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ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 3  |  Page : 122-127  

Determination of uranium concentrations and 234U/238U activity ratio in some granitic rock samples by alpha spectrometry: Application of a radiochemical procedure


Nuclear Materials Authority, El-Maadi, Cairo, Egypt; Department of Chemistry, Laboratory of Radiochemistry, University of Helsinki, FI 00014, Helsinki, Finland

Date of Web Publication30-Nov-2016

Correspondence Address:
Mahmoud R Khattab
Nuclear Materials Authority, P.O. Box 530, El-Maadi, Cairo, Egypt; Department of Chemistry, Laboratory of Radiochemistry, University of Helsinki, P.O. Box 55, FI 00014, Helsinki, Finland

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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.194961

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  Abstract 

The present study is an application of a radiochemical procedure using alpha spectrometry technique for determination of uranium isotopes 238 U,234 U, and 235 U on 13 granitic samples. These samples were collected from Gabal Gattar area, Northeastern Desert, Egypt. The collected samples were digested using microwave technique with aqua regia and spiked with 232 U for chemical yield and activity calculation. Separation of uranium isotopes from the samples was done by Dowex 1 × 4 (50–100 mesh) resin followed by source preparation using microprecipitation technique. The concentrations of 238 U were ranged between 28.9 ± 0.9 and 134.8 ± 1.8 Bq/g, and the 234 U concentrations were between 24 ± 0.6 and 147.7 ± 2.2 Bq/g. For the 235 U, the activity concentrations were between 1.3 ± 0.2 and 6.7 ± 1.2 Bq/g. The activity ratio of 234 U/238 U was calculated and varied from 0.80 to 1.30.

Keywords: Granite, isotopic composition, radiochemical separation, uranium concentrations


How to cite this article:
Khattab MR. Determination of uranium concentrations and 234U/238U activity ratio in some granitic rock samples by alpha spectrometry: Application of a radiochemical procedure. Radiat Prot Environ 2016;39:122-7

How to cite this URL:
Khattab MR. Determination of uranium concentrations and 234U/238U activity ratio in some granitic rock samples by alpha spectrometry: Application of a radiochemical procedure. Radiat Prot Environ [serial online] 2016 [cited 2019 Jun 25];39:122-7. Available from: http://www.rpe.org.in/text.asp?2016/39/3/122/194961


  Introduction Top


Uranium is accumulated in granites and sedimentary rocks by the geological process. The most common uranium-containing minerals are; uraninite which is a complex of uranium oxides (UO2/UO3), autunite (a hydrated calcium uranium phosphate) Ca (UO2)2(PO4) 2.10-12H2O, brannerite (a uranium calcium cerium-titanium iron oxide) UTi2O6, and carnotite (a hydrated potassium uranium vanadate) K2(UO2)2(VO4)2.1-3H2O.[1]

Natural uranium has three alpha radioactive isotopes: 99.2745% of 238 U, 0.7200% of 235 U, and 0.0054% of 234 U.[2],[3] The activity of 1 Bq of 238 U corresponds to 81.6 μg of natural uranium (1 μg natural U = 12.3 mBq of 238 U).[4] The specific activity of 238 U is much lower (1.24 × 104 Bq/g) in comparison with 234 U (2.30 × 108 Bq/g).[5]

The uranium concentration and U-isotopic ratios are detected and determined in various environmental and geological samples by different nondestructive and destructive techniques. The nondestructive techniques such as gamma-spectrometry (NaI and HPGe-detector) which are carried out on the bulk samples without the need for complicated and time-consuming radiochemical methods,[6] while the destructive techniques are carried out through several analytical methods (e.g. alpha-spectrometry, inductively coupled plasma mass spectrometry, kinetic phosphorescence, etc.).[6] Among these techniques, alpha spectrometry is the common one that measures isotopes and can detect low uranium levels. It's detection limit is 100–1000 times lower than gamma-spectrometry.

