Ranyah province has attracted many companies for the exploitation of granites and ornamental stones, used as building materials that are sold locally and internationally. All the igneous rocks used as ornamental stone in the Kingdom are from the Proterozoic Arabian Shield. Figure 1 shows the geological map of the area surrounding the Arabian shield along with Ranyah location [15] . A report was prepared by United States Geological Survey (USGS) about Precambrian geology of the Ranyah quadrangle [16] . They indicated that nearly all of the western half and the northeastern quarter of the quadrangle is underlied by intrusive granitic rock containing local residual blocks of older metabasalt and minor intrusive gabbro and felsic dikes. Perthite granite forms a prominent mountain range across the central part of the Ranyah quadrangle. Most of the granite is coarse grained and perthitic feldspar is ubiquitous. Some granites, especially near the edges of the mountain range and near the contact with the felsic volcanic rock are fine to medium grained. Approximately the northern half of the pluton is alkali granite [16] (and references included therein).

In this study, 26 raw granite samples were collected from different sectors of Ranyah near granite exploitation sites (quarries). The covered area was 35 km long and 15 km wide. A portable Scintillation Gamma Radiameter (SGR) was used to record the radiation

background level at different locations and to look for possible elevated radioactivity. Figure 2 shows the prominent mountain range across centrlal part of Ranyah along with several sampling locations. The four anomalous granites (AG1, AG2, AG3 and AG4) were collected from a different area called “Taghdoua”. The granitic rock samples, each about 1.5 kg in weight, were first crushed, homogenized then sieved in order to have the same matrix as the reference sample. All samples were tightly sealed for a minimum of four weeks before measurements in 0.5 l Marinelli beakers, to reach secular equilibrium between ²³²Th, ²²⁶Ra and their short-lived progenies.

Measurement of the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K was performed with high-resolution gamma ray spectrometry.A Canberra n-type hyper-pure Germanium (HPGe) detector (GR5021) of 50% relative efficiency was used for the analysis with a resolution of 2.1 keV and 1 keV (FWHM) for the 1.332 MeV gamma ray of ⁶⁰Co and 122 keV of ⁵⁷Co, respectively, equipped with model 747 Canberra lead shield system. The Data Acquisition system consists of Digital Spectrum Analyzer (DSA-2000), which is a fully integrated high performance multichannel analyzer. Analysis of spectra was performed by Genie 2000 software. The Standard radioactive source for efficiency calibration was prepared in the spectrometry laboratory at King Abdulaziz University using multi-gamma emitter ¹⁵²Eu. Such standard source was prepared in 0.5-liter Marinelli beakers having the same geometry as the samples. The 1460.8 keV line of Potassium ⁴⁰K was similarly considered in this respect. A full description of the procedure is

given in [12] . The background spectrum was acquired for 864,000 s (24 hours) while all the samples were measured for 36,000 s. The ²²⁶Ra activities were estimated from the gamma rays of ²²²Rn decay products ²¹⁴Pb (295.2, 351.9 keV) and ²¹⁴Bi (609.3, 1120.3 keV). The ²³²Th activities were estimated from the gamma rays of ²¹²Pb (238.6 keV), ²²⁸Ac (338.4, 911.2 keV) and ²⁰⁸Tl (583.2 keV). The ⁴⁰K activity was determined from its own gamma ray (1460.8 keV).

The calculated indoor absorbed gamma dose rate, D_ind and annual effective dose rate, E_ind for anomalous granites due to external gamma exposure when used as superficial building material in the previously specified room model are given in Table 2. The arithmetic mean and the standard deviation (σ) are given for normal (NG) and anomalous (AG) granites separately. The last column indicates the classification of each

granite according to Table 1. It is clear that samples AG1 and AG2 exceeded the reference level for indoor external exposure to gamma rays of 1 mSv・y⁻¹ with 1.13 and 1.45 mSv・y⁻¹ values, respectively. The mean value and the standard deviation for all anomalous granites (0.91 ± 0.46 mSv・y⁻¹) are much bigger than those for normal granites (0.15 ± 0.02 mSv・y⁻¹). Based on the activity concentration indices, all NG samples except one classified as “normal” were categorized as A1 and A2 having ACI < 1 (suitable for being used as bulk and surface materials without restriction). Normal granite NG16 was classified as A1/B1to indicate a potential radiological hazard in bulk utilization of this granite, and should be subject to control (having ACI-1 within the computed uncertainty). Three out of four anomalous granites were categorized as B1 having ACI > 1 (restricting their use as bulk materials and should be subject to control). However, they can be used superficially without restrictions having ACI < 6 (A2 category). Anomalous granite AG2 was categorized as B2 material forbidding completely its use as construction material having ACI > 6 (I = 6.35). In column 5 of Table 2, the radium equivalent activity (Ra_eq) defined by Beretka and Mathew [21] , has been included to show that all four anomalous granites far exceed the permissible value from building materials of 370 Bq・kg⁻¹ with an average value of 1168 Bq・kg⁻¹ and a range extending from 604 to 1859 Bq・kg⁻¹.

