|Year : 2011 | Volume
| Issue : 4 | Page : 270-274
Measurement of natural activities of 238 U, 232 th and 40 k in tin ore
Nassif A Mansour
Zagazig University, Faculty of Science, Physics Department, Zagazig, Egypt
|Date of Web Publication||17-Jan-2013|
Nassif A Mansour
Zagazig University, Faculty of Science, Physics Department, Zagazig
Source of Support: None, Conflict of Interest: None
Tin ore is widely used in many industrial purposes. Such as tin ore contain natural radioactive nuclides at various concentrations. If this ore contain high concentrations of natural radioactive nuclides, workers handling them might be exposed to significant levels of radiation. Therefore it is important to determine the radioactive nuclides in this ore. The natural radionuclides ( 238 U, 232 Th and 40 K) and their daughter products contents of tin ore have been estimated in gamma-ray spectrometry and their levels using 70% Hyper-Pure` Germanium (HPGe) detector. The mean activities due to the three radionuclides ( 238 U, 232 Th and 40 K) were found to be 40 ± 3, 19 ± 2.2 and 97 ± 19 Bqkg−1 , respectively. The radium equivalent activity varied from 77.87 ± 11-12.03 ± .27 Bqkg−1 . The representative external hazard index values for the corresponding Tin Ore are also estimated. The results of this assessment obtained by the gamma-ray spectroscopic analysis, have indicated that the levels of natural radioactivity were lower than the international recommended limits.
Keywords: Annual effective dose, equivalent activity, gamma spectrometer, NAA, natural radionuclides, radiation hazard index
|How to cite this article:|
Mansour NA. Measurement of natural activities of 238 U, 232 th and 40 k in tin ore. Radiat Prot Environ 2011;34:270-4
| 1. Introduction|| |
An established fact that over 60 radionuclides can be found in the natural environment, and they can be classified into three general categories i.e., Primordial - formed before the creation of the Earth, Cosmogenic - formed as a result of cosmic ray interactions, and human produced - enhanced or formed due to human actions. Radionuclides are found naturally in air, water and soil. They are even found in us, being that we are products of our environment. Every day, we ingest and inhale radionuclides in our air, food and the water. Natural radioactivity is common in the rocks and soil that makes up our planet, in water and oceans, and in our building materials and homes.  There is nowhere on Earth that we cannot find natural radioactivity.  Some radioactive nuclides are detectable in soil. They belong to the natural radionuclides like the members of the uranium and thorium decay series. More specifically, natural environment radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions, and appear at different levels in the soils of each region in the world. , The specific levels of terrestrial environmental radiation are related to the geological composition of each lithological separated area, and to the content of the rock from which the soils originated in each area in the radioactive elements of thorium (Th), uranium (U) and potassium (K). It is well known, for instance, that igneous rocks of granitic composition are strongly enriched in Th and U, as compared to rocks of basaltic or ultramafic composition. ,, For that reason, higher radiation levels are associated with igneous rock and lower level with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of those radionuclides. , Uranium minerals are chemically weathered to soluble U(VI) complexes and carried by river water downstream to the oceans, while the primary mode for transport of thorium from the continents to the oceans follow the detrital phase. , The residence time for thorium in sea water is only approximately 300-350 years. Uranium remains soluble in the sea (as carbonate and other complexes) and has a residence time in sea water of some 500000 years.  Precipitation of uranium can occur easily by reduction to insoluble U(IV). Thus, environments in which carbonaceous and bituminous shales form are particularly favourable for U removal by reduction of U(VI) to U(IV). Lignites are also enriched in uranium, some as U(IV), as expected, and some as U(VI) because the latter form can easily be scavenged by 'coaly' material without reduction. This accounts, in part, for the high U content of such rock. In the case of phosphotic rocks, coprecipitation of U(IV) with Ca2 + is likely owing to their very similar ionic radii, but the exact mechanism for the reduction of U(VI) to U(IV) in these rocks is not known.  Th/U ratio in nature varies widely. In rocks from which U has been removed, high Th/U ratio results; conversely, in rocks precipitated under chemically reducing environments far from suspected rock source, U is enriched over thorium. Thus, above average Th/U ratios are observed in continental sediments, especially in laterites and other residual deposits. Low Th/U ratios are found in chemically precipitated marine sedimentary rock, such as evaporate sand, limestone, and extremely low Th/U ratios are found in carbonaceous rock.  Human beings have always been exposed to natural radiations from their surrounding. The exposure to ionizing radiations from natural sources occurs because of naturally occurring radioactive elements in the soil and rocks, cosmic rays entering the earth's atmosphere from outer space and the internal exposure from radioactive elements through food, water and air. Therefore, the assessment of gamma radiation dose from natural sources is of particular importance as natural radiation is the largest contributor to the external dose of the world population. , wherever the tin ore is used in many industrial purposes, such as paints, plastics industry, container, bronzealloys, electroplating and various alloys of tin. Consequently there are specific measures to protect the working in tin ore-related industry such as reducing exposure levels and time of exposure and the use of exhaust ventilation. So the aim of this work is to determine the concentration of natural radioactivity uranium, thorium and potassium in tin ore and to measure the surface radiation dose rate and the radium equivalent activity and radiation hazard index. Finally determination of the elements presented in tin ore samples by NAA using two identical isotopic radioactive Am-Be neutron sources.