Alpha spectrometry is a versatile and sensitive technique which finds application in many aspects of the nuclear energy field. In particular, many studies have used this technique for uranium determination in environmental samples. The detection limit of alpha-spectrometry is considered as 100–1000 times lower than gamma-spectrometry.[7] The advantage of using alpha spectrometry is resulting from the observation of the high yield alpha-decay process, low intrinsic detector backgrounds, and the elimination of competing radiation by chemical processing. Besides, the use of an alpha-emitting isotope as a tracer makes the technique more reliable.[8]

In general, a procedure for alpha spectrometry involves a number of separate steps; (i) sample pretreatment that includes physical and chemical processes to select adequate aliquots and dissolve or preconcentrate the element of interest. The sample preparation aims to convert the sample into a thin layered, chemically isolated form that can be counted with a minimum of self-absorption and interferences. This stage includes sample digestion acids in a microwave digester (also uses acid for digestion). Using the microwave reduced the usage of acids and other reagent, hazardous emission, and the waste generated during sample preparation, (ii) chemical separation. Solvent extraction and ion-exchange separation are two commonly used techniques for achieving efficient chemical separation,[9] and finally (iii) source preparation; - two main methods are commonly used for the preparation of the sample on a disc microprecipitation and electrodeposition.

One of the radiochemical procedures that are used for the determination of uranium isotopes is based on some steps as follow; (i) isotopic dilution using of 232 U half-life of 70.6 a, Eα =5320.24 KeV, probability = 69.1%) tracer for chemical yield estimation; (ii) full acid digestion in several consecutive steps with HF, HNO3, and HCl; (iii) radiochemical separation of uranium from other matrix elements by ion exchange resin (Bio-Rad AG1-X8) and further purification by recently developed chromatographic materials UTEVA; (iv) source preparation for alpha spectrometry by microprecipitation.[10]

Another radiochemical method for the determination of uranium isotopes in environmental samples by alpha spectrometry is based on the preconcentration of uranium by co-precipitated with iron (III) hydroxide at pH 9–10 using an ammonia solution and the precipitate is dissolved in HNO3 and the mineralized with H2O2 and HF; uranium in biological samples was ashed at 600°C, leached with Na2 CO3 solution and mineralized with HNO3, HF, and H2O2; uranium in soil samples was fused with Na2 CO3 and Na2O2 at 600°C and leached with HCl, HNO3, and HF. The mineralized or leaching solution in 2M HNO3 was passed through a microthene trioctylphosphine oxide column; after washing uranium was directly eluted into a cell with ammonium oxalate solution, electrodeposited on a stainless steel disc and measured by alpha spectrometry.[7]

The ratios of 234 U to 238 U as well as uranium to its daughters are used for evaluation of geological processes. Studies of uranium behavior in uranium deposits are used as analogs for the final disposal of spent nuclear fuel from nuclear power plants and dissolution, reprocessing as well as final disposal of spent fuel.[11] The 234 U/238 U activity ratio in environmental components would be expected usually to be close to one as long as the uranium stays locked inside an undisturbed crustal rock in equilibrium with its progeny, but obtained study showed that this ratio is different than one. This disequilibrium in activity between 234 U and 238 U occurs when the rock is disturbed by chemical or physical changes involving water, and these processes can change the uranium isotope ratios in air, soil, and water.[1]

The present work was done at the Laboratory of Radiochemistry, Helsinki University according to the cooperation between Finland and Egypt and aims to apply a simple and an accurate radiochemical procedure for the determination of uranium isotopes instead of the complicated and tough one that were used before.


  Materials and Methods Top


Geology of the study area

The Gabal (mountain) area is located about 35 km Southwest of Hurghada city, Red Sea coast at the intersection of latitude approximately 27°7' 30'' N and longitude 33° 17' 5'' E [Figure 1].[3] The Gabal Gattar area encloses several rock units. The oldest are the Hammamat sedimentary rocks and Dokhan volcanics, while the youngest are granites and related dikes, in addition to Wadi deposits. The Gabal Gattar granite is the host rock for uranium mineralization which is localized in seven prospects, starting from G I to G VII.[12],[13] Earlier study estimated as much as 4000 tons of uranium in the mineralized granites in the Gabal Gattar area.[14]
Figure 1: Geological map of Gabal Gattar area, Northeastern Desert, Egypt[3]

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The samples of this study are collected from GII uranium prospect which is located in the Northwestern part of Gabal Gattar [Figure 1]. All the samples were collected from the surface with the exception of the samples (SQ-2, SQ-4, SQ-6, and SQ-7) which were collected from GII Trench (35 m underground).

