1 IAEA-CN-184/177 Method development for analysis of single hot particles in Safeguards swipe samples Zs. Mácsik 1, N. Vajda 2, É. Széles 1, R. Katona 1 1 Institute of Isotopes, Hungarian Academy of Sciences, Budapest, Hungary 2 RadAnal Ltd., Budapest, Hungary macsikzsu@gmail.com Abstract. A method consists of several, individual procedures has been developed for the particle analysis of Safeguards swipe samples. The paper introduces the present state of the on-going development. For the identification and localization of hot particles, solid state nuclear detectors (CR-39) were used. The location of particles containing alpha emitting materials was determined with an accuracy of better than 20 μm in case of particles with 80...120 μm size. The described method offers the opportunity of micro-manipulation and the examination of individual particles by scanning electron microscopy (SEM). The U isotopic composition of the particles of interest was determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 1. Introduction Swipe sampling has been routinely used as an effective inspection tool by the International Atomic Energy Agency (IAEA) since 1996 to verify the absence of undeclared nuclear materials and activities [1]. Bulk and particle analysis can be applied for the determination of actinides in swipe samples as well. Bulk analysis provides information only about the average composition of the samples however, the artificial fissile material in the environment mostly occurs in the form of solid, radioactive particles surrounded by a large number of particles containing fissile material of natural origin. Fissile and fertile materials (U, Pu, Th) and their transmutation products (Np, Am, Cm) in localized and micro-manipulated hot particles originating from swipe samples can be examined with high sensitivity excluding the effect of the sample as matrix. Therefore the use of particle analysis for Safeguards swipe samples is essential to achieve as accurate results as possible. The information about single hot particles (such as isotope ratios of actinides) given by particle analysis offers a great challenge. According to the results obtained by the analysis of different single particles, various origin scenarios of particles can be distinguished in the same swipe sample. Moreover, the more precise information can improve the verification process of IAEA Safeguards. A novel analytical method has been developed for the determination of actinides (Th, U, Np, Pu and Am) in hot particles. In the present paper, procedures developed and adapted for the identification of hot particles using solid state nuclear track detectors, localization and micro-manipulation of the identified particles, examination of particles by SEM and analysis of the U content using LA-ICP-MS are to be discussed. 2. Experimental 2.1. Samples Test particles containing U and Pu were made of anion exchange resin (BioRad Ltd., USA) beads to simulate hot particles for testing the adapted and developed procedures. Uranyl chloride (Reanal Ltd., Budapest) and plutonium nitrate ( 239 Pu standard reference material produced by CERCA LEA, Framatome Ant, France) were loaded onto anion exchange resins to produce U and Pu test particles, respectively. Each anion exchange bead with an average diameter of 140 μm contained approximately 13 mbq 238 U (1 μg U) or approximately 43 mbq (19 pg) 239 Pu.
Particles originating from confiscated nuclear fuel pellets and monazite sand particles containing Th originating from Sri Lanka were also analyzed. Environmental as well as nuclear samples were investigated since these types of materials can occur in Safeguards swipe samples. Real Safeguards samples in the phase of development have not been analyzed yet. The size of the investigated particles varied between 80...120 μm, 150...200 μm and 40...210 μm in case of test, monazite and pellet particles, respectively. 2.2. Applied procedures 2.2.1. Identification of particles containing alpha emitting material using solid state nuclear track detectors The procedure for the identification of particles containing alpha emitting material is based on alpha track analysis (Fig. 1) using CR-39 type solid state nuclear track detector (RadoSys Ltd., Hungary). Nuclear track detectors were placed on the sample holders containing particles. The detectors were exposed to the samples for several days depending on the type of the sample. After unpacking the samples, the detectors were rinsed with ethyl alcohol, etched in 6.25 M NaOH at 90 ± 3 ºC for 4 h, rinsed with distilled water and dried in air. Alpha tracks were observed with optical microscopes (the optical microscope of New Wave Research UP-213 laser ablation system and Zeiss Axio Imager.M1m optical research microscope (Zeiss, Germany) equipped with AxioCam MRc 5 video camera). Figure 1: A typical particle originating from nuclear fuel pellet and the image of the alpha track formed by it (in the red cross-hairs) 2.2.2. Localization and re-localization of the identified particles The exact location of single particles containing alpha emitting materials is needed for further examination. The so-called 6-point algorithm calculation method was used for the determination of the particle coordinates in the coordinates system of the given microscope [2]. Three reference marks were placed on the sample holder and three reference marks were defined on the nuclear track detectors, as well. According to the coordinates of the reference marks and the coordinates of the nuclear tracks, the coordinates of the particles can be calculated. In case of using different equipment, the re-location of the given particle is necessary. The so-called 3-point algorithm calculation method [2] was used to determine the coordinates of the particle in the coordinates system of the new equipment. 2.2.3. Micro-manipulation of the localized particles and examination by SEM-EDX (scanning electron microscopy combined with energy dispersive X-ray spectrometry) Some of the localized particles had to be removed from the original sample holder because either the particle density was too high or the original sample holder was not appropriate for the examination by SEM-EDX (JEOL JSM-5600LV SEM equipped with energy dispersive X-spectrometer (EDS 2000, IXRF Systems Inc., USA)). The micro-manipulation was carried out using the Zeiss Axio Imager.M1m optical microscope (Zeiss, Germany) equipped with a micro-manipulator operating system (Mitutoyo, Japan). Steel needles were used as micro-manipulator. Either the phenomenon of static electricity or liquid glue (Loctite Super Attack Power Gel) was used to remove the particles depending on their size. Morphological examination and the determination of the main components of the particles were executed with SEM-EDX. These examinations were used in the optimization of the laser ablation parameters.
