Radiation activity exists everywhere on the surface of Earth and its interior. Uranium-238, thorium-232 (and their progenies) and potassium-40 are most important sources of radiation. These nuclei are found in any type of rocks, especially in igneous and sedimentary rocks. U-238 decays by ejection an alpha particle to generate daughter radionuclide Th-234 which followed by other decays to produce other radionuclides such as Ra-226 and its progeny Rn-222. Similarly, Th-232 disintegrates to produce Rn-228 and followed by other radionuclides . These radionuclides constitute risks by the external exposure to gamma radiation emissions and internally by radon and its progenies. Radon is a human carcinogen, and it is considered the second leading cause of lung cancer . Knowledge of radionuclide distribution is important because it gives helpful information in the observation of natural environmental radioactivity and connected external exposure that resulting from gamma radiation primary based on the geological and geographical conditions and can be seen at various levels in the rocks of each area of the world . The aim of this study is to determine the radionuclides activity concentration of Ra-226, Th-232 and K-40 for igneous and sedimentary rock samples collected from Al-Atawilah (north of Al-Baha region), and to estimate the doses and hazard indices originate from the existence of the natural radionuclides in the surrounding area. Moreover, Ra-226, Th-232 and K-40 activity concentrations were used to simulate and predict the range of decay by using an appropriate program. The results are used to assess the future effect of these radionuclides and evaluate the future of radiation hazards.
2.1. Sampling and Samples Preparation
Eighteen igneous rock samples and twelve sedimentary rock samples were collected from Al-Atawilah (20.273351˚N, 41.358325˚E), north of Al-Baha region, southeast of Saudi Arabia, Figure 1.
Figure 1. Locations of the igneous and sedimentary rock samples of Al-Atawilah region.
The samples were collected from the study region at depth ≈ 5 cm, by cracking the rock with hammer after removing a thin layer of the mother rock. Then, the collected samples were dried, pulverized and then sieved through less than 1mm-mesh size . All crushed sieved rock samples were filled into Polyethylene Marinelli beakers. The samples were pulverized into fine-grained powder for uniform distribution of radon and its decay products and to avert any accumulation in the top . The weight of each sample was recorded and then hermetically sealed from outside using a thick tape for more than a month so as to establish the secular equilibrium between Ra-226 and Th-232 (and their decay products)  .
2.2. Measurements Equipment
Measurement of radionuclides activity concentrations in the rock samples were evaluated by using high-resolution gamma-ray spectrometry system includes hyper-purity closed-end coaxial germanium detector. The detector had resolution of 2 keV at 1332.5 keV of Co-60 and 25% relative efficiency and peak to Compton ratio of 50:1. The detector is placed in a cylinder-heavy lead shield to minimize the radiation background values. Activity concentration calculations were based on the establishment of secular equilibrium in the measured sample between 226Ra and 232Th and their progenies of smaller lifetime . 226Ra activity concentration was evaluated from the peak energies of 295.2 keV and 351.9 keV at 214Pb and 609.3 keV, 1120.28 and 1764.5 at 214Bi. The activity concentration of 232Th was assessed from γ-ray peaks of 338.3 keV, 911.1 keV and 968.96 keV at 228Ac, 238.6 keV at 212Pb, 727.3 keV at 212Bi and 583.1 keV and 860.5 at 208Tl. The activity concentration of 40K was determined directly from the γ-ray line of 1460.8 keV at 40Ar, and 137Cs activity concentration was evaluated from peak energy of 661.6 keV at 137Ba.
2.3. Equations for Calculations
The radionuclide activity concentration (Ac) in the investigated rocks samples were determined in Bq∙kg−1. The activity concentration calculations were carried out using the following formula  :
where: Nc is the net peak area per unit time (second), m is sample mass in kg, ε is the detector absolute efficiency at the photo-peak energy and β is the branching ratio of gamma radiation.
Radium equivalent (Raeq) is calculated by applying the following equation  :
where: CRa, CTh and CK are 226Ra, 232Th and 40K activity concentrations, respectively.
External hazard index (Hex) can be calculated from the following equation  :
To keep the radiation hazard insignificant, the calculated value of the external hazard index must be less than unity .
