Assessment of radiological parameters and metal contents in soil and stone samples from Harrat Al Madinah, Saudi Arabia

Graphical abstract


Method details
Natural background radiation are the main sources of outdoor terrestrial gamma dose as humans are continuously exposed to ionizing radiation from natural radionuclides like 226 Ra, 232 Th and 40 K from the soil.
[1] A person is exposed approximately eighty percent of the total radiation dose in a year [2,3]. Soil is one of the most common sources of natural radionuclides. The activity concentrations of radioelements and chemical elements depend on the geological setting and geochemical properties of each region caused by surrounding environment. Thus, the information of the contents of radionuclides is necessary to estimate the radiation risk on environment [4][5][6][7][8][9].
Al Madinah El Monawara is one of the most important cities in Saudi Arabia where people visit every year from all over the world. There is a lack of data about the contents of natural radioactivity on the studied area. This research is considered the first study in that region. However, a continuous monitoring and assessment of radionuclides fingerprints and contamination is advocate.
The current study focuses on radiometric and chemical analysis of soil and stone samples collected from Al Madinah city in Saudi Arabia using X-ray fluorescence (XRF) and NaI (Tl) scintillation detector.

Geological setting
The study area lies between longitudes 34 to 46 and latitudes 17 to 32 in the western region of Saudi Arabia Fig. 1. The most important characteristic of Harrat Al Madinah from the geological point of view is the existence of volcanic eruptions. The soil and stones found in the area are dark basaltic rocks formed by the eruption of lava from the ground to the surface [10].

Samples preparation
About 0.5-1.0 kg of twelve (12) soil and fifteen (15) stone samples were collected between 0 and 10 cm of land surface from different locations in Al Madinah, KSA. There are many steps to samples preparation before radiometric and chemical analysis as follows: (1) Soil and stone samples were dried at 105 C to remove moisture completely, and then split by quartering to ensure the distribution of the elemental contents. (2) Crushed and sieved through a 200 mesh to become homogenous.
(3) For radiometric analysis, each sample was weighed and placed in a 350 cm 3 beaker, and then sealed tightly for four (4) weeks to allow for secular equilibrium to ensure that radon gas is confined within the volume in the sample [11]. (4) For X-ray fluorescence (XRF), about 8 g from powder sample and 1.6 g of wax were taken and pressed under suitable pressure to prepare discs for elemental measurements [12][13][14].

Instrumentation and calibration
A gamma ray scintillation spectrometry NaI(Tl) detector model A320 and SN A3200829 was used to determine activity concentrations of radionuclides. The hermetically sealed assembly is coupled to a personal computer-multichannel analyzer (Canberra AccuSpec) model MCA2500R and serial 25,066. The detector was shielded to reduce background radiation using lead shield (100 mm thick) and copper shield (0.3 mm thick). Quantum Gold version 4.04.4 PGT (Princeton Gamma-Tech) was used to analyze gamma ray spectrum [4]. An empty beaker was used in the same condition of samples measuring to estimate the background radiation around the work environment. The accumulated spectrum of background was subtracted from specified photo-peak energy of each sample to get accurate measured activity.

Calculation of activity
The measured activity in (Bq/Kg) for soil environmental samples was calculated using the following equation:- where N E is (CPS) specified line energy for samples, N B is (CPS) specified line energy for background, e is the abundance of the gamma-peak in a radionuclide, h is the measured efficiency for specified gamma-peak energy, and m is mass of sample in (Kg) [11]. The uncertainty of activity u(A) was calculated using square limit equation as follow:-  calculated assuming that a maximum inaccuracy of 2% due to contribution of other nuclides and it should be low average value. The total uncertainty of systematic and statistical efficiency was 5% [1].

