Assessment of Natural Radioactivity Levels and Radiation Dose Rate in Some Soil Samples from Historical Area, AL-RAKKAH, Saudi Arabia ()
1. Introduction
During the past few years, growing attention has been devoted to studying the effect of natural radioactivity. We are primarily exposed to ionizing radiations emitted from natural and man-made sources. Natural radiations originate from the interaction of cosmic ray particles with the atmospheric elements and naturally occurring radioactive elements found in the earth’s crust [1] . The exposure of the public to natural sources of radiation has been estimated by the [2] UNSCEAR, 1988 which concluded an effective average annual dose equivalent to 2.4 mSv・y−1 per person.
In soil, the natural radioactivity concentrations determine both natural and man-made sources which are important in radiological monitoring and radiation dose assessment for public. There have been many surveys conducted to determine the background levels of radionuclides in soils, which can in turn be related to the absorbed dose rates in air. All of these spectrometric measurements indicate that the three components of the external radiation field, namely from the gamma-emitting radionuclides in the 238U and 232Th series and 40K, make approximately equal contributions to the externally incident gamma radiation dose to individuals in typical situations both outdoors and indoors [3] UNSCEAR 2000. Studies of natural radioactivity are necessary not only because of their radiological impact, but also because they act as excellent biochemical and geochemical tracers in the environment. Also, studies of U-series radionuclides present in nature have been of particular interest due to their relatively high biological mobility [4] . There is a lot of literature on the levels of natural radiation from soil [5] - [12] . However, the number of studies made in the Arabian Gulf is comparatively limited [13] - [17] .
The aim of the present work was to determine the activity concentrations of 226Ra, 232Th and 40K in the soil samples from a historical area which was discovered in east of Saudi Arabia at 2009 G, Called Al-RAKKAH. In order to understand the occurrence and distribution of natural radionuclides of soil samples in area under investigation and evaluate potential health hazards, the representative level index, Iγr, the radium equivalent activity (Raeq), and the annual effective dose equivalent (AEDE) in air for all soil samples were estimated to assess the contribution of this radionuclide to public exposure.
2. Materials and Methods
2.1. Samples Collection and Preparation
In order to measure the natural radioactivity in soil, surface soil (about 0 - 10 cm) 33 samples were collected using hand auger from 7 regions ((a) North, (b) Center, (c) South, North-East (NE), North-West (NW), South-East (SE), South-West (SW) and East (E)).
AL-RAKKAH is an ancient village dating back to the period of early Islam, located on an area of 108,000 square meters in the middle of the road between the cities of Al Khobar and Dammam, eastern Saudi Arabia and it is one of the most important archaeological sites in the Eastern Province, and also located between two roads coastal and speedy, that link the two cities. It is also located near the fertilizer plant (SAFCO) previously and this was known as the site during a comprehensive survey of sites relics in 1976 and holds the No. (105/208). (Dr Ali Ghabban, Vice President for Antiquities and Museums of the General Authority for Tourism and Antiquities at a news conference Monday, 08/02/2010 in the city of Khobar) [18] . He noted the site discovery time on 12/08/1976 G during the survey work carried out by the agency of Antiquities and Museums, and took No. (105/208) as income property within the site Aramco several years ago. The site covers some sand dunes which are also covered with desert shrubs and devoid of rock formations; it is easy flat, with rising higher hills about 12.46 meters above sea level, while the low-lying areas rises at a rate of 8.5 meters.
Location is mainly residential belonging to the period of early Islam and the Umayyad period and perhaps the beginning of the Abbasid period, through reading pieces of pottery, porcelain, glass, steatite and metal pieces that can be returned for two centuries, the first and second Islamic.
The effects of the site:
can distinguish three hills head of archaeological excavation site; first is located on the north side has been given the name of the area (a), and the second in Center and carry the name of the area (b), and third in the South has been named area (c).
spread on the surface of the site, particularly in the area of the ancient hills of breaking pottery, porcelain, glass, can the percentage back to the early Islamic period, as you can see, extensions built with stone walls in all directions.
Exploration
to document the process of capturing and processing the surface and the process of archaeological excavation contour map was prepared for the entire site; other retinal map divides the target site to dig a network boxes, each rib 22:00 with a break all boxes with width of one meter.
archaeological survey was conducted to the surface of the site which picked up samples of surface in the light of the results of this survey were selected on the region by the most points to start the drilling process.
