SIMULTANEOUS DETERMINATION OF BETA NUCLIDES BY LIQUID SCINTILLATION SPECTROMETRY

Transcription

1 SIMULTANEOUS DETERMINATION OF BETA NUCLIDES BY LIQUID SCINTILLATION SPECTROMETRY Cordula Nebelung 1 Peggy Jähnigen Gert Bernhard Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, Dresden, Germany. ABSTRACT. This work evaluated 2 types of liquid scintillation (LS) spectra analysis. 1) Quench correction with the transformed spectral index of external standard (tsie) in combination with automatic efficiency control (AEC) is used to measure up to 3 nuclides in 1 sample with the assay type dpm (single, dual, or triple) with good accuracy even for large activity differences and beta energies close together. For this method, it is necessary to generate quench curves for each nuclide to calculate the dpm (disintegrations per minute) from the measured cpm (counts per minute). 2) The multinuclide spectra measured with a logarithmic x axis are deconvoluted with the knowledge of the shape of the single-nuclide spectra. In this case, the difference between the measured and fitted counts (cpm) and the real activity (dpm) has to be considered subsequently. The spectra deconvolution allows the determination of up to 7 nuclides in 1 sample. The uncertainty even for 3 and more nuclides in 1 sample is <10% for both methods, combined with a fast measuring process. INTRODUCTION For the assessment of exposed radioactivity coming from nuclear facilities, weapons test sites, and installations for uranium mining and fuel reprocessing, sensitive and economic methods are needed for measuring radionuclides. To study the sorption and migration behavior of radionuclides under environmental conditions, it is necessary to measure the composition of multinuclide systems. The common way to analyze several nuclides in 1 sample involves time-consuming chemical separations followed by activity measurements of the isolated nuclides (Saito et al. 1990; Küppers and Erdtmann 1992; Niese and Gleisberg 1995; Wallner 1997; Benitez-Nelson and Buesseler 1998; Rodriguez et al. 2000; Park et al. 2006). An alternative method is simultaneous measurement of the nuclides by recording and analyzing the complete LS spectra. Kashirin et al. (2000) set up a spectra library with quench curves for some nuclides, together with a special software (spectradec). For model mixtures, they describe results with uncertainties between 16 and 52%. A calculation with counting efficiencies in 2 or 3 energy windows similar to the implemented calculation for the Tri- Carb 3100 have been described (Abrams et al. 1981; Hong et al. 2001; Miyazawa et al. 1991). Finally, the CIEMAT/NIST calculation use atomic data like half-life, maximum beta energies, fluorescence yield, electron capture probabilities, photon energies, and Auger electron energies (Carles 1994; Carles et al. 1994a,b; Ceccatelli et al. 1999; Altzitzoglou 2004; Carles and Kossert 2007). In this work, 2 methods to analyze multinuclide samples were compared. The first method is the device-implemented calculation for up to 3 nuclides simultaneously (single, dual, triple mode). The transformed spectral index of external standard (tsie) is the optimal quench correction. The optimal channel adjustment is provided by the automatic efficiency control (AEC). The second method is the deconvolution ( fitting ) of LS spectra measured on a logarithmic energy axis. This method requires no extra time-consuming sample preparation. EXPERIMENTAL Defined nuclide mixtures were prepared for the verification of both methods. Nuclides relevant for nuclear facilities were used. The applied nuclides are described In Table Fe is an electron capture nuclide while all other nuclides are β emitters. 1 Corresponding author c.nebelung@fzd.de by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p