Samples preparation

Microwave digestion

Leachability of uranium from soil, sediment, and rocks is variable and depends largely on its own set of well-defined properties or compositions of the mineral. Therefore, to get accurate results the mineralization or leaching techniques used in an analytical procedure become very critical.

Each sample is crushed to (200 mesh) and mixed well. Using a microwave significantly reduced the usage of acids and other reagent, hazardous emission, and the waste generated during sample preparation of uranium and analysis by alpha spectrometry. The digestion process was performed by Microwave Accelerator reaction system MARS 5™ using OMNI vessels. One gram of the sample was weighted into precleaned vessels and 20 ml Aqua Regia (5 ml HNO3 + 15 ml of HCl). The vessels were then sealed, tightened using a torque wrench, then they were placed inside a rotor of the microwave digestion system, and finally applied to a microwave dissolution program (pressure 55, 19, power 1600 W bar; stage 1, 0–20 min: 165°C, stage 2, 20–35 min (hold 10 min): 195°C). After cooling, the digest was filtered with 0.45 µm polypropylene filter. The microwave digests were stored in Nalgene Containers in the refrigerator for further analysis.[15]

Separation of uranium

The uranium was separated from the solutions obtained from digestions using anion exchange resin Dowex 1 × 4, 50–100 mesh (strongly basic gel type polystyrene resin) with appropriate functional groups, filled in a column by 10 ml of 9 M HCl.

Uranium and other interfering elements in the samples were transferred to a column in 9 M HCl, which is used to activate the resin. To elute uranium from the column, 10 ml 0.1 M HCl is passed through the column and then the eluted uranium is evaporated to dryness. The evaporated sample was then dissolved in 5 ml of 1 M HCl, 0.3 ml of Ce (≈100 µ/ml) which is added as a carrier, few drops of TiCl3 for reduction of uranium (VI) to uranium (IV) and finally, 0.5 ml of HF was added.[16] The 232 U tracer is added for chemical yield and activity calculation. Therefore, the amount of the 232 U tracer added is determined according to the expected concentration of uranium in the samples.

Source preparation and alpha-counting

Although several methods for the preparation of alpha source are reviewed, certain essential properties for a suitable source are common for all preparation's methods. Probably, the most important quality is essential for a suitable from two standpoints; the loss of active material will cause both contamination and a change in the emission rate of the source. Good uniformity of thickness is also highly desirable, especially with alpha sources since variations in thickness lead to changes in the alpha spectrum because of changes in self-absorption.[17]

The techniques most widely used to prepare sources for alpha spectrometry include direct evaporation (with or without the use of a spreading agent), electrodeposition, vacuum evaporation, and electrospraying. The source preparation technique chosen reflects the experimental conditions and data requirements. Sources prepared for calibration purposes or to measure alpha particle energies and study alpha particle fine structure, must have the highest possible resolution. To achieve this, repeated chemical purifications are carried out before source preparation to ensure complete removal of inactive contaminants and the sources are invariably prepared by vacuum evaporation, which is the best technique consistently giving alpha source of the highest quality.[9]

A simple method for the preparation of alpha sources was applied in the present work. This method was the precipitation method where either aqueous or organic solution of the source materials is evaporated to dryness on a substrate. Although the method may not produce as adherent a deposit as that of electrodeposition, vacuum evaporation, or electrospraying methods, but it gives 100% yields. The membrane filter used is 0.1 µm VCWP with a reference VCWP02500 and Lot number R5DA90839.

The prepared alpha spectrometry sources were then placed and counted for 24 h in the chambers of alpha spectrometry which have ion house built vacuum chamber equipped with 450 mm 2 Canberra PIPS/Ortec ultra-As silicon detectors. Signal processing electronics is built of standard Ortec/Canberra NIM modules, and the data acquisition is implemented with ortec ethernium 920 E modules and Maestro software (- ORTEC® 801 south lllinois Ave., Oak Ridge, TN 37830 USA.). The system energy calibration is performed with a mixed source containing 239 Pu (Eα =5.1 MeV),241 Am (Eα =5.48 MeV), and 244 Cm (Eα =5.8 MeV) radionuclides. They have the same chemical composition, the same concentration, the same geometry, and the same counting configuration.