Intensity [cps] 2.2.4. Determination of the U content of the localized particles by LA-ICP-MS The localized particles were analyzed by LA-ICP-MS. The mass spectrometric analysis was carried out using an Element 2 ICP-MS (Thermo Electron Corp., Germany) with magnetic sector field and single electron multiplier. The laser ablation was carried out using an UP-213 laser ablation system (New Wave Research, USA) equipped with a Nd:YAG laser at a wavelength of 213 nm. A NIST 612 glass reference material (NIST, USA) was used for the optimization of the LA-ICP-MS system. The achieved sensitivity was approximately 5 10 4 cps for 37.1 mg/kg 238 U. The mass bias factor was determined by the measurement of the 235 U/ 238 U ratio of the NIST 612 glass reference material and a highly enriched U-oxide pellet (from an interlaboratory exercise of Nuclear Smuggling International Technical Working Group (ITWG), 2001). Low (R=300) and medium (R=4000) mass resolutions were applied. The used laser ablation technique was based on the method described by Z. Varga [3]. Spot ablation (spot size (laser beam diameter): 40 or 60 μm, laser energy flux: 0.04 mj/cm 2, repetition time: 10 Hz) was used for the laser ablation. The intensities of the U isotopes were recorded in the chromatographic mode of the ICP-MS during the laser ablation of the particles (Fig. 2). Three measurements were applied on each sample. The background level was measured in 20-25 measuring points by starting the record of the chromatograms before starting the laser ablation. The U isotope ratios ( 235 U/ 238 U, 234 U/ 238 U, 236 U/ 238 U) were calculated from the average of a given time interval of the U isotope signals after the background correction. The U isotopic composition (the mass ratio of one isotope relative to the sum of all isotopes of the element) of the particles was evaluated, as well. The results for particles originating from the U oxide pellets were compared to the results of an analysis done in our laboratory by LA-ICP-MS measurement of the whole pellets (The whole pellet was placed in the chamber of the laser ablation system and line scan ablation was applied during the ICP-MS measurements of the samples.). 100000000 10000000 1000000 100000 10000 1000 100 10 1 0 10 20 30 40 50 60 70 80 90 100 110 Time [s] U234 (LR) Low Intensity [cps] U235 (LR) Low Intensity [cps] U238 (LR) Low Intensity [cps] U236 (LR) Low Intensity [cps] U233 (LR) Low Intensity [cps] Figure 2: The chromatogram of a LA-ICP-MS measurement of the particle in Fig. 1 3. Results and discussion 3.1. Identification and localization of hot particles The parameters of alpha track analysis (exposure time, etching time, etching temperature) were optimized to achieve as precise localization as possible. In case of single particles the nuclear tracks form clusters. The exposure time is needed to allow for each cluster to have enough single tracks to be able to define the centre of the cluster. The optimal exposure time depends on the size of the particle and the alpha emitting material content. For example 5 days and 1 day were the optimal exposure times in case of U and Pu test samples, respectively. The alpha energies of actinides (U, Pu and Th) vary between 3.9 and 6.0 MeV. Nanometer size alpha tracks
produced by them can be enlarged to a diameter of approximately 30 μm applying the above mentioned etching parameters. The calculation methods for localization and re-localization of the identified particles were also tested. Test particles were fixed onto sample holders and alpha track analysis was executed. Six-point algorithm was used to calculate the coordinates of the particles, and then the centre of the particle was determined. The difference between the calculated and the exact coordinates was defined as the precision of the calculation method. In case of re-location the same sample holders were examined in different positions or instruments. The achieved precision of the 6- and the 3-point algorithm was 10±7 μm and 7±6 μm, respectively in case of 80...120 μm particle size. 3.2. Examination of the localized particles by SEM-EDX Besides the morphological examinations, the main components of the monazite sand and test particles were determined by SEM-EDX. Thorium could be unambiguously determined only in monazite sand particles that produced nuclear track clusters with more than 1000 single tracks. In the monazite sand particles that produced nuclear track clusters with less single tracks, Th was a trace component. Uranium and Pu test particles were also analyzed. Plutonium could not be determined since each particle had less than 0.00001% of Pu content. Uranium could be identified in the X-ray spectrum. According to the results the U content of the test particles was approximately 17%...23%. 