24 were drilled square in the region (a) and a similar number in the region (b), has been digging Saudi team from the Archaeological Office in Dammam and under the supervision and follow-up of the sector of Antiquities and Museums Authority.
the are close to each cluster of homes of the three that make up the village water system consisting of a well-circular shape folded stones medium-sized and irregular in its upper part and stones freely (Frosch) in its lower portion which has been brought in from the sea coast near, a well in Qatar two meters of inside an irregular understeer. Other buildings consist of an elliptical basin provider-channel rainwater to drain water flowing one to the north and the other to the south-east has been introduced in the system to build channels pottery jars to control the amount of water outflows. Table 1 shows the presents distribution of samples in regions under study.
To get moisture free samples, they were dried in an oven at 110˚C for 24 hours until constant dry weight [19] . The dried samples were crushed and allowed to pass through micro sieves to maintain the uniform grain size to obtain a fine-grained homogenous soil sample for the measurement [20] IAEA, 1989. About 1000 gm of the homogenized soil samples were transferred into 1000 ml Marinelli containers. They were carefully sealed and stored for at least 30 days before gamma ray analysis to allow 226Ra and its short-lived progenies to reach secular equilibrium [21] . Each sample was measured for 12 hours, by using gamma-ray spectrometry. The system consists of an 80% relatively efficiency high-purity germanium (HPGe) detector from Ortec and has a resolution of 2.0 at the 1.33 MeV line of 60Co. The system was calibrated using a mixture source in soil matrix covering energies range from 59.5 to 1836 keV in 100 ml cylindrical plastic beaker. In order to determine the background distribution due to naturally occurring radionuclides in the environment around the detector, an empty polystyrene container was counted in the same manner as the samples. Reducing background radioactivity was achieved by surrounding the detector with a cylindrical passive lead shield of about 10 cm thickness. After measurement and subtraction of the background, the activity concentration was calculated [14] [22] .
2.2. Samples Analysis
The analysis focuses on measuring 226Ra, 232Th, and 40K activities in the soil samples. Due to the difficulty of determination the activity of 226Ra and 232Th directly from their very weak energy’s lines, the concentrations for both of them are determined from their decay products assuming secular equilibrium between parents and daughters. The peak analysis was conducted for the most significant energy lines for each radioisotope. The determination of activity concentration of 226Ra was by using the most abundant gamma rays from the lead 214Pb at 295 keV (18.15%) yield and at 352 keV (35.10%) yield, and from 214Bi being at 1764 keV (15.1%) yield, where the other energy lines yields are below 5% which are too small to detect with enough confidences. The activities obtained from these three lines were averaged and the mean value was considered as the activity concentration of 226Ra. The specific activity of 232Th was evaluated from gamma-ray lines of 228Ac at 338.4, 911.1 and 968.9 keV. The specific activity of 40K was determined directly from its 1460.8 keV gamma-ray line. The activity concentrations of the natural radionuclides in the measured samples (AS) were computed using the following
![]()
Table 1. Presents distribution of samples in regions under study.
relation [23] - [26] .
AS(Bq・kg−1) = Ca/εPrMs (1)
where Ca is the net gamma counting rate (counts per second), ε the detector efficiency of the specific γ-ray, Pr the absolute transition probability of Gamma-decay and Ms the mass of the sample (kg).
3. Result and Discussion
3.1. Activity Concentrations of 226Ra, 232Th and 40K
By using gamma-ray spectrometer, activity concentrations of the natural radionuclides were investigated in the soil samples. The three most important primordial radionuclides investigated in the area of interest were 226Ra, 232Th and 40K [7] . The activity concentrations of 226Ra, 232Th and 40K radionuclides in soil samples are calculated and presented in Table 2. The activity of 226Ra in the soil ranged from 17.34 Bq/kg to 28.34 Bq/kg with a mean 23 ± 1.6 Bq/kg, 232Th ranged from 1.1 ± 1.8 Bq/kg to 81.0 ± 1.7 Bq/kg with a mean of 20 ± 1.4 and 40K ranged from 218 ± 11 Bq/kg to 255 ± 18 Bq/kg with a mean of 233 ± 12 Bq/kg. The 40K activity is higher than 232Th and 226Ra in all measured samples. Figure 1 shows, the comparison of the mean activity concentrations values of 232Th, 226Ra and 40K (Bq/kg) in different regions of soil samples. Also, the activity concentrations of 226Ra, 232Th and 40K in soil samples from the studied areas was compared with those from similar investigations in other countries and summary results were given in Table 3. The comparison shows that the values of soils under consideration are extremely low in accordance with others. It is found that the mean value of 232Th, 226Ra and 40K in the present study were lower than reported for soils of Yemen, China, Turkey and Sweden, but 226Ra and 232Th is found that it is higher than reported for Jeddah and Riyadh, Saudi Arabia. Where 40K is found that it is lower than reported for Jeddah, Saudi Arabia. The comparison of 40K activity concentration shows that the values of this radionuclide in the soil of United State, Yemen, Nigeria, Algeria and China are lower than the present study mean value. The variations in the concentrations of the radioactivity in the soil of the various locations of the world, depend upon the geological and geographical conditions of the area and the extent of fertilizer applied to the agriculture lands [3] [27] [28] .