2 194 C Nebelung et al. Table 1 Nuclide descriptions. Nuclide Producer decay Max. energy (kev) Daughter nuclide decay Energy (kev) 3 H Amersham Biosciences β He stable 14 C Eckart & Ziegler β N stable 55 Fe Eckart & Ziegler EC Mn stable 60 Co Eckart & Ziegler β Ni stable 90 Sr a AEA Technology β Y β Tc Amersham β Ru stable 137 Cs b AEA Technology β m Ba IT a90+ Sr: 90 Sr in equilibrium with its daughter 90 Y. b137+ Cs: 137 Cs in equilibrium with its daughter 137m Ba. Radionuclide stock solutions were prepared from purchased standard solutions by dilution with MilliQ water. Activities between 1 and 5000 Bq in various combinations were measured. The sample volume was 110 μl with 5 ml of Utima Gold as scintillation cocktail in each sample. The liquid scintillator consists of 65 70% di-isopropylnaphtalene, 12 15% ethoxylated alkylphenol, 9 12% mono- and di-phosphate ester, 1 2% sodium di-octylsulphosuccinate, <1% 2,5-diphenyloxazole, and <1% 1,4-bis (2-methylstyryl) benzene (PerkinElmer). The LS systems used were a Tri-Carb 3100 with a linear energy x-axis and internal calculation in the tsie/aec mode and Wallac 1414 low-level α/β with a logarithmic scale x-axis for spectra deconvolution (both devices from PerkinElmer). The spectra are measured over 5 min at activities 100 Bq and over 20 min at activities 100 Bq. RESULTS First, all single nuclides were measured to determine the spectra shape of each nuclide. Knowledge of these shapes was used for the calculation of multinuclide samples. The LS spectra measured with the Tri-Carb 3100 (Figure 1) were close together with visually low shape differences for some nuclides. For spectra determined with the Wallac 1414, typical shapes for each nuclide were observed (Figure 2). To distinguish between some pairs, such as 3 H and 55 Fe, 99 Tc and 60 Co, or 137 Cs and 90 Sr, may be difficult because the peak shapes were similar and strongly overlapping. The dual or triple mode in the Tri-Carb 3100 TR works only in the dpm (disintegrations per minute) mode. The measured counts (cpm = counts per minute) were internally corrected with the counting efficiency. This correction required quench curves for each nuclide. The result of this correction was the dpm value, i.e. the absolute activity. In this paper, all samples were measured in water and quench curves were determined in water (Figure 3), measured with tsie/aec. The middle- and high-energy nuclides showed no dependence on the volume. The low-energy nuclide 3 H and the electron capture nuclide 55 Fe, however, showed a strong dependence on the sample volume (Jähnigen 2007). For a sample volume of 110 μl (see black vertical line in Figure 3), efficiencies of 50% for 3 H and 63% for 55 Fe were determined. The spectrum was partitioned in 2 energy ranges for the measurement of 2 nuclides in 1 sample. Four quench curves were used for the calculation: a low-energy nuclide in the low-energy range, low-energy nuclide in the high-energy range (near zero), high-energy nuclide in the low-energy range, and high-energy nuclide in the high-energy range. The activity of both nuclides was calculated with the quench curves. Three energy ranges and 9 quench curves were necessary for the calculation of 3 nuclides in 1 sample.

3 Simultaneous Determination of Beta Nuclides by LS Spectrometry 195 Figure 1 LS spectra, measured with the Tri-Carb 3100 TR Figure 2 LS spectra, measured with the Wallac 1414

4 196 C Nebelung et al. Figure 3 Volume quench curves, measured with the Tri-Carb 3100 The deconvolution of the spectra, measured with the Wallac 1414, required fit functions to describe the peak shapes (Nebelung and Baraniak 2007). The peak fitting module of the graphic program Origin 5.0 (microcal Inc.) contains several internal fit functions. The low-energy nuclides 3 H and 55 Fe were described very well by the asymmetric double sigmoidal function, Equation 1 (Figure 4). 1 1 yx ( ) = A e x xc+ w w e x xc w w 3 (1) where y(x) are the counts in channel x; A is the amplitude; xc is the peak center (energy, channel); w1 is width 1: full width of half maximum; w2 is width 2: variance of low-energy side; and w3 is width 3: variance of high-energy side. A Gaussian function with an exponential term at low energies (Equation 2) were added to the originfit function library (Nebelung and Baraniak 2007; Westmeier and van Aarle 1990) for the description of the high energy nuclides 137+ Cs, 90+ Sr (Figure 5) and 60 Co. In the case of 90+ Sr, it could be distinguished between 90 Sr and its daughter 90 Y. ( x x yx ( ) H 1 * 0 ) 2 π 2*ε *σ 2 -- *σ *H 2 T1 *H T1 *( x x 0 ) + σ 2 = exp + 1exp * 2*ε T1 erfc ε T1 * ( x x 0) + σ *σ*ε T1 (2)

5 Simultaneous Determination of Beta Nuclides by LS Spectrometry 197 where y(x) are the counts in channel x; H 1 the counts of peak maximum; x 0 the position (energy, channel) of peak maximum; H t1 the degree of tailing (height relative to H 1 ); ε T1 the tailing-gradient (slope); and σ 2 is the variance of Gaussian distribution. Figure 4 Asymmetric double sigmoidal function for 3 H and 55 Fe fitting Figure 5 Combined Gaussian with exponential term for 90+ Sr and 137+ Cs fitting.