| 2. Materials and Methods|| |
Tin ore were collected from Nuweiba area in the Eastern Desert of Egypt, Egyptian General Authority for Mineral wealth Resources, Ministry of Petroleum. The tin put into clean containers. Sample were stored for 30-day-period to achieve the secular equilibrium between radium and its products and then measure the samples 18000 sec. Indium foils were also included in the irradiation containers for monitoring the neutron flux variation along the vertical axis. The energy and intensity of various gamma-ray lines have been measured using a system consist of Canberra coaxial High-Purity Germanium detector (HPGe) which has a photo peak efficiency of 70% . The energy resolution of 2 keV full-width at half maximum (FWHM) for the 1332 keV gamma-ray line of 60 Co. A cylindrical lead shield of 5-cm thickness, which contains inner concentric cylinder of Cu with thickness of 10 mm, was used to shield the detector and to reduce the effect of background. The detector was cooled to liquid nitrogen temperatures and coupled to a PC-based 8K multichannel analyzer and an ADC with Genie 2000 for data acquisition and analysis. The calibration of the detector was carried out by using standard point sources 60 Co (1173.2 and 1332.5 keV), 133 Ba (356.1 keV), 137 Cs (661.9 keV). And 22 Na (1368.6 keV) besides 226 Ra (186.2 keV). Absolute efficiency calibration curves are calculated for activity determination of the sample by using standard 226 Ra, contained in the same cylindrical bottles as the samples. The samples were prepared with a uniform geometry. An empty bottle with the same geometry was measured for subtracting the background. The gamma-ray transitions of energies 1120.3 keV ( 214 Bi) and 1764 keV ( 214 Bi) were used to determine the concentration of the 238 U series. The gamma-ray transitions of energies 911.1 keV ( 228 Ac) and 2614 keV ( 208 Tl) were used to determine the concentration of the ( 232 Th) series. The 1460 keV gamma-ray transition of 40 K was used to determine the concentration of 40 K in the samples as shown in [Table 1] and their intensities. The spectra of the samples were perefectly analyzed using a special PC Genie 2000 software to calculate the concentrations of 238 U, 232 Th and 40 K and their decay products.
|Table 1: The Natural Radionuclides, their Gamma Lines used and their Intensities|
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The present facility plans to use two identical isotopic radioactive neutron sources of Am-Be type. Each source has a cylindrical shape with dimensions of about 22-cm long and 4.4 cm in diameter, and has an activity of about 5 curies (185 GBq).The sources are put in an iron cylindrical barrel which filled with melted pure paraffin where the sources are isolated from the inner vertical central irradiation tube with the same paraffin material as shown in [Figure 1]a,b.
|Figure 1: (a) Describes the neutron irradiation facility used through the investigation. (b) Blocked diagram of HPGe g-ray spectrometer system|
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| 3. Results and Discussion|| |
3.1 Natural specific activity measurement
The activity levels for radionuclides in the measured samples are computed using the following equation  :
A = The activity level of a certain radionuclide (Bq/kg)
C = The net count rate of the sample (counts/seconds)
ε = The detector efficiency for the specific gamma ray energy
I = The intensity of gamma-line in a radionuclide
m = The mass of dried sample (kg).
Activities due to the presence of 226 Ra, 232 Th and 40 K radionuclides have been determined in the samples. The minimum, maximum and mean activity values of 226 Ra, 232 Th and 40 K found in these samples are listed in [Table 1]. As may be seen in this table the measured values of activity in the samples due to 232 Th vary from 21.9 Bqkg−1 to 9.9 Bqkg−1 , 226 Ra activities vary from 42.77 Bqkg−1 to 21.43 Bqkg−1 and variation in 40 K activities ranges from 145.33 Bqkg−1 to 16.22 Bqkg−1 .