The activity concentrations of 238 U,235 U, and 234 U are calculated through two main equations as follows:

  1. Calculation of the tracer yield (YTracer) [Figure 2] and [Figure 3]
    Figure 2: Uranium spectrum for a granite sample (SQ-1) of high uranium concentration

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    Figure 3: Uranium spectrum for a granite sample (SQ-11) of high uranium concentration

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    where NTracer is the count of the tracer used (NBackground) is the background count t is the counting time (s), ATracer is the activity (Bq) of the tracer and (eff) is the efficiency of the detector which is about 0.30.

  2. Calculation of the final activity concentration of uranium (ANuclide)




where NNuclide is the count of the studied nuclide, (NBackground) is the background count t is the counting time (s), M is the mass of the sample (kg), and (eff) is the efficiency of the detector which is about 0.30.


  Results and Discussion Top


Recent surface exploration activities led to the discovery of new promising uranium mineralized zones on the surface of the G-II occurrence. These zones were investigated at depth by mapping the surface features and projecting the geometry to the subsurface. Primary uranium mineralizations were first time discovered in the tunnel by the geological team of Gabal Gattar prospect. However, the field geological studies indicated that visible secondary uranium mineralization of bright yellow and orange colors are recorded at various levels of exposures.

One standard ore sample from IAEA was re-analyzed and used as a reference sample to control the analytical procedure. Several experiments were performed to indicate the suitable volume of the digested samples to give the accurate concentration of uranium isotopes in the collected rock samples. For the IAEA uranium reference sample (RGU-1), the certified value of 238 U concentration is 5.1 ± 0.2 Bq/g while measuring it with alpha spectrometry, in turn, gave a value of 4.8 ± 0.1 Bq/g which is only 7% lower than certified value and is slightly within the uncertainties.

In this procedure,232 U is added to the sample (spike) with activity concentration (0.3 Bq/ml) after the microwave digestion stage, and the amount of the 232 U tracer was about 0.4 ml for each sample.232 U has the advantage that it is alpha energy (5.32 MeV) is well separated from those of 238 U,235 U, and 234 U. Use of a tracer provides the most accurate estimation of the chemical yield for every sample. The Chemical yield is ranged between 70% and 85%.

The obtained results in terms 238 U,234 U, and 235 U isotopic activity concentrations (Bq/g −1) are listed in [Table 1]. From the granite rock samples listed in [Table 1] from SQ-1 to SUIII-1, the concentration of 238 U were ranged between 28.9 ± 0.9 and 134.8 ± 1.8 Bq/g. The 234 U concentrations were between 24 ± 0.6 and 147.7 ± 2.2 Bq/g. For the 235 U, the activity concentrations were between 1.3 ± 0.2 and 6.7 ± 1.2 Bq/g [Figure 4].
Table 1: Uranium concentrations in (Bq/g), 234U/238U and the chemical recovery (%) of the studied area

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Figure 4: 238U and 234U concentration (Bq/g) in the studied samples

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234 U/238 U are in radiological equilibrium, namely activity ratio of 234 U to 238 U being equal to unity if the uranium was placed in the closed system. Isotopic fractionation of heavy elements such as uranium usually is less than significant compared with that of light elements such as hydrogen and oxygen.

If the systems are exposed to weathering and groundwater circulation, the different physicochemical conditions affecting 238 U and 234 U will result in their fractionation and, thus, the respective activity ratios will, therefore, be greater or less than unity. The isotopic variation of U results from selective leaching of 234 U itself and the direct a-recoil transfer of a 234 U precursor (234 Th).234 U atoms are more susceptible to leaching than 238 U atoms as a result of a-recoil within the mineral lattice. The preferential leaching of 234 U will result in 234 U/238 U < 1.0 in the weathering mineral. This results in the enrichment of 234 U in groundwater and 234 U depletion in the mineral. An additional mechanism is the process of daughter product emplacement from pore waters containing dissolved 234 U and 238 U, resulting in rocks that are enriched in 234 U, and 230 Th relative to 238 U in reducing environments.