Representative level index (Iγ) is determined with following equation :
The absorbed dose rate (DR) in air at 1m overhead the ground level was evaluated from the activity concentrations of the relevant natural radionuclides according to the following equation :
2.4. Decay Simulation of Natural Radionuclides
The mean activity concentrations in the rock samples of Al-Atawilah were used to simulate and predict the range of decay. The decay of the radionuclides U-238 (Ra-226), Th-232 and K-40 of the rocks samples were simulated using written MATLAB R2020ascript according to the exponential law of radioactive decay :
where: A is the change of the radioactive nuclei number with time, A0 is the initial activity, t is the time and λ is the constant of the decay.
In this study, Forward Different Interpolation Method was applied to reconstruct the activity concentrations of radionuclides. The term e−λt of the radionuclide decay equation “ ” (n to a 4th order) was used in Taylor polynomial form. The decay factor e−λt was approximated to a polynomial form by the following analysis for the fourth order:
this yields the polynomial as:
The coefficients a, b, c and e were calculated for the equation (8), and it was used with MATLAB R2020a to simulate the decay of the radionuclides 238U, 232Th and 40K using their half-lives .
3. Results and Discussion
3.1. Activity Concentrations
Analytical results for the samples have been applied to evaluate the activity concentration of 226Ra, 232Th and 40K, and the artificial radionuclide (137Cs) in Bq∙kg−1 together with their total uncertainties. The results are presented in Table 1. In the igneous rock samples, the activity concentrations of 226Ra ranged from 5.86 to 17.50 Bq∙kg−1 with average value of 11.00 Bq∙kg−1, 232Th from 21.42 to 5.12 Bq∙kg−1 with average value of 11.50 Bq∙kg−1. For 40K activity concentrations varied from 328.08 to 4854.00 Bq∙kg−1 with average value of 1172.71 Bq∙kg−1. The concentrations of the fallout nuclide 137Cs varied from 5.63 to 0.90 Bq∙kg−1 and the average value is 1.47 Bq∙kg−1. 226Ra activity concentrations in the sedimentary rock samples varied from 5.23 to 54.85 Bq∙kg−1 with average value of 12.04 Bq∙kg−1. The highest value of 232Th is 61.26 Bq∙kg−1 and lowest value is 5.15 Bq∙kg−1, the average value is 13.18 Bq∙kg−1. 40K activity concentrations ranged from 110 Bq∙kg−1 to 6070.75 Bq∙kg−1 with mean value of 1131.36 Bq∙kg−1. The maximum value of 137Cs is 4 and the lowest value is 0.84 Bq∙kg−1, and the average value is 1.60 Bq∙kg−1. 137Cs radionuclide is found in the most of the studied igneous and sedimentary rocks samples. Radium and Thorium activity concentrations of the studied igneous and sedimentary rocks varied depending on the types of the rocks. However, the average values of 226Ra and 232Th activity concentrations are less than the world average (50 Bq∙kg−1) reported in UNSCEAR 2008. Potassium activity concentration (CK) values are higher than recommended value in all igneous rock samples, except for two samples. The mean value of 40K the igneous rock samples is 2.35 times greater than the acceptable average value of 500 Bq∙kg−1 that recommended by UNSCEAR 2008. 40K is the most important radionuclides identified, it is the extremely abundant natural radionuclide in all igneous rock samples under investigation. The average contribution of 40K in the igneous rock samples is 98.1% while the average contributions of 226Ra and 232Th are 0.92% and 0.96%, respectively.
40K activity concentration are measured higher than the acceptable value in all the sedimentary rock samples in this study except for four samples. The calculated mean value of 40K is 2.2 times higher than the recommended value reported in UNSCEAR 2008 (500 Bq∙kg−1). 40K radionuclide is the most significant radionuclide had measured, it is most abundant natural radionuclide in all sedimentary rock samples under studied. The mean contribution of 40K in the sedimentary rock samples is 97.82% while the mean contributions of 226Ra and 232Th are 1.04% and 1.14%, respectively.
Table 1. Values of activity concentrations, radium equivalent, external hazard index, γ-ray representative level index and absorbed dose rate, and their minimum, maximum and mean values of the rock samples.
aSD: standard deviation; bND: not determined.