3.3. Measurement of U content by LA-ICP-MS The described method was applied to U test and nuclear fuel pellet particles. Natural isotopic U composition was predicted in case of U test particles since natural composition of the uranyl chloride was assumed. Low and medium mass resolutions were also applied during the LA-ICP-MS measurements. Typical results are shown in Table 1 and compared with the values of the isotopic composition of natural U recommended by IUPAC (International Union of Pure and Applied Chemistry) [4]. The results obtained by using medium mass resolution agreed well with the values of IUPAC. In case of low mass resolution, the isotope ratio of 235 U/ 238 U (0.0079 ± 0.0001) was significantly higher than that of natural U. This caused major bias in the isotopic compositions of results obtained by the measurements using low mass resolution. Spectral interferences at the mass range of 235 U could cause the significantly higher 235 U/ 238 U ratio. Table 1: The calculated isotopic composition of the U test particles Test Sample A Test Sample A IUPAC Low Resolution Medium Resolution [%] [%] [%] 234 U 0.0055±0.0005 0.0061±0.0007 0.0054±0.0005 235 U 0.7200±0.0012 0.7879±0.0092 0.7406±0.0190 238 U 99.2745±0.0060 99.2060±0.0092 99.2540±0.0190 The calculated U isotopic composition of the nuclear fuel pellet particles are shown in Table 2 and compared with the results originating from the LA-ICP-MS measurement of the whole pellets (marked us ). Good agreement can be observed between them. The lateral dimensions of the analyzed particles are also listed in the table. According to the measured 235 U/ 238 U isotope ratios (Sample A: 0.00294 ± 0.00013, Sample B: 0.00724 ± 0.00004, Sample C: 0.0257 ± 0.0016), depleted (DU), natural (NU) and low enriched (LEU) U could be distinguished.
Table 2: The calculated isotopic composition of the nuclear fuel pellet particles Sample A Sample A (DU) Sample B Sample B (NU) Sample C Sample C (LEU) [%] [%] [%] [%] [%] [%] 234 U 0.0017±0.0001 0.00145±0.0001 0.00524±0.0001 0.00480±0.0002 0.036±0.001 0.0335±0.0026 235 U 0.2883±0.0071 0.2866±0.0146 0.7083±0.0043 0.7189±0.0074 2.529±0.019 2.494±0.302 236 U n.m. 1 0.00661±0.00005 n.m. 1 (1.7±0.004) 10-4 0.474±0.024 0.401±0.050 238 U 91.7100±0.0072 99.7053±0.0145 99.2865±0.0044 99.2762±0.0074 96.961±0.020 97.072±0.354 (43 55 μm) (100 200 μm) (30 30 μm) 1 : not measured The results are calculated at 95% confidence level. 4. Conclusions The method developed for the analysis of single hot particles in Safeguards swipe samples consist of several, individual procedures. Particles containing alpha emitting materials can be identified using solid state nuclear detectors. The location of the identified particles can be determined with an accuracy of better than 20 μm in case of particles with 80...120 μm size. The method offers the opportunity of micromanipulation and simultaneous examination of the particles by SEM. The main components of the particles obtained by examination with SEM-EDX can help optimize the parameters of the LA-ICP-MS system. E.g. in case of detection of nuclides which can form species (e.g. molecule ions) responsible for spectral interferences at m/z=234, 235, 236 and 238, the use of medium mass resolution mode of ICP-MS can be recommended (instead of low mass resolution). Thus the U isotope ratios and isotopic composition of the particles of interest can be determined by LA-ICP-MS with high accuracy. The procedure has to be further developed for the effective removal of particles from swipe samples and the LA-ICP-MS method has to be extended for the determination of Pu isotope ratios and isotopic composition. Further developments are also needed to extend the method for particles under 10 μm size. Acknowledgements This study was financially supported by the Hungarian Atomic Energy Authority. Tamas Biro (Institute of Isotopes of the Hungarian Academy of Sciences) and Zsolt Stefanka (Hungaian Atomic Energy Authority) are thanked for helpful discussions. The authors are grateful to Erik Hulber (RadoSys Ltd, Hungary) for the nuclear track detectors and Peter Hargittai for the examination by SEM-EDX. References [1] Donohue, D. L. Strengthening IAEA safeguards through environmental sampling and analysis, Journal of Alloys and Compounds, 271-273, 11-18, 1998 [2] Admon U., Single particles handling and analyses; Chapter of Radioactive Particles in the Environment book, Springer Publisher, 2007, ISBN 978-90-481-2949-2 (e-book) [3] Varga Zs., Application of laser ablation inductively coupled plasma mass spectrometry for the isotopic analysis of single uranium particles, Analytica Chimica Acta, Vol. 625, pp. 1-7, 2008 [4] IUPAC, Isotopic compositions of the elements, 1989, Pure &Appl. Chem., Vol. 63, No. 7, pp. 991-1002,1991.