Variations on this AR have been used as sensitive chemical indicator for identifying isotopically distinct groundwater and geochemical processes. Origin of variation of AR of 234 U/238 U is from preferential leaching of 234 U from the rock matrix which is due to the crystal defects from the alpha particle recoil [12] which concerns only the isotopes 234 U generated from 238 U via decay to 234 Th and 234 Pa and is independent of geochemical conditions. The simultaneous dissolution of all uranium isotopes is a chemical process, which is dependent on geochemical conditions.[12]

The 234 U/238 U activity ratios varying from 0.80 to 1.30 in the studied samples [Figure 5]; we could observed from these results that there are disequilibrium between 234 U and 238 U as the results were above or less than unity.
Figure 5: 234U/238U activity ratio in the studied samples

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Results of samples analysis for 238 U,234 U indicate five predominant geochemical processes that affected uranium in the rock: (1) uranium leaching where 238 U and 234 U were removed with little or no fractionation; (2) preferential 234 U leaching by alpha recoil displacement (234 U recoil loss) with lesser 238 U loss; (3)234 U recoil loss with little or no 238 U loss; (4) uranium assimilation where both 238 U and 234 U were added with present-day,234 U/238 U activity ratios varying from 0.8 to 1.3; (5) addition of 234 U and 230Th by daughter emplacement processes (234 U + 230 Th recoil again). The 234 U displacement mechanism operates when 234 U, produced by decay of 238 U.[18]238 U atoms decay by alpha disintegration and recoiling daughter nuclides of 230 Th,234 Th,234 pa, and 234 U are absorbed or embedded in particular matter at the solid-liquid interface. After sufficient geologic time, (234 U + 230 Th recoil again), results in solids in which 234 U/238 U is greater than unity.[18]

The 234 U is preferentially leached from rocks because 234 U is located in crystals of rocks in damaged lattice sites. It occupies the sites of a 238 U atom that has undergone alpha-decay. The alpha-particle and the recoil of the nucleus damage this site. Since it occupies damage site, it is more easily removed from the crystal by weathering than 238 U.


  Conclusions Top


The application of this radiochemical procedure to the granitic rock samples allowed estimating of the uranium isotopic concentration in these samples. This radiochemical procedure is accurate and suitable for this type of granite rock samples.

The concentrations of 238 U were ranged between 28.9 Bq/g and 134.8 Bq/g. The 234 U concentrations were ranged between 24 Bq/g and 147.7 Bq/g for. For 235 U, the activity concentrations were between 1.3 and 6.7 Bq/g. The 234 U/238 U activity ratios varying from 0.80 to 1.30 that evidence for disequilibrium between 234 U and 238 U concentrations due to the oxidation-reduction potential.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Dimova N, Kinova L, Beleva B, Slavchev B. Radiochemical procedures for determination of naturally occurred uranium isotopes in environmental samples, university of mining and geology, annual. Geol Geophys Sofia 2003;46:241-6.  Back to cited text no. 10
    
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Hussein HAM, Sayyah TA. Uranium potential of the younger granites of Egypt: IAEA-TECDOC-650, 1992. p. 58-64. International Atomic Energy Agency, Vienna. Austria: Nuclear Energy Agency. 75 - Paris, France: 258 p; ISSN 1011-4289; 1992. p. 58-64; Technical committee meeting on new developments in uranium exploration, resources, production and demand. Vienna, Austria; 1991. p. 26-29.  Back to cited text no. 14
    
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Hanna T, Daniela V, Esa P, David R, Dina S, Jukka L. A comparison of analytical methods for determining uranium and thorium in ores and mill tailings. J Geochem Explor 2015;148:174-80.  Back to cited text no. 15
    
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Juntunen P, Ruutu A, Suksi J. Determination of 226 Ra from Rock Samples Using LSC. Proceedings of the International Conference on Advances in Liquid Scintillation Spectrometry, Karlsruhe, Germany; 2001. p. 299.  Back to cited text no. 16
    
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Deal R, Chanda R. A rapid source preparation technique for high resolution alpha particle spectroscopy. Nucl Instrum Method 1969;69:89-92.  Back to cited text no. 17
    
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Rosholt JN. Isotopic composition of uranium and thorium in crystalline rocks. J Geophys Res 1983;88:7315-30.  Back to cited text no. 18
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1]


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