3.2. Radiological Parameters
3.2.1. Radium Equivalent Activities (Raeq)
The Gamma-ray radiation hazards due to the specified radionuclides 226Ra, 232Th and 40K were assessed by different indices. The most widely used radiation hazard index Raeq is called the radium equivalent activity Raeq [36] [37] . Raeq is a weighted sum of activities of the above three radionuclides based on the estimation that 370
![]()
Table 2. Activity concentrations (226Ra, 232Th and 40K) in soil from a historical area Al Rakkah, Saudi Arabia.
![]()
Figure 1. Average activity of 226Ra, 232Th and 40K (Bq/kg) in soil from different regions of a historical area, Al Rakkah, Saudi Arabia.
![]()
Table 3. Comparison of natural radioactivity concentration (Bq・kg−1) in the soil Samples for present study with previous study reported from different countries of the world.
Bq/kg of 226Ra, 259 Bq/kg of 232Th and 4810 Bq/kg of 40K produce the same g-ray dose rates. Raeq is given by
Raeq = ARa + (ATh ´ 1.43) + (AK ´ 0.077) (2)
The formula is based on the assumption that 370 Bq・kg−1 of 226Ra, 259 Bq・kg−1 of 232Th and 481 Bq・kg−1 of 40K produce the same gamma-ray dose rate [38] A value of 370 Bq・kg−1 corresponds to 1 mSv・y−1. The radium equivalent concept allows a single index or number which is a widely used hazard index to describe the gamma output from different mixtures of uranium, thorium and potassium in the soil samples from different locations. The calculated values are varied from 35.8 to 159.9 Bq/kg with an average of 69.52 Bq/kg (Table 4). These values are lower than the permissible maximum value of 370 Bq・kg−1 [2] .
3.2.2. Hazard Index (H)
The external hazard index for samples under investigation is given by the following equation [36]
Hex = CRa/370 Bq・kg−1 + CTh/259 Bq・kg−1 + CK/4810 Bq・kg−1 (3)
In addition, the external irradiation radon and its short-lived products are also hazardous to the respiratory organs. The values of outdoor radiation hazard index (Hex) vary from 0.10 to 0.43 with a mean value of 0.2, which all values are less than the critical value of unity. Therefore, based on these results of radium equivalent activity and external hazard indices, one can conclude that there is no health hazard from the soil of Al-Rakkah region. External and internal hazard indices calculated for soils. The health hazard for the population in the environment results mostly from the alpha-radioactive nuclides (radon, thorn and their decay products) being present in the air. To estimate the population internal exposure associated with the natural radio nuclides in the soil, the internal hazard index (Hin) was calculated according to the following equation [39] :
Hin = CRa/185 + CTh/259 + CK/4810 < 1 (4)
The criterion of this model considers that the maximum value of Hin equal to unity. The values of indoor radiation hazard index (Hin) vary from 0.17 to 0.5 with a mean value of 0.2, which all values are less than the critical value of unity and are presented in Table 4.
3.2.3. Representative Level Index
Radiation hazards due to the specified radionuclides of 226Ra, 232Th and 40K were assessed by another index called representative level index, Iγr. The following equation was applied to calculate Iγr for soil samples under investigation.
Iγr = (1/150)CRa + (1/100)CTh+ (1/1500)CK (5)
![]()
Table 4. Average radiological hazards (Hex, Hin, Iγr and Raeq,) in soil from Al-Rakkah.
where CRa, CTh and CK are the specific activities of 226Ra, 232Th and 40K in Bq/kg, respectively [22] . Iγr varies from 0.31 to 1.14 with a mean value of 0.5 as shown in Table 3. The average hazard index (Hex, Hin) and representative level index, Iᵧr are plotted in Figure 2.
