples in (kg), β is the transition probability of gamma-decay, ε is the detector absolute efficiency at the specific gamma-ray energy. The measured dry weight activity concentrations of the gamma emitting radionuclides of the ²²⁶Ra series, ²³²Th series and ⁴⁰K in 16 sandstone samples are reported in Table 2 and Figure 2.

In region 1 (SS: 1 - 7), the measured activity concentration averages of ²²⁶Ra, ²³²Th and ⁴⁰K Bq/kg were 10.97 ± 0.43, 27.68 ± 0.37 and 64.56 ± 0.74 Bq/kg respectively. These values are less than the recommended reference Levels (50, 50 and 500) (UNSCEAR, 2000). In region 2 (SS 8 - 16), the average of measured activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K Bq/kg were 2465.49 ± 0.01, 2042.02 ± 0.10 and 2259.65 ± 0.64 Bq/kg respectively. These values are higher than the recommended reference Levels (50, 50 and 500) (UNSCEAR, 2000), the variations in the concentrations of the radioactivity in region 1 and region 2 depend upon the geological and geochemical conditions of the areas. These results are given in Table 2 and shown in Figure 2.

The variations in the concentrations of the radioactivity in sandstone samples depend upon the geological and geographical conditions of the area (UNSCEAR, 2000).

Exposure to radiation has been defined in terms of the radium equivalent Ra_eq Bq/kg which is calculated from equation (Jose, Jorge, Cleomacio, Sueldo, & Romilton, 2005).

Ra eq = C Ra + ( C Th × 1 . 43 ) + ( C K × 0.0 77 ) (2)

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 The concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K were determined from the average concentrations of gamma ray lines of energies tabulated in Table 2.

⁴⁰K (Bq/kg) dry weight was calculated using the equation (Jose, Jorge, Cleomacio, Sueldo, & Romilton, 2005).

D ( nGy / h ) = 0. 462C Ra + 0. 6 0 4C Th + 0.0 417C K (3)

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 (Krieger, 1981).

D eff ( mSv / y ) = D ( nGy / h ) × 8766h × 0. 7 ( Sv / Gy ) × 0. 2 × 1 0 − 6 (4)

where: D (nG/h) is the total air absorbed dose rate in the outdoor. 8766 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 the equation (Krieger, 1981).

H ex = C Ra / 37 0 + C Th / 259 + C K / 481 0 (5)

Internal exposure to radon and its progeny can be quantified using the index H_in, which was estimated by the following expression (Krieger, 1981). Results are given in Table 3 and Figure 3.

H in = C Ra / 185 + C Th / 259 + C K / 481 0 (6)

In region 1, (Al Wajh, Al Ula). Radium equivalent (Ra_eq Bq/Kg) average value in sandstone samples) is 121.13 Bq/Kg. This value of Ra_eq is lower than the limit of

370 Bq/kg recommend by (UNSCEAR, 2000). The Absorbed dose D (nGy/h), External hazard index (H_ex) and Internal index (H_in) average values are 27.22, 0.07 and 0.10, these values are lower than the limit 65, ≤1, ≤1 recommend by (UNSCEAR, 2000). For the Annual effective dose D_eff (mSv/y) average value is 11.75 (mSv/y) this value is higher than the limit 1 (mSv/y) recommend by (UNSCEAR, 2000). As shown in Table 3. In region 2, (N. Al Ula, AzZbira-wasia). Radium equivalent (Ra_eq Bq/Kg), the absorbed dose D (nGy/h), the Annual effective dose D_eff (mSv/y) and External hazard index (H_ex) and Internal index (H_in) average values are 5775.19 Bq/Kg, 1787.78 D (nGy/h), 846.58 D_eff (mSv/y), 11.57 (H_ex) and 15.64 (H_in), these values for all samples are higher than the limit 370, 65, 1, ≤1 and ≤1 recommend by (UNSCEAR, 2000). As shown in Table 3. In region 2, (N. Al Ula, AzZbira-wasia). Radium equivalent (Ra_eq Bq/Kg), the absorbed dose D (nGy/h), the Annual effective dose D_eff (mSv/y) and External hazard index (H_ex) and Internal index (H_in) average values are 5775.19 Bq/Kg, 1787.78 D (nGy/h), 846.58 D_eff (mSv/y), 11.57(H_ex) and 15.64 (H_in), these values for all samples are higher than the limit 370, 65, 1, ≤1 and ≤1 recommend by (UNSCEAR, 2000). As shown in Table 3. In region 1, the results of the radiological hazard, the values of Radium equivalent (Ra_eq), absorbed dose (D), the annual effective dose (D_eff), external (H_ex) and internal hazard of sandstone samples show that there is no health hazard. It is less threat to the environment and to the human health. In region 2, these values of sandstone samples are higher than the limit 370, 65, 1, ≤1, ≤1 recommend by (UNSCEAR, 2000). These calculated dose rates in sandstone samples put the users, dwellers and people around the area on radiological hazard. This study has refers to background guideline on the natural radioactivity levels in region 2 which will be database to the population.

Sandstone is sedimentary rock that is mainly composed of quartz or feldspar (both silicates-SiO₂). Sandstones are resistant to weathering and are very much easy to work. This makes sandstone commonly used as building and tiling material (Mubiayi, 2013). Sex elements (Al, Ca, Fe, K, Mg, Bi) of (16) sandstone samples in two regions were measured by atomic absorption spectroscopy (AAS).

Sandstone may be any colour due to impurities within the minerals. Sandstone that contains more than 90% Silicon (Si) is called Quartz sandstone. When the sandstone contains more than 25% Silicon (Si), it is feldspar sandstone. If sandstone contains 5.0% Iron (Fe), it is Hematite and sandstone with 2.9% Aluminium (Al), it is Kaolinite Clay, when there is a significant with 11% others, it is Impurities (Mundra et al., 2020). Elemental analysis of Al, Ca, Fe, K, Mg %. detection limit lies in %. In region 1, the averages concentrations of Al, Ca, Fe, K, Mg, Bi % are 4.42, 0.41, 1.37, 0.04, 0.03 and ND % respectively. In region 2, the averages concentrations of Al, Ca, Fe, K, Mg % are 12.50%, 10.05%, 1.01%, 1.19% and 0.04% respectively. The colour of sandstone varies, depending on its (elemental composition). The results are listed in Table 4 and Table 5.

ND: Not Detected.

The obtained results show that, in region 1 (Al Wajh and Al Ula), the major element in sandstone samples (except SS2 and SS5) is Aluminium (Al) with content more than 2.9%, the sandstones are Kaolinite-clay. SS2 content is more than (2.9%) and (5.9%) the sandstone is Kaolinite-clay and Hematite. SS5 contents are minor and trace elements. Sandstone description of sample 5 is impurities. In region 2 (North Al Ula and Az Zabira Wasis), the major element in all sandstone samples is Aluminium (Al) with content more than 2.9%, the sandstones are Kaolinite-clay, with elements are dominantly controlled by clay minerals and/or by minerals associated with clays during sedimentation (Boggs, 2006).