Radiometric analysis
In the current study, the measured activity concentrations of 226 Ra, 232 Th and 40 K in 27 samples from different area of Harrat Al Madinah, Saudi Arabia are presented in Table 1 and showed in Fig. 2 These results were compared with published global limits of 226 Ra, 232 Th and 40 K by [15,16] which these values 35, 30 and 400 Bq Kg À1 respectively (Fig. 2).
Radium equivalent in Bq Kg À1 was calculated according to references of [11,12]. As shown in Table 1. We found that the Ra eq varies between 7.53 and 174.82 Bq Kg À1 with average 100.67 Bq Kg À1 and the obtained results are lower than the global value 370 Bq Kg À1 which recommended by UNSCEAR [15].
The frequency distribution curve of radium equivalent is plotted as shown in Fig. 3. That is clear that a nearly symmetric distribution with skewness equal -0.68, kurtosis equal -0.011and mean equal 100.67.
The calculated external and internal hazard indices for each sample are presented in Table 1 using equation published by [17][18][19][20]. H ex ranged from 0.02 to 0.47 with average 0.27 and H in ranged from 0.03 to 0.65 with average 0.37. It is clear that all samples indicated values less than unity as shown in Table 1 Activity of Ra-226, Th-232 and K-40 (Bq/Kg), radium equivalent, the external and internal hazard index and level index.   Table 1 and Fig. 4 using equation published by [21]. I g ranged from 0.05 to 1.27 with average 0.73. These results are close or less than unity except one sample that have sample code (S3) represented by black arrow in Fig. 4. Table 2 represents the calculated radiation hazard parameters for investigated samples. According to the recent [22] the dose rates values lie within the worldwide range (18-93 nGy h À1 ) with average (55 nGy h À1 ). In the current work the dose rates lie between 3.53-81.12 with average 45.34 nGy h À1 which they are concordant with the worldwide ranges. Fig. 5 represents the frequency distribution curve of dose rate. As it is clear that a nearly symmetric distribution with skewness equal À0.457, kurtosis equal À0.404 and mean equal 45. 34     Excess lifetime risk was calculated and noted in Table 2 using the following equation:- where AEDE is annual effective dose equivalent, D L is duration of life (70 year) and R F is risk factor (Sv À1 ). Fatal cancer risk per Sievert for stochastic effects, ICRP 60 uses the magnitude of 0.05 for the public [23,24]. The calculated lifetime risk ranges from 0.30 to 6.97 with average 3.89. Table 3 lists the comparison of mean radioactivity concentrations of 226 Ra, 232 Th and 40 K in the present study and other locations in different countries in the world. In some countries in Asia, the mean activity concentrations range from 12.53 to 70 for 226 Ra, 10.5 to 64.9 for 232 Th and 138.1 to 436.1 40 K. In some countries in Europe, the mean activity concentrations range from 25.2 to 37 for 226 Ra, 28.9 to 40 for 232 Th and 384.4 to 667 40 K. In some countries in Africa, the mean activity concentrations range from 12.24 to 46.1 for 226 Ra, 8.46 to 65.73 for 232 Th and 136.3 to 412.5 40 K. It is clear that the results of different continents are in the Global permitted ranges which published with [8,15] except India.

Chemical analysis using XRF
Oxides of major elements of studied samples were carried out using EDXRF (ARL Quant'X manufactured by Thermo Fisher Scientific Seller, USA) and listed in Table 4. There is a strong 140-850 (400) [8]. correlation between rich uranium content and chemical composition especially iron and manganese oxides due to their high ability to absorb uranium [25,26]. The content of Fe 2 O 3 ranges from 5.5% to 13.6% with average 9% and the content of MgO range from 5% to 8.3% with average 6.2%. All element contents are concordant with the obtained results by [26].

Conclusions
Radiometric and chemical analysis was carried out in soil and stone samples collected from Harrats Al Madinah in western region of the kingdom of Saudi Arabia. The contribution of radionuclides in Al-Madina city represents 37% for 226 Ra, 36% for 232 Th and 27% for 40 K. The results of the current study are within the global allowable limits, so this area of samples is safe for human beings that they live in. When we compared the content of elements in this study with other countries in different continents, we found that the concentrations of elements agree with them. Some of these elements have strong ability to sorption uranium, thorium and potassium. The current study is considered as the first baseline reference data about the natural radionuclides and elemental contents in the area of the study. The researchers recommend that follow-up of the study area should be raised to record the changes and develop a pollution control strategy.