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| Fig. 1: A close-up of a granite rock. (Source: Wikimedia Commons) |
Many naturally occurring rocks contain radioactive elements such as K-40, Th-232, U-238, and Ra-226. The radioactive elements in these rocks contribute to background radiation levels. One example of a radioactive rock is granite, shown in Fig. 1, used for cladding in city buildings and as architectural pieces in homes. [1] Granite has an enhanced elemental concentration of uranium and thorium compared to the low concentration of these elements in the Earth's mantle and crust. Geologists say this behavior is a result of partial melting and fractional crystallization of magma - this allows uranium and thorium to be concentrated in the liquid phase of granite and become incorporated into the granite product. [2] In the surface soil worldwide, radium concentration is relatively low in minerals of normal radioactive activity. However, in regions with a high count of volcanic rocks and uranium mining regions, the earth's crust is radioactive and constantly leaks radon gas into the atmosphere. [1]
Radon gas leaks are associated with the presence of radium and uranium in the ground. Inhaling radon and it's daughter products can have harmful effects. In uranium mining environments, radon doses may be high enough to cause lung cancer. The measurement of natural radioactivity in soil is important for environmental protection. Two of the consequences of natural radioactivity in the soil of mines are: the effect of γ-rays on the body and the effect of Rn-222 decay products on lung tissue. [3]
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| Fig. 2: Satellite view of the Eastern Desert in Egypt (Source: Wikimedia Commons) |
Southern Egypt deviates from normal radioactive background levels due to a concentration of natural radioactive minerals in soil in the mining area in southern Egypt. In a study conducted by Sroor et al., thirty soil samples from 6 locations in Southern Egypt were collected. [3] The soil samples, which weighed about 1kg, were ground, homogenized, and sieved to about 100 mesh by a crushing machine. Next, moisture was completely removed from the samples by drying them at 110°C for 48 hours. These dry samples were placed in polyethylene bottles and completely sealed for a month in order to allow radioactive equilibrium to be reached. This process ensures that radon gas is confined within the bottle and so that the radon decay daughters will remain in the sample. [3]
γ-ray spectroscopy was used to measure the activity concentration of Tl-208, Bi-212, Ac-228, Bi-214, Pb-214, Ra-226, and K-40 isotopes in the 30 soil samples. Radon concentration and exhalation were measured for the samples using solid state nuclear track detector Cr-39. [3] Each of the thirty samples were placed in a glass cylinder with a radius of 3.5 cm and a length of 10 cm. Dosimeters were prepared by inserting two Cr-39 detectors in the bottom of the chamber cover. [3] A dosimeter is useful for measuring radiation dose from ionizing radiation including but not limited to beta and gamma rays as well as non-ionizing short wave radiation, such as X-rays. The dosimeter absorbs radiation and indicates radiation dose. [4] The cylindrical containers containing the samples were sealed and stored for 30 days. [3]
The results indicated that from the 6 locations, locations 2 and 6 had the highest average Ac-228 radionuclide concentrations. Locations 3 and 6 had the highest average Bi-214 radionuclide concentration. And location 3 had the highest K-40 and U-238 radionuclide concentrations. Location 6 also had the highest Th-232 radionuclide concentration. Location 3 had the highest radioactivity level (measured in Bq kg-1), the highest effective radium content ( measured in Bq kg-1), and the highest exhalation rate of radon ( mesured in Bq m-2 d-1).
These results were then compared with published activity level results in the soil from different areas in Egypt. These comparisons indicated that 5 of the 6 locations had normal levels of natural background. However, location 3 had a high value of U-238 concentration. Although location 3 had a high concentration of K-40, it was still within natural background levels for this region. Lastly, location 3 even had a much smaller radon concentration than the average for Southern Egypt. [3]
In a study conducted by Arafa, fifty granite rock samples - 10 samples from five different locations in the central part of the Eastern Desert of Egypt, pictured in Fig. - were collected and analyzed for K-40, Th-232, and U-238 concentrations. In order to attain these measurements, various techniques were applied - solid state nuclear track detectors and alpha particles spectroscopy and gamma spectroscopy using high resolution hyper- pure Ge detectors. Using the hyper-pure GE detector allows for high precision measurements of the energy of gamma lines. Concentrations of U-238 and Th-232 were computed by counting gamma rays from Bi-214 and Ti-208. [1]
The five chosen regions and the types of rocks present in each region are as follows: [1]
Gabal El Majal - younger granite rocks
Gabal El Misikat - jasperoid veins and gneissose quartz diorite
Gabal El Aradiya - metamorphic and igneous rocks
Homret Waggat North and South - basement rocks such as gabbro, diorite and granitic rocks
The rock samples were crushed, dried, weighed, and packed in marinelli beakers for 4 weeks. The results of the experiment indicated that the average values of activity in some locations were relatively higher than normal. In Gabal El Majal, the average 238U concentration was 198.3 Bq kg-1, in Gabal El Misikat it was 1184.2 Bq kg-1, in Gabal El Aradiya it was 125.4 Bq kg-1, in Homret Waggat North is was 488.5 Bq kg-1 and in Homret Waggat South it was 787.42 Bq kg-1. El Misikat had the highest observed concentration of U-238. Another notable result was that average U-238 concentration in Homret Waggat South was nearly twice the U-238 concentration in Homret Waggat North. The reason for this gap is that the granite in Homret Waggat South is enriched with sphene and zircon. [1]
The normal value of Th-232 concentration is 50 Bq kg-1. Only Homret Waggat North and South had values above the normal value. These two regions also had high K-40 concentrations - the normal value of K-40 concentration is 500 Bq kg-1. The K-40 concentrations in El Majal, El Misikat, and El Aradiya were found to be around normal. In terms of radium equivalent activity, El Misikat, Homret Waggat North, and Homret Waggat South were higher than the recommended maximum radium equivalent activity. All five locations were found to have higher average absorbed dose rates than the international recommended value.[1]
The soil samples tested in Southern Egyptian Soil were found to have normal levels of natural backgrounds, except for one location, location 3, which had a high U-238 concentration. Although location 3 also had a high K-40 concentration, it was within the natural background level. All 6 locations had lower indoor radon concentration levels than the global average. Therefore, the tested soil samples from the 6 regions in southern Egypt were found to of typical level for the region. [3] Eastern Egypt has a different story. 2 locations, Gabal Homret Waggat South and North had higher than recommended Th-232 and K-40 levels. El Majal and El Misikat also had higher than recommended K-40 levels. All 5 tested regions had average dose rates higher than international recommended value.[1]
© Juan Leis-Pretto. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] W. Arafa, "Specific Activity and Hazards of Granite Samples Collected from the Eastern Desert of Egypt," J. Environ. Radioactiv. 75, 3315 (2004).
[2] A. M. El-Arabi, "Ra-226, Th-232 and K-40 Concentrations in Igneous Rocks from Eastern Desert, Egypt and its Radiological Implications," Radiat. Meas. 42, 94 (2007).
[3] A. Sroor, et al., "Natural Radioactivity and Radon Exhalation Rate of Soil in Southern Egypt," Appl. Radiat. Isotopes 55, 873 (2001).
[4] S. D. Miller, "Composite Material Dosimeters," U.S. Patent US5569927A, 29 Oct 96.