Measurements for Natural Radioactive Materials at the East & West banks of The River Nile

Soil samples were collected along the East and West banks of the River Nile region starting from Bait Al-mal in the West side and the second East bank of the River Nile Shambat. Gamma radiation measurements were performed using high resolution HPGe detector with low background PC multichannel spectrometer. The gamma measurements of the soil revealed the presence of the natural long-lived radioisotopes U, Th and K. The ranges of their activity concentrations were 3.32±1.1~6.3±2.1 Bqkg, 5.61±1.7~10.76±2.13 Bqkg and 180±7.77~291±11.6 Bqkg ‘respectively’. The other factors controlling the distribution of the detected natural radioisotopes have been discussed. Absorbed dose rates have been calculated for each location with range 15.27±0.82~21.07±1.53 nGyh. Also, the estimated activity utilization indexes have been presented. The ratios between the detected radioisotopes have been calculated.


Introduction
There are two kinds of radiation according to their origin: natural and artificial radiation. Natural radiation comes from cosmic rays from outer space and naturally occurring radioactive materials (NORMs) that exist in food, air and our natural habitat while exposure in medical practice mostly due to diagnostic X-rays, contributes the largest fraction of human's exposure to artificial radiation. About 90% of human radiation exposures arise from natural sources [3].
Significant naturally occurring radionuclides present in the soil are 238 U, 232 Th and 40 K [4].
These radionuclides are not uniformly distributed hence, the knowledge of their distribution in soil plays an important role in radiation protection.
Soil not only acts as a source of continuous radiation exposure to humans but also as a medium of migration for transfer of radionuclides to biological systems [5]. Hence, the basic indicator of radiological contamination in the environment is the soil [6]. The understanding of the decay chain where series of stepwise decay in terms of half-lives of nuclei of unstable radionuclides lead to stable states in form of other nuclides is very paramount. Alpha (α) and beta (β) particles are still emitting with the gamma but their travel distance in the air is limited while gamma (γ) radiation is emitted. High levels of uranium and thorium and their decay products in rock and soil are the main sources of high natural background radiation. There is a continuous bombardment of human and his environment by these (radionuclides) ionizing radiations [7]. The long term exposure to uranium and its daughters such as radium and radon through inhalation has several health effects such as chronic lung diseases, acute leucopoenia, anemia and necrosis of the mouth. Radium causes bone, cranial and nasal tumors. Thorium exposure can cause lung, pancreas, hepatic, bone, kidney cancers and leukemia [8].
Therefore, gamma dose rates and radionuclides activity concentrations should be monitored because of their health implications.
The Nile River is the longest river in the world. In ancient times, the Nile River flooded annually, caused by rains in central Africa and in Ethiopian highlands carrying great quantities of silt in its water. [12].
The Nile River has supported many civilizations of Egypt and Sudan throughout history and continues to play a vital role in supplying precious water for drinking, irrigation, and industry to the people of the Sudan. The Nile River plays an essential role in Sudanese life; it is the life line of Egypt and Sudan, the study of the natural radioactivity of the coasts of the River Nile is very important, and the assessment of natural dose rates will be of some interest to regional health.
The Nile is a major north-owing river in north eastern Africa. It is 6650 km long. It runs through the 10 countries of Sudan, South Sudan, Burundi, Rwanda, the Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda, and Egypt. During the last decades, there has been an increasing interest in the study of radioactivity in River Nile soil [9 , 10, 11].

Sampling
A total number of 30 surface soil (dry and out of the Nile water) samples were collected along the East and West banks of the River Nile region from 30 successive locations at a chosen point, data from Global Positioning System (GPS) were used for tracking the data recorded starting from Bait Al-mal in the West (15°39'0.20"N ~ 32°30'25.30"E) to the Shambat at East side (15°38'57.30"N~ 32°30'44.40"E).
The samples locations are listed in Table 1 and showed in( Figure. 1)and ( Figure 2) The study area was divided into Two sections First the East bank and second West bank of the River Nile.