3.2. Radiation Hazards
Table 1 summarized the estimated values of radiation hazard indices (radium equivalent (Raeq), external hazard index (Hex), gamma ray representative level index (Iγ) and absorbed dose rate (DR)) for all igneous and sedimentary rock samples, with the average recommended values reported by UNSCEAR 2008 (370 Bq/kg for Raeq, and 1 for Hex and Iγ). In the igneous rock samples, (Raeq) values are ranged from 42.53 to 406.73 Bq∙kg−1, and the mean value is 163.83 Bq∙kg−1. (Hex) are ranged between 0.11 and 1.10 with average value of 0.44. The lowest value of (Iγ) is 0.33, while the highest value is 3.46. And the mean value is 1.36. The estimated total absorbed dose rate for the igneous rock samples are varied from 28.57 to 216.86 nGy∙h−1, and the mean value is 86 nGy∙h−1. The proportions of radionuclides contributions to the total absorbed dose rate in the igneous rock samples are 5.84% of 226Ra, 8.13% of 232Th and 86.04% of 40K.
In the sedimentary rock samples, the lowermost value of (Raeq) equal to 23.45 Bq∙kg−1 and the uppermost value is 610 Bq∙kg−1, and the average value is 118.01 Bq∙kg−1. (Hex) values are varied between 0.06 and 1.64 respectively, with mean value of 0.31. The lowest value of (Iγ) is 0.18, where the highest value equal to 5.02, and the mean value of (Iγ) in the sedimentary rock samples is 0.96. The total absorbed dose rate values are ranged between 11.37 and 315.5 nGy∙h−1, with a calculated mean value equal to 60.70 nGy∙h−1, and the ratios of radionuclides contributions to the total absorbed dose rate in the sedimentary rock samples are 9.16%, 13.18% and 77.71% of 226Ra 232Th and 40K, respectively.
3.3. Natural Radionuclides Decay Simulation of the Rock Samples
The decay of 226Ra (238U equivalent), 232Th and 40K of the rock samples were predicted over 103 years, 106 years, 109 years and finally over 1010 years using MATLAB R2020a script based on their present mean activity concentrations. Figures 2-5 display the exponential decay graphs of the natural radionuclides. The mean activity concentrations for all rocks samples under investigation are 11.35 Bq∙kg−1, 12.17 Bq∙kg−1 and 1516.17 Bq∙kg−1 for 226Ra, 232Th and 40K, respectively. The graphics have been zoomed in to clarify 238U (226Ra) and 232Th decay curves. The following observations from the simulation outcomes can be indicated as:
1) According to the Equation (6), the decay curves were expected to give an exponential graph, the lines approach zero if the background radiation is ignored.
2) The decay will be almost constant in the several next years in the study area, as the decay was not observed in Figure 2 and Figure 3, this due to the extremely long half-lives of the terrestrial radionuclides, thus the decay that the radionuclides will undergo during thousands or millions years will be insignificant.
3) From Figure 4 and Figure 5, a significant collapse was observed in the decay curve of 40K compared to 232Th and 238U (226Ra), this due to its shorter half-life compared to 238U and 232Th half-lives.
Figure 2. The decay simulation of the natural radionuclides of rock samples over ×103 years.
Figure 3. The decay simulation of the natural radionuclides of rock samples over ×106 years.
Figure 4. The decay simulation of the natural radionuclides of rock samples over ×109 years.
Figure 5. The decay simulation of the natural radionuclides of rock samples over ×1010 years.
4) This study evaluates the future effect of natural radionuclides on the population of this region and any increase in the concentration of radioactivity should be due to human inputs.
Gamma-ray spectroscopy of hyper-purity Germanium (HPGe) detector is a good experimental tool for studying levels of the radioactivity in various environmental samples such as rocks. The mean activity concentrations of 226Ra, 232Th and 40K in the igneous rock samples are 11, 11.5, 1172.71 Bq∙kg−1, respectively. For the sedimentary rock samples, the activity concentrations are found to be 12.04, 13.18, 1131.36 Bq∙kg−1 for 226Ra, 232Th and 40K, respectively. Fallout nuclide (137Cs) was found in the most of the rock samples under investigation, the low mean values of 137Cs are not of radiologically significant. The calculated mean values of radium equivalent (Raeq), external hazard index (Hex), representative level index (Iγ) and absorbed dose rate (DR) are within the suggested limit values, except for a slight increase of Iγ in igneous rock samples. This study also evaluated the radioactivity levels in the future and their effects on the population of the study region and any increase in the concentration of radioactivity should be due to human inputs. The results of the present study can help us to understand the distribution of natural radionuclides in the environment of Al-Baha region and provide a main map of radioactivity levels in Saudi Arabia.
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