USE OF GAMMA-RAY SPECTROMETRY FOR URANIUM ISOTOPIC ANALYSIS IN ENVIRONMENTAL SAMPLES

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1 USE OF GMM-RY SPECTROMETRY FOR URNIUM ISOTOPIC NLYSIS IN ENVIRONMENTL SMPLES Y.Y. EBID Physics Department, Faculty of Science, Fayoum University, Fayoum, Egypt Received February 18, 2009 Gamma-ray spectrometry was employed to determine the uranium isotopic ratios ( 235 U/ 238 U) in the environmental samples. simple mathematical formula was derived and used to easily determine the 235 U in any environmental sample, regardless of 238 U equilibrium status. lso the adopted formula could be used for all types of environmental samples with different uranium isotopic ratios. Key words: Uranium isotopic ratio, gamma-ray spectrometry 1. INTRODUCTION Uranium is a rare earth element found in the earth s crust with an average of 3 ppm. Natural uranium is a mixture of three radioactive isotopes 238 U (99.27% abundance), 235 U (0.72% abundance) and 234 U (0.0054% abundance). It has been employed over the last seven decades as a source of energy and weapon. Enrichment processes were widely practiced to produce the necessary fuel. On the other hand, depleted uranium (DU) was also produced as a by-product of this enrichment processes. It was successfully used in many fields e.g. for shielding gamma radiation, penetrating calibers and as ballasts in aircrafts. ccordingly, monitoring of the isotopic 235 U/ 238 U ratio could be a good indicative of the origin and/or activities associated with any uranium containing samples. Gamma-ray spectrometry is a powerful non-destructive analytical tool to determine the gamma emitters both qualitatively and quantitatively. Hyper-pure germanium detectors (HpGe) are widely used for gamma spectrometry measurements. They are favored over other detectors due to their distinctive resolving power. However, some photo-peaks from different radionuclides could still overlap together because of the proximity in their energy values so that the difference between them is less than the resolving power. n explicit example of that is the interference between the kev photopeak of the 235 U and the kev photopeak of the 226 Ra with branching ratios (f-value) of 57.2 % and of 3.5% for 235 U and 226 Ra respectively [1]. This problem could be easily handled if the radionuclide of interest Rom. Journ. Phys., Vol. 55, Nos. 1 2, P , Bucharest, 2010

2 70 Y.Y. Ebaid 2 has another gamma transitions with reasonable f-values. dditionally, the occurrence of secular equilibrium between certain radionuclide and its respective progenies could be very helpful to determine the parent's activity whenever they have the appropriate gamma transitions e.g. 226 Ra and its progenies 214 Pb and 214 Bi. Environmental samples could pose a potential challenge when they are measured for both natural uranium and radium. This is because of the explicit interference between the kev photopeak of the 235 U and the kev photopeak of the 226 Ra. So, a mathematical treatment should be performed to extract the net counts contributed by each radionuclide [2]. This study is going to mathematically treat the relations between the 226 Ra, 235 U and 238 U count rates due to their gamma transitions in order to easily calculate both uranium isotopes and to distinguish between samples containing normal, enriched and depleted uranium. In order to accurately determine the 235 U/ 238 U isotopic ratio in environmental samples using gamma-ray spectrometry, careful measurement using precise efficiency calibration should be performed. Uranium-238 could be estimated precisely using the kev (0.837 %) of 234m Pa. The most predominant gamma transition kev (57.2 %) is preferred to measure the 235 U. The reason that the kev (10.96 %), kev (5.08 %) and kev (5.01 %) energy transitions are not commonly used to determine 235 U in environmental samples is mainly due to their relatively lower branching ratios compared to that of the kev energy transition. ccording to the average normal concentrations of 235 U in the environmental samples, counting rates due to the kev, kev and kev energy transitions would be below the detection limits ranges for the HpGe detector. So it is more practical to use the kev energy transition to assess the 235 U. ccordingly it is necessary to set up some equations to express for the concentration of 226 Ra, 235 U and 238 U isotopes separately. 2. THEORETICL CLCULTIONS The activity of a specific radionuclide with a gamma energy transition could be expressed using the following equation: CEn (, ) (1) tf. ( En, ). ε ( En, ) where: : activity concentration of radionuclides n C: the net photopeak count t: counting time, s, f: branching ratio, number of photon with energy E per hundred disintegration, ε : is the detection efficiency.