Study Area Description
The study area is located on the banks of the Nile River north of Khartoum, starting from Bait Al-mal in the West (15°39'0.20"N ~ 32°30 '25.30"E) to the Shambat at East side (15°38'57.30"N~ 32°30'44.40"E). This area was chosen because it is an agricultural area and provide the Khartoum markets with the necessary vegetables and the agricultural land at two sides are used fertilizers from originally from Phosphate.
The significance of the current study relies on its contribution to the efforts being done to generate a radiation map for Sudan and on the targeted area, The River Nile which is of great importance due to the heavy agricultural activities involved. Level of natural radioactivity in surface soil samples collected along the east and west banks of the River Nile will be evaluated using high resolution HPGe detector. The study measures the activity concentrations of 238 U, 232 Th and 40 K across all of the soil samples. The radiological hazard to humans due to the radioactivity arising from radionuclides contained in soils samples is to be assessed and the absorbed dose rates based on soil radioactivity is to be estimated.

Radioactivity Measurements
The soil samples were collected from the upper layer at depth 0-30 cm using plastic cup and then placed in plastic bags and prepared. These were first placed a dry area for few minutes and then placed in an oven to be heated at a temperature of 60°C for 24 hours. This was to remove all moisture found in the samples. The samples were then sieved using 2mm 500µm mesh to differentiate particle size. A Marinelli beaker (size 500ml) was used to seal the soil samples, weighted and labelled. The samples were stored for at least one whole month to reach secular equilibrium. The samples filled in the beaker are of the same volume and are assumed to be homogenous with respect to parameter to be measured. Marinelli beakers are chemically resistant polypropylene, light weight and seamlessness which eliminate leaks. They are used for gamma-spectral analysis because of their high counting efficiencies and because they minimize gamma-ray attenuation. The details of the sample characteristics are shown in Table 2 [40]. For gamma analysis, the soil samples were placed directly over the detector. The measured activity concentrations were presented as Bqkg -1 .
The (1460.83 keV) gamma transition was used to determine the concentration of 40 K in different samples. The gamma transitions used to determine the concentrations of 238 U and 232 Th were listed in Table 3.
The gamma ray measurements were performed with a low background spectrometer with PC multichannel analyzer and HPGe detector high resolution.
The details of the coaxial hyper-pure germanium detector (HPGe) and the parameters used for this experimental project are as follows: 1.

Quality Control
Three types of calibrations were carried out for HPGe detector. These are energy, resolution and efficiency calibrations. Efficiency calibration was performed using 152 Eu standard source. 152 Eu was manufactured by Amersham International plc. Precision and accuracy were checked by parallel measurements of IAEA inter comparison soil samples; IAEA-300 and IAEA-315. In order to determine the background due to the existence of natural radionuclides in the environment, an empty polyethylene container with the same geometry and the same measuring conditions as for the samples, was used. The uncertainties in the calibration of the peak areas of these photo peaks were within ±2%.

Energy Calibration
The initial energy calibration was performed using a standard source ( 152 Eu) with an activity of 2.549kBq. Source is placed on the detector for a preset time and counts for seconds. The energies of 152 Eu source, ranging from 121.78, 244.96, 344.27, 778.90, 964.07 and 1408keV were used to calibrate the detector. The linear relationship between energy and channel is shown in Figure 2.1. The displayed equation on the plot is used to find other peaks that do not belong to standard source 152 Eu. The equation relates to the gamma energy at any given channel x. The spectra energy lines corresponding to the counts are shown in Figure 3.

Efficiency Calibration
In efficiency calibration, four standard sources 226 Ra, 232 Th, NG3 and 152 Eu were used. The gamma ray energy lines ranges from ~100 to 2.6MeV were used to analyze the absolute full energy peak detection efficiency. The NG3 source consisting of 241 Am, 57 Co, 60 Co, 85 Sr, 88 Y, 109 Cd, 137 Cs, 139 Ce, and 203 Hg provided by Hi-tech sources. The values obtained are used to plot the efficiency curve as shown in Figure 5.