The activity concentration is shown in [Table 2] where all activity are lower than world average except value for 238 U activity concentration was determined by measuring the 295.2 keV (18.7%) and 351.9 keV (35.8.1%) gamma rays from 214 Pb and the 609.3 keV (45%) and 1120.3 keV (14.9%) gamma rays from 214 Bi. 232 Th activity was determined from the gamma rays of 238.6 keV (45%) from 212 Pb and 338.4 keV (12%), 911.1 keV (29%) and 968.6 keV (17.5%) from 228 Ac and 583.1 keV (30%) gamma-rays from 208 Tl. 40 K concentration was measured from its 1460 keV (10.67%) gamma-ray line.
|Table 2: The minimum, Maximum and Average Values of the Activities due to 226Ra, 232Th and 40K in Tin Ore Sample|
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The obtained spectrum of the background gamma radiation was subtracted from the measured gamma ray spectra of the samples. The characteristic gamma-ray emitters are marked above the corresponding peaks. A selected one of the obtained spectrum for sample is shown in [Figure 2] and [Figure 3].
To assess the radiological hazard of the tin ore sample, it is useful to calculate an index called the radium equivalent activity, Ra eq , defined according to the estimation that 1 Bq/kg of 226 Ra, 1.43 Bq/kg of 232 Th and 0.077 Bq/kg of 40 K produce the same g-ray dose.  This index Ra eq is given as:
|Figure 2: Portion of gamma ray spectrum irradiation 2days, decay 0 sec, 900 sec counting illustrates the analysis of cement sample by NAA Technique|
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|Figure 3: Portion of gamma ray spectrum irradiation 2 days, decay 900 sec, 900 sec counting illustrates the analysis of cement sample by NAA Technique|
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Where A Ra , A Th and A K are the activity concentration in Bqkg−1 of 226 Ra, 232 Th and 40 K, respectively. The maximum value of Ra eq in Tin ore samples must be less than 370 Bq kg-1 for safe use (UNSCEAR, 1993), i.e., to keep the external dose below 1.5 mSv y-1. The values of Ra eq are below this criterion limit. In tin ore samples, the Ra eq activity are within the recommended safety limit when used in industrey. The calculated values of the radium equivalent Ra eq for the studied tin ore samples are given in [Table 3].
|Table 3: The Minimum, Maximum and Average Values of the Activities due to Radium Equivalent Activity (Bqkg−1), External Annual Dose (mSv/y) and Gamma-Radiation Hazard (Hex)|
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Another radiation hazard index, the representative level index, Iγr, used to estimate the levels of gamma radiation hazard associated with the natural radionuclides in specific tin ore samples, is defined as. 
Where A Ra , A Th and A K are the activity concentrations in Bq/kg of 226 Ra, 232 Th and 40 K, respectively. The values of H ex for the studied samples are given in [Table 3]. It is clear that the studied tin ore samples do not exceed the upper limit for the representative level which is unity.
3.2 Radiation hazard index
This factor is used to estimate the level of gamma radiation hazard associated with natural radionuclides in specific tin ore samples. The external hazard index is obtained from Ra eq expression through the assumption that its maximum value allowed (equal to unity) corresponds to the upper limit of Ra eq (370 Bqkg−1 ) according to UNSCEAR, 1993. This index value must be less than unity in order to keep the radiation hazard insignificant; then, the external hazard index (H ex ) can be defined as, 
Where A Ra , A Th and A Kare the specific activities of 226 Ra, 232 Th and 40 K (in Bq.kg-1) were calculated for the investigated samples to indicate different levels of external gamma radiation due to different combinations of specific natural activities in specific tin ore samples.
3.3 External annual dose
The external annual effective dose (EAD) is calculated. The equation used to calculate the annual effective dose may be defined as, 
Where, 0.92, 1.1 and 0.08 are the specific dose rates of Ra, Th and K, respectively; with an estimated indoor occupancy factor of 0.8.
| 4. Conclusions|| |
To conclude, radium equivalent activities calculated from measured values of 226 Ra, 232 Th and 40 K activities in the tin ore samples are lower than the upper recommended limit of 370 Bqkg−1 . These data serve as a basis for the assessment of radiological hazard to the workers involved in tin ore mining, transportation and industrial applications in Egypt. The calculated external hazard indices are less than unity. Therefore, the tin ore of studied is acceptable for use. The results of this assessment obtained by the gamma ray spectroscopic analysis, have indicated that the levels of natural radioactivity were lower than the international recommended limits, Therefore, safety rules and precautions should be applied for those working in this field.
| 5. Acknowledgement|| |
I would like to thank Prof. Dr. Mohamed Ahmed Ali Prof. of Nuclear Experimental Physics, Experimental Physics Department, EAEA, and Prof. Dr. Mohamed Fayez Hassan, the Acting Head of Experimental Nuclear Physics Department, Egyptian Atomic Energy Authority of Egypt (EAEA), for his useful comments and assistance.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]