Fumitaka
Esaka
*,
Daisuke
Suzuki
,
Takumi
Yomogida
and
Masaaki
Magara
Research Group for Safeguards Analytical Chemistry, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai, Naka, Ibaraki 319-1195, Japan. E-mail: esaka.fumitaka@jaea.go.jp; Tel: +81-29-282-6165
First published on 20th January 2016
The isotope ratio analysis of individual uranium particles in environmental samples taken at nuclear facilities is important to clarify their origins for nuclear safeguards. Secondary ion mass spectrometry (SIMS) is often used for this purpose. An automated particle measurement (APM) screening software was recently developed for SIMS instruments, which enables us to obtain scanning ion images of uranium isotopes over the sample in short duration. The positions and approximate isotope ratios of each uranium particle can be determined from the images. This makes SIMS more effective because a few uranium particles with irregular isotopic compositions among thousands of uranium particles with normal isotopic compositions can be screened prior to precise isotope ratio analysis. However, the formation of molecular and/or hydride ions often leads to spectral interferences and inaccurate results in SIMS. In the present study, APM screening was applied to select uranium particles prior to precise isotope ratio analysis by thermal ionization mass spectrometry (TIMS). As a result, it was demonstrated that the APM-TIMS method eliminated molecular ion interferences in the uranium mass region in the analysis of real inspection samples, while higher and unreasonable 234U and 236U atomic ratios for some particles were obtained by APM-SIMS.
Thermal ionization mass spectrometry (TIMS) is often utilized for this purpose in combination with a particle screening technique using fission tracks.4–8 When the sample contains uranium particles, thermal neutron irradiation results in fission tracks in a nuclear track detector. The positions of each uranium particle are identified by observing the fission tracks. Furthermore, highly 235U enriched uranium particles can be selectively sampled for subsequent isotope ratio analysis by TIMS because the number of fission tracks depends on the number of 235U atoms in each particle. The detection of highly 235U enriched uranium particles is critical to detect undeclared nuclear activities for nuclear safeguards. One drawback in the fission track-TIMS technique is that it requires a nuclear reactor to perform thermal neutron irradiation. The other is “particle mixing”, due to which an average isotope ratio is obtained when two or more uranium particles are measured at one measurement. In a previous study, we overcame this particle mixing problem by adding a microsampling process to the fission track-TIMS procedure.8
Secondary ion mass spectrometry (SIMS) is also used for the isotope ratio analysis of individual uranium particles.9–11 Since individual uranium particles are randomly analyzed in a conventional SIMS technique without screening capability, it is difficult to detect and analyze a few uranium particles with undeclared isotopic compositions among thousands of uranium particles with normal isotopic compositions. The automated particle measurement (APM) screening software developed by Hedberg and Peres et al.12,13 significantly improves the SIMS performance and enables the selective analysis of particles of interest.14,15 In APM screening, scanning ion images of isotopes such as 235U and 238U are measured over the sample using a rastered primary ion beam (e.g., 500 μm). As a result, the positions and approximate isotope ratios of each particle are identified in short duration. Precise isotope ratio analysis is then performed for individual particles by SIMS using the instrument's microprobe mode. A drawback in SIMS is that the formation of molecular and/or hydride ions produces spectral interferences in the uranium mass region. For example, 207Pb27Al+ and 208Pb27Al+ molecular ions produce peaks at m/z 234 and 235, respectively.16 The separation of 207Pb27Al+ and 234U+ ion peaks requires a mass resolution (M/ΔM) of approximately 2800. Although the separation is possible by narrowing the entrance and exit slits in small geometry SIMS (SG-SIMS) instruments, the resulting reduction in the signal intensity makes it impossible to perform precise isotope ratio analysis of uranium minor isotopes (234U and 236U) in a small particle. When large geometry SIMS (LG-SIMS) instruments are used, most of the molecular ion interferences can be eliminated due to the inherently higher transmission of secondary ions and higher mass resolution. However, the hydride ion interference of 235U1H+ on 236U+ is still present even if LG-SIMS instruments are used. From this perspective, TIMS has an advantage because the molecular and hydride ion interferences are almost negligible.
In the present study, APM screening was combined with isotope ratio analysis using TIMS and applied to the analysis of quality control and real inspection samples. The APM screening was utilized for the selection of uranium particles to be analyzed. The selected particles were sampled using a scanning electron microscope (SEM) and measured by TIMS to determine precise isotope ratios. The analytical performance of the APM-TIMS was compared with that of the APM-SIMS through the analysis of real inspection samples.
Fig. 1(a) shows 235U+ and 238U+ ion images with a raster size of 500 μm in an analytical area obtained by APM screening for an inspection sample. The presence of one uranium particle in the lower left position was indicated in these images. The CCD image of this area was obtained with a camera attached to the SIMS instrument as shown in Fig. 1(b). The sample was removed from the SIMS instrument and introduced into the SEM instrument. The positions of the analytical areas were calculated from the positions of the centers of areas (X, Y) = (01, 01), (01, 20), (20, 01) and (20, 20). By this calculation, the analytical positions were easily identified in SEM observation