238 U, 232 Th and 40 K Activity Concentrations
The activity concentrations of the detected radionuclides 238 U, 232 Th and 40 K are presented in Table 6 and showed in Figure 6 Elemental concentrations are calculated from measured radiological concentrations in the soil samples. First, the radiological concentration of nuclide i, C, in units of Bq kg -1 of soil, is calculated using, where      The study area was divided into two sections along the East and West banks of the River Nile. Table 6 summarizes the activity concentration ranges for the radionuclides under considerations in each section.
A. The first section: the East bank of the Nile River The first section is extended from station SE1 to SE15 as listed in Table 6. This section includes the highest and lowest concentration values of 232 Th on the whole study area. Relatively high level of 238 U concentration has been observed at SE8. This is could be due to the soil type. Irregular spatial distribution has been observed for 238 U and 232 Th radioisotopes.
Decline trends of 40 K values were observed in the first at SE3 and second sections SW5 as shown in Figure 6 The first section showed the decline trend of 40 K values decreasing from SE9 (291 Bqkg -1 ) to SE3 (180Bq kg -1 ). This could be due to the change of the soil type with the increase of the river flow.
B. The second section: the West bank of the Nile River This section is extended from station SW1 to SW15 as listed in Table 6.
Relatively high level of 238 U concentration has been observed at SW10. This is could be due to the presence of phosphate fertilizers in the soil because it is an agricultural area.
This section was characterized by irregular distribution patterns for 238 U, 232 Th and 40 K radioisotopes.
This could be due to the change of the soil type with the increase of the river flow. The confluence of two Niles (blue and white Niles) play an important role for increase the levels of 238 U and 232 Th due to the erosion processes.
C. Effective factors: Many factors could be distinguished to play an effective role in the distribution of the detected natural radioisotopes along the study area. The First and the most important that the presence of fertilizers in the soil because it is an agricultural area. The Second factor the soil types. Thirdly could be the locations of the soil sample. The samples collected along the East and West banks of the Nile River.
The other factor could be due to the human activities and its corresponding waste. Moreover, the contamination from the drainage of fertilizers hold radioactive materials, the mineralogical of the sites such as the presence of calcium carbonate and the chemistry of the Nile water could be added as another factors controlling the distribution of the radioisotopes under consideration.
The geographical features of the Nile are also considered as important factors controlling the distribution of the radioactive isotopes under consideration. Radioactive materials could be transported with the flow of the Nile water from the central Africa and Ethiopia such as the black sand, which is characterized by relatively high radioactivity concentrations [14]; [13].
The other factor erosion processes can play important role to remove the upper light sediments layer and expose the old soils, which contains black sand. Table 7 shows the concentration ranges obtained from each section compared with the other referenced values. The detected ranges of 238 U and 232 Th are in agreement with the other referenced data for the other parts of Nile River [16]; [18]; [14]; [17]; [20].

Absorbed Dose Rate (DR)
The absorbed dose rates due to gamma radiations in air at 1 m above the ground surface for the uniform distribution of the naturally occurring radionuclides ( 238 U, 232 Th and 40 K) were calculated which based on the guidelines provided by UNSCEAR [11]. The conversion factors used to compute the absorbed gamma dose rate (DR) in air per unit activity concentration in Bqkg −1 (dry weight) corresponds to 0.428 nGyh −1 for 238 U, 0.666 nGyh −1 for 232 Th and 0.042 nGyh −1 for 40 K. Therefore DR can be calculated as follows: DR(nGyh −1 ) = 0.428A U + 0.666 A Th + 0.042A K (2) Table 6 shows the absorbed dose rates calculated from 238 U, 232 Th and 40 K activity concentrations for each location based on the conversion factors given by [20]. The range of the total absorbed dose rates was from at East side 15 The values are decreased according to the fractional utilization of the materials. This could be due to the spatial variations of minerals holding in the soil.
All the samples exhibit dose rates that range from 15 to 21 nGyh −1 .