3 3 Uranium isotopic analysis in environmental samples 71 In this study we use the assumption that secular equilibrium is not necessarily existed between 238 U and its progeny 226 Ra. Determination of the radionuclide 226 Ra is often performed using the most intensive gamma transitions of its progenies, 214 Pb and 214 Bi, following a secular equilibrium of at least 30 days after samples being sealed. The energy transitions used are kev (18.5%) and kev (35.8%) of the 214 Pb isotope and kev (44.8%) and kev (14.8%) of the 214 Bi isotope. So a good sealing of the samples container for at least 30 days should be essential so that no radon escape during this 30 day is allowed to avoid disequilibrium problems between 226 Ra and its respective progenies. lso, 238 U is frequently estimated using the energy transitions of 63.3 kev (3.6%) and 92.6 (4.9%) of the 234 Th (Direct daughter of 238 U). Several restrictions draw attention to the use of these two lines to determine 238 U due to the interferences and self absorption [3]. simple mathematical calculation was provided by [2] to determine 238 U, 235 U and 226 Ra in the environmental samples concluded that. CR Ra /CR U Ra (2) Where CR Ra : is the count rate (counts.sec -1 ) due to 226 Ra in the 186 kev energy peak. CR U5 : is the count rate (counts. sec 1 ) due to 235 U in the 186 kev energy peak. Ra : is the activity of 226 Ra in the sample U5 : is the activity of 235 U in the sample This resulted in the formulae (3) and (4) for samples with secular equilibrium. CR Ra CR T (3) and CR U CR T (4) Where CR T : is the total count rate (counts. sec 1 ) in the 186 kev energy peak. Ebaid, et al., 2005, assumed that the samples do not contain uranium isotopic ratios anomalies [2]. In this study we need to deduce a simple formula to calculate the 235 U activity in any environmental sample using its most predominant gamma transition kev (57.2%). CRU 5 U 5 (5) ξ f U 186keV (185.7 kev ) CR CR CR (6) while Total (186.0 kev ) Ra(186.0 kev ) CR CR Total (186.0 kev ) Ra(186.0 kev ) So U 5 ξ186kev f(185.7 kev ) (7)

4 72 Y.Y. Ebaid 4 lso CR Ra(186.0 kev) could be calculated using the activity of the 214 Bi isotope in equilibrium with its parent the 226 Ra using the kev (44.8%) energy transition. ccordingly, the equation will be as follows; CR ξ f ξ f Total (186.0 kev ) Bi kev Ra(186.0 kev ) 186keV (185.7 kev ) (8) nd CR CR ξ f ξ f Bi(609.3 kev ) Total (186.0 kev ) 186 kev Ra(186.0 kev ) ξbi(609.3 kev ) fbi(609.3 kev ) 186keV (185.7 kev ) By using the reference values for the f-values in the equation, the equation will be; (9) nd CR So the formula will be; CR ξ ξ Bi(609.3 kev ) Total (186.0 kev ) 186keV ξbi(609.3 kev ) fbi(609.3 kev ) 186keV CRTotal (186.0 kev ) CR Bi(609.3 kev ) ξ ξ f 186 kev Bi(609.3 kev ) Bi(609.3 kev ) (10) (11) CR Total nd finally (186.0 kev 1.75 ) 0.063( ) U5 Ra 226 ξ186kev Using equation (12) we can easily calculate the 235 U content of any sample containing 226 Ra as an interfering radionuclide. dditionally, Uranium isotopic ratios could be calculated using the 238 U results calculated from the 1001 kev transition energy. It is worthy to mention that both the detection efficiencies (ε ): at kev and were considered the same for simplicity since the difference would be minimal. (12) 3. EXPERIMENTL Three different samples were measured for their uranium isotopic contents and ratios in this study. Uranium ore reference (IE-RGU-1) soil sample, natural uranium standard solution, and commercial Uranyl nitrate solution were measured to estimate their uranium isotopic ratios. Three identical aliquot (75 ml) were used from each sample to minimize the statistical error.