3.2.4. Absorbed Dose Rate
The absorbed gamma dose rates DR(nG・h−1) in air at 1 m above the ground surface for the uniform distribution of radionuclides were calculated based on guidelines provided by [40] UNSCEAR (2000).
DR(nG・h−1) = 0.427CRa + 0.623CTh +0.043CK (6)
where CRa, CTh and CK are the activity concentrations (Bq・kg−1) of 226Ra, 232Th and 40K respectively, in the samples. The absorbed dose rate expresses the received dose in the open air from the radiation emitted from radionuclides concentration in Environmental materials. Also, it is the first major step for evaluating the health risk and is expressed in gray (Gy).
The calculated total absorbed dose of samples are tabulated in Table 5. It is observed that the absorbed dose rate calculated from activity concentration of 226Ra, 232Th and 40K ranges between 20.06 to 71.28 with an average 32.30 Bq・kg−1.
3.2.5. The Annual Effective Dose Equivalent (AEDE)
The annual effective dose equivalent (AEDE) was calculated from the absorbed dose by applying the dose conversion factor of 0.7 Sv・Gy−1 with an outdoor occupancy factor of 0.2 and 0.8 for indoor [3] [39] UNSCEAR, 1993 & UNSCEAR, 2000.
(AEDE)outdoor = D (nG・h−1) × 8760 (h・y−1) × 0.7 × (103 mSv/nGy 109) × 0.2 (7)
Equation (7) simplifies into (AEDE)outdoor = D × 1.226 × 10−3 (mSv・y−1)
(AEDE)indoor = D (nG・h−1) × 8760 (h・y−1) × 0.7 × (103 mSv/nGy 109) × 0.8 (8)
Equation (8) simplifies into (AEDE)indoor = D × 4.905 × 10−3 (mSv・y−1)
To estimate annual effective doses, account must be taken of (a) the conversion coefficient from absorbed dose in air to effective dose and (b) the indoor occupancy factor. The average numerical values of those parameters vary with the age of the population and the climate at the location considered. A report by the UNSCEAR Committee 1993) [39] , used 0.7 Sv・Gy−1 for the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.8 for the indoor occupancy factor, i.e. the fraction of time spent indoors and outdoors is 0.8 and 0.2, respectively. The resulting worldwide average of the annual effective dose is 0.48 mSv, with the results for individual countries being generally within the 0.3 - 0.6 mSv range. For children and infants, the values are about 10% and 30% higher, in direct proportion to an increase in the value of the conversion coefficient from absorbed dose in air to effective dose [3] .
![]()
Figure 2. Average activity of Hex, Hin, (Bq/kg) and Iᵧr in soil from different regions of a historical area, Al Rakkah, Saudi Arabia.
![]()
Table 5. Air-absorbed dose rates and annual effective doses calculated for surface soil samples collected from Al-Rakkah, Saudi Arabia.
As shown in Table 5, the annual effective dose equivalent from outdoor terrestrial gamma radiation ranged from 0.02 mSv・y−1 to 0.09 mSv・y−1 with a mean value of 0.04 mSv・y−1. Also, for indoor exposure, the annual effective dose equivalent had a range from 0.10 mSv・y−1 to 0.35 mSv・y−1 with a mean value of 0.16 mSv・y−1. The corresponding world average value is 0.41 mSv・y−1 of which 0.07 mSv・y−1 is from outdoor and 0.34 mSv・y−1 from indoor exposure [2] . Therefore, the study area is still in the zones of normal radiation level, which leaves the soil radioactivity there less a threat to the environment as well as the human health.
4. Conclusion
It is important to determine background radiation levels in order to evaluate the health hazards. The method of gamma spectrometry had been used to measure the radioactivity concentration of 33 soil samples collected from a new historical area which was discovered in east of Saudi Arabia at 2009 G, Called Al-Rakkah. From this study, the mean activity concentrations for 226Ra, 232Th and 40K are 23, 20 and 233 Bq・kg−1 respectively. Overall, the study showed that the measured values are lower than those in the world wide soil. The mean value of total absorbed dose rate is 32.69 nGy/h, which is below the corresponding population-weight (world average) value of 65 nGy・h−1. Annual effective gamma doses are lower than the world’s average. The value of Raeq activity was found to be less than 370 Bq・kg−1; the external hazard indices were found to be less than the acceptable limit of unity. Therefore, the study area is still within the zone of normal radiation level, which leaves the soil radioactivity there less of a threat to the environment as well as the human health. However, this data may provide a general background level for the area studied and may also serve as a guideline for future measurement and assessment of possible radiological risks to human health in this region.