It is remarkable that most of the granite samples with high radioactivity examined by Chen and Lin [22] were also red coloured as samples AG1 and AG2. The red colour of their highly radioactive granites (Balmoral and African red) is due to the presence of abundant feldspars that are reddish in colour as reported by Pavlidou et al. [23] . However, all four “anomalous” granites were collected from a different region where the small rocky mountain of the anomaly area is not subject to exploitation.

Trevisi et al. [24] noted in their database, that among the 621 superficial materials used in 15 MS (Member States) of EU (European Union), only two had ACI (I_max) higher than 6 (Italian basalt I ≈ 6.10 and a granite, commercially named as Café Brown, imported to Greece I ≈ 7.03). In our present study, one out of twenty six (granite AG2) had exceeded this critical value for superficial building materials. Table 3 shows a comparison between the results from the EU database, few recent studies worldwide

^aAverage values not available. ^bHPGe measurements were considered.

and the measurements for our normal and anomalous raw granites, which were collected from Ranyah region.

Other “anomalies” with much higher level of activity concentration values were found in young granites in the Egyptian desert at “Um Taghir”. The average values of ²²⁶Ra, ²³²Th and ⁴⁰K activity concentration were 3732, 1683 and 4801 Bq・kg⁻¹, respectively [30] .

As mentioned early, the calculations for radon concentration were performed for two values for the air removal rate due to ventilation: = 0.1 h⁻¹ for poor ventilation and = 0.5 h⁻¹ for adequate ventilation. The results are shown in Figure 3 and Figure 4 for all samples including the mean value for normal granites (MNG). It is worth mentioning that the reference level of 300 Bq・m⁻³ for radon in dwellings represents approximately 10 mSv・y⁻¹ [31] . It is clear that all 22 granites classified as “normal”, are safe in both circumstances. For a poor ventilated condition, all four anomalous granites clearly exceed the reference level. However, when the ventilation is adequate, only samples AG1 and AG2 exceed this limit and should not be used as superficial building materials (same red colour in both figures). By comparison, Anjos et al. [14] found that all their investigated granites had a value below 100 Bq・m⁻³ in case of adequate ventilation. In case of poor ventilation, 9% of 71 analysed Brazilian commercial granites showed a value above 300 Bq・m⁻³ with a maximum value of (~400 Bq・m⁻³). In our case, the maximum value (~2800 Bq・m⁻³) is seven times higher in case of poor ventilation due to the relatively high ²²⁶Ra activity concentrations in anomalous granites.

The collected anomalous granites are characterized by their high ²²⁶Ra and ²³²Th activity concentrations. At the same time, they have relatively low ⁴⁰K activity concentration by comparison with other collected granites. A recent work with plutonic bodies used as decorative building material in Greece investigated to correlate the mineralogical composition, the chemical composition (major oxides), groups of minerals and ratios of major oxides, age and grain size with ²²⁶Ra and ²³²Th activities [32] . However, obvious correlations were not found as the R² values were below 0.1 in all cases, even for SiO₂. Therefore, we restricted our analysis to the four anomalous granites along with granite NG6 (which has the highest ⁴⁰K activity concentration (1531 Bq・kg⁻¹) to look for possible correlation with their elemental composition. Energy dispersive X-ray Spectrometry (EDS) was conducted at Taif University JEOL scanning electron microscope (JEOL SEM 6390 LA). An example of SEM image and associated standard-less EDS spectrum is shown in Figure 5 for hazardous graniteAG2 that had the highest level of natural radioactivity in our present work. Very similar spectra were obtained for other samples. Silicon (Si) is the dominant element in all samples. Other common elements are potassium (K), aluminium (Al), Sodium (Na), Calcium (Ca), iron (Fe) and Magnesium (Mg). In addition, Titanium Ti is found in granite NG6 in small amount whereas as traces in others. These elements belong to the following oxides found in general in granites as pointed out by Krmar et al. [33] : SiO₂, K₂O, Al₂O₃, Na₂O, CaO, Fe₂O₃ and MgO. The final quantitative analysis of all four anomalous granites along with granite NG6 are summarized in Table 4, where the mass percentages of different elements present in the samples are shown. The correlation between K content and ⁴⁰K activity concentration is obvious. The correlation coefficient R² reached a high value of 0.95 as shown in Figure 6.