6 198 C Nebelung et al. The spectra of 14 C and 99 Tc could be described by both fit functions (Equations 1 and 2). Each nuclide had a typical shape and the parameters of the fit function except the amplitude (A) and peak height (H 1, H t1 ) were fixed for the deconvolution. A lower efficiency was measured for 3 H (49.7%) and for 55 Fe (52.4%) in the Wallac 1414 system. The fitted values for both nuclides had to be corrected with these efficiencies. Various combinations of nuclides with activity ratios up to 1/50 were measured: Dual: 3 H/ 14 C, 14 C/ 60 Co, 14 C/ 90+ Sr, 14 C/ 99 Tc, 137+ Cs/ 60 Co, 55 Fe/ 3 H, 99 Tc/ 60 Co; Triple: 55 Fe/ 3 H/ 14 C, 55 Fe/ 14 C/ 99 Tc, 3 H/ 14 C/ 60 Co, 3 H/ 14 C/ 137+ Cs, 3 H/ 14 C/ 90+ Sr, 14 C/ 99 Tc/ 60 Co; More than 3: 55 Fe/ 3 H/ 14 C/ 99 Tc, 55 Fe/ 3 H/ 14 C/ 137+ Cs, 3 H/ 14 C/ 99 Tc/ 137+ Cs, 3 H/ 14 C/ 137+ Cs/ 90 Sr/ 90 Y, 3 H/ 99 Tc/ 60 Co/ 90 Sr/ 90 Y, 55 Fe/ 3 H/ 14 C / 99 Tc/ 137+ Cs/ 60 Co/ 90 Sr/ 90 Y. In Table 2, the results for the measurement of single and multiple labeled samples are summarized. The variances were lower at high activities. The high variances in the dual mode were caused by a lot of combinations of nuclides with similar shapes. Table 2 Comparison of measured vs. expected activities in %. Single Dual Triple Four and more Tri-Carb 3100 TR Low activity a ± ± ± 9.5 High activity a ± ± ± 3.6 Wallac 1414 Low activity a,b a Low activity: Bq; high activity: Bq. b90 Sr and 90 Y separately, 3 H, 55 Fe low-energy nuclides ± ± ± ± 7.6 High activity a,b 98.6 ± ± ± ± 6.8 scaling up with the efficiency. The samples with similar shapes and energies close together were closely examined. In Table 3, the results of the pairs 55 Fe and 3 H, 99 Tc and 60 Co, and 137+ Cs and 90+ Sr are shown in detail. The combination with lower activity of the low-energy nuclide and high activity of the high-energy nuclide gave higher differences between the measured and the expected values. In the combination 137+ Cs and 90+ Sr, the 137 Cs peak at kev can be used with an activity percentage of 6.9 ± 0.3% for the deconvolution. Table 3 Comparison of measured vs. expected activities in Bq (Tri-Carb 3100 TR, low activities). Measured activity in Bq Added activity in Bq 55 Fe / 3 H 99 Tc / 60 Co 137+ Cs / 90+ Sr 100/ / / / / / / / / / / / / / / / / / / / /1 9.80/ / / / / / /20.72 In Figure 6, some samples with 3 nuclides are shown. In the case of 90+ Sr, 4 nuclides were fitted. Figure 7 includes spectra with 4, 5, and even 7 nuclides with equal activities (each nuclide 10 or 100 Bq). Also, the spectrum including all nuclides, each 100 Bq, can be fitted with maximal devia-

7 Simultaneous Determination of Beta Nuclides by LS Spectrometry 199 tions of 15% relating to the added activity. Larger deviations were expected for 4 and more nuclides in 2 samples, if the sample contains an unknown number of nuclides or if the activity ratio between the nuclides is larger than 1:5, especially for nuclides with similar shapes and small activity for the nuclide with lower energy. Figure 6 LS spectra for triple nuclides, measured in the Wallac 1414 system, spectra deconvolution Figure 7 LS spectra for 4, 5, and 7 nuclides, measured in the Wallac 1414 system, spectra deconvolution