238 U and 232 Th Activity Ratios
The investigation of 238 U: 232 Th activity ratio calculations revealed that 232 Th activity concentrations are seen to be on the average 1.6 times higher than the 238 U activity concentration in the measured at East bank of the Nile River soil and at the West bank of the Nile River the investigation of 238 U: 232 Th activity ratio calculations revealed that 232 Th activity concentrations are seen to be on the average 1.7 times higher than the 238 U activity concentration.
This indicated that relatively equal amount of 238 U (5.72 Bqkg -1 ) exist than 232 Th (5.61 Bqkg -1 ). This is could be due to the presence of the human activities and its corresponding waste. Moreover, the contamination from the drainage of fertilizers hold radioactive materials, and the soil type loamy soil at SE8 which have relatively high amount of 238 U. On the other hand, SE11 showed the lowest U/Th ratio (0.4). This indicated that this location has 232 Th activity concentration (10 Bqkg -1 ) higher than 238 U (3.91 Bqkg -1 ). At one samples (SE8), equality has been detected between 238 U and 232 Th activity concentrations. This could be due to the differences of the mineralogy of the soil type. Also, it could be due to the geological and solubility differences between 238 U and 232 Th.
From the other side, Th/U ratios have been calculated and listed in Table 6. The presence of calcium carbonate sediments could be playing an important role for depletion of the 232 Th relative to 238 U [16]. [30] have mentioned that 0.7-0.4 is the typical range for igneous zircon. [31] enlarge the previously mentioned range to 0.3-2 for xenolith zircon. All soil samples of the coasts of the Nile River have Th/U ratio within this range. The majority of the measured samples have Th/U ratios match with this range. This could be indicated that the present study confirmed the presence of zircon in the banks of the Nile River soil.

Conclusion
a) The detected natural radionuclides 238 U, 232 Th and 40 K showed irregular distribution patterns along the study area. The investigation of the spatial distribution of the banks of the Nile soil revealed that there are many factors affecting the distribution of the natural radioisotopes in the area under consideration.
These factors could be as the following: (1) The phosphate fertilizers contain high concentrations of uranium compared with other types of Fertilizers, because there is a companion uranium phosphate.
(2) the presence of fertilizers in the soil because it is an agricultural area; (3) the geographical feature of the stream such as the confluence of two Niles; (blue and white Niles) (4) chemistry of the Nile water (5) human activities and its corresponding waste.
b) The significance of the current study relies on its contribution to the efforts being done to generate a radiation map for Sudan and on the targeted area, The River Nile which is of great importance due to the heavy agricultural activities involved.
c) The activity concentrations and the absorbed dose rates, due to 238 U, 232 Th and 40 K, and annual effective dose rate (AEDR). These values obtained from the coastal soil were less than the recommended safe and criterion limits given by UNSCEAR.
d) The activity concentration measurements showed that 232 Th activities are higher than 238 U activity concentrations by 1.4 fold along the study area.
e) The study uses Spectral HPGe detector to evaluate the activity concentrations of 238 U, 232 Th and 40 K in 30 soil samples taken from the Nile east and west banks northern Khartoum. The activity concentrations may be limited to the randomly taken samples and the results may not be generalized to the whole Nile banks. More samples are needed to evaluate f) Results of the study could serve as an important baseline radiometric data for future epidemiological studies and monitoring initiatives in the study area. The statistical methods were applied to study the relationship between all the calculated natural radionuclides g) The Absorbed Dose Rate in Air, Effective dose rate in Air and was calculated from the activity concentrations of ( 238 U) series, 232 Th series and 40 K. All results were found to be less than world maximum values. h) All soil samples of the banks of the Nile River have Th/U ratio within this range (1 -2.56). The majority of the measured samples have Th/U ratios match with this range. This could be indicated that the present study confirmed the presence of zircon in the coasts of the Nile River soil.
The data collected in this experiment could be used for various purposes. It could be used to determine whether or not the radiation levels in the River Nile basin pose a health hazard to the local population or to wildlife. It could also be used to determine the chemical composition of the soil, and by comparison to soil samples that are not subject to the effects of fertilizers, the effects of fertilizer on the chemical composition of soil could be determined.