5 5 Uranium isotopic analysis in environmental samples 73 Gamma-ray spectrometer with a GX Sl CNBERR extended range electrode germanium detector with a CNBERR model 2002CSL preamplifier was used for this work. The HpGe detector had a relative efficiency of 40% and full width at half maximum (FWHM) of 1.9 kev for 60 Co gamma energy transition at kev. Samples were counted for 86,400 seconds to achieve minimum photo-peak fitting errors. The system was calibrated for photopeak efficiency using standard soil sample in the same geometry for the studied samples. The energy transition of kev were used to estimate 235 U contents, while the energy transition of 1001 kev was used to calculate the 238 U contents. Uranium- 235 was calculated for the RGU-1 samples after using our current equation. Correction for both matrix and density were performed according to methods adopted by Khater and Ebaid, 2008, and Ebaid, 2009 [4, 5]. The uranyl nitrate sample was also measured using alpha spectrometry technique. Diluted aliquot of the original solution was used to determine the isotopic uranium contents. Sample was first co precipitated using ammonium phosphomolebdate to precipitate the uranium from the solution. The samples were dissolved using 6 M Nitric acid and then run through strong anion exchange resin followed by electro-chemical plating. Silicon Surface barrier detector was used to count the samples for 24 h to reach minimum counting error. 4. RESULTS ND DISCUSSIONS Table 1 shows the calculated 235 U and 238 U in the studied samples in Bq.kg 1 using gamma-ray spectrometry. Only RGU-1 sample contains 226 Ra in equilibrium with 238 U. It could be noticed that the obtained isotopic uranium ratios are in accordance with the certified values. On the other hand, both the natural uranium and Uranyl nitrate solutions contain uranium only with the absence of 226 Ra. The formula was successful to be used for all types of samples. It can also be noticed that the resultant isotopic uranium ratio for the natural uranium solution is also in accordance with the certified value. Table 1 Specific activities (Bq.kg -1 ) of 235 U and 238 U and 235 U /238 U ratios in the studied samples using gammaray spectrometry Sample 235 U 238 U 235 U /238 U Ratio Certified Calculated Bias % RGU ± ± Natural Uranium 11.1 ± ± Standard Solution Uranyl Nitrate Solution ± ± 13.1 N N

6 74 Y.Y. Ebaid 6 Table 2 The calculated 234 U and 238 U in two different concentrations of uranyl nitrate solutions in mbq.kg -1 using alpha spectrometry Sample 234 U 238 U 234 U /238 U Uranyl Nitrate ± ± Uranyl Nitrate ± ± ccording to this formula we were able to assess how far the commercial uranyl nitrate is depleted. The isotopic ratio was found to be compared to a normal ratio of This formula might help is the assessment of the uranium isotopic ratio in any environmental samples regardless of both their 238 U- 226 Ra equilibrium status and their uranium isotopic ratios On the other hand, alpha spectrometry results shown in Table 2 were very efficient in the determination of both 234 U and 238 U in the samples. However, the determination of 235 U was so crucial because it has several alpha emissions. mong those emissions, there are four major alpha particle energies of , kev , kev , and kev kev with intensities of 6.4%, 17%, 57%, and 5.6% respectively. ccordingly, it was a challenging process to calculate the total counts due to the 235 U. CONCLUSION It is recommended to use gamma-ray spectrometry as a non-destructive and relatively simple technique to assess the uranium isotopic ratios in environmental samples in order to investigate their status regarding being normal, enriched or depleted. This technique is highly recommended to perform environmental safeguard analyses for all types of environmental samples. REFERENCES 1. J.U. Delgado, J. Morel, M. Etcheverry, Measurements of photon emission probabilities from the decay of 226 Ra and daughters, ppl. Radiat. Isot., 56, , Y.Y. Ebaid, S.. El-Mongy, K.. llam, 235 U γ emission contribution to the 186 kev energy transition of 226 Ra in environmental samples activity calculations, International Congress Series 1276, , Z. Papp, Z. Dezso, S. Daroczy, Measurement of the Radioactivity of 238 U, 232 Th, 226 Ra, 137 Cs and 40 K in Soil using Direct Ge(Li) γ-ray Spectroscopy, J. Radioanal. Nucl. Chem (1 2), , E.M. Khater, Y.Y. Ebaid, simplified gamma-ray self-attenuation correction in bulk samples, ppl. Rad. Isot., 66, , Y.Y. Ebaid, On the use of reference materials in gamma-ray spectrometric efficiency calibration for environmental samples, J. Radioanal. Nucl. Chem., 280, No. 1, 21 25, 2009.