8 200 C Nebelung et al. SUMMARY The determination of multinuclide samples was successful with both methods, the device-integrated calculation (Tri-Carb 3100) and the spectra deconvolution (Wallac 1414). In the Tri-Carb 3100, up to 3 nuclides can be determined together in the dpm mode. It is a very fast method and only requires the measuring time. Also, nuclides with energies close together can be measured. For these determinations, accurate quench curves are necessary. Multinuclide determination after α/β separation is not possible. The spectra measured in the Wallac 1414 system in cpm mode were used for the spectra deconvolution. This method demands more time: measuring time and 15 min for deconvolution. The deconvolution of up to 7 nuclides was tested successfully. A prior knowledge of the peak shapes of all single nuclides was required. Equal conditions (volume, liquids, scintillator) for all measurements were also essential because the shape varied with these parameters. The correction of the fitted values of low-energy nuclides ( 3 H, 55 Fe) with its efficiency was necessary. The deconvolution after α/β separation is possible for α and β spectra (Nebelung and Baraniak 2007). Both methods produced results of multinuclide samples in solution in a short time, with deviations below 10%, and are therefore of interest for further investigations in radionuclide analytics. REFERENCES Abrams ND, Mcquarrie AS, Ediss C A novel method for the simultaneous determination of I-125 and C-14 activities using liquid scintillation counting. Journal of Radioanalytical Chemistry 65(1 2): Altzitzoglou T Analysis of triple-label samples by liquid scintillation spectrometry. Applied Radiation and Isotopes 60 (2 4): Benitez-Nelson RC, Buesseler OK Measurement of cosmogenic P-32 and P-33 activities in rainwater and seawater. Analytical Chemistry 70(1): Carles GA A new linear spectrum unfolding method applied to radionuclide mixtures in liquid scintillation spectrometry. Applied Radiation and Isotopes 45(1): Carles GA, Kossert K Measurement of the shapefactor functions of the long-lived radionuclides Rb- 87, K-40 and Be-10. Nuclear Instruments and Methods in Physics Research A 572(2): Carles GA, Barquero RL, Malonda GA. 1994a. Deconvolution of Tl-204/Cl-36 and Pm147/Ca-45 dual mixtures. Nuclear Instruments and Methods in Physics Research A 339 (1 2):71 7. Carles GA, Malonda GA, Barquero RL 1994b. Standardization of I-125, Sr-85 and Cd-109 by CIEMAT/NIST method. Applied Radiation and Isotopes 45(4): Ceccatelli A, Felice DP Standardisation of Sr-90, Ni-63 and Fe-55 by the 4 pi beta liquid scintillation spectrometry method with H-3-standard efficiency tracing. Applied Radiation and Isotopes 51(1): Hong HK, Cho HY, Lee HM, Choi SG, Lee WC Simultaneous measurement of Sr-89 and Sr-90 in aqueous samples by liquid scintillation counting using the spectrum unfolding method. Applied Radiation and Isotopes 54(2): Jähnigen P Simultanbestimmung von Beta-strahlern in Nuklidgemischen mittels Flüssigszintillationsspektrometrie [diploma thesis]. TU Dresden, Faculty of Mathematics and Natural Sciences, Department of Chemistry. Kashirin AI, Ermakov IA, Malinovskiy VS, Belanova VS, Sapozhnikov AY, Efimov MK, Tikhomirov AV, Sobolev IA Liquid scintillation determination of low level components in complex mixtures of radionuclides. Applied Radiation and Isotopes 53(1 2): Küppers G, Erdtmann G Determination of sub-ppb contents of uranium and thorium in high-purity aluminum by RNAA. Journal of Radioanalytical and Nuclear Chemistry 160(2): Miyazawa Y, Sakai N, Murakami N, Konishi T Application of simultaneous determination of H-3, C- 14, and Na-22 by liquid scintillation counting to the measurement of cellular ion-transport. Analytical Biochemistry 198(1): Nebelung C, Baraniak L Simultaneous determination of Ra-226, U-233 and Np-237 by liquid scintillation spectrometry. Applied Radiation and Isotopes 65(2): Niese S, Gleisberg B Determination of radioisotopes of Ce, Eu, Pu, Am, and Cm in low-level-wastes from power-reactors. Journal of Radioanalytical and Nuclear Chemistry 200(1): Park DS, Lee NH, Ahn JH, Kim SJ, Han HS, Jee YK Distribution of C-14 and H-3 in low level radioactive wastes generated by wet waste streams from pressurized water reactors. Journal of Radioanalytical and Nuclear Chemistry 270(3): Rodriguez BM, Tome VF, Lozano CJ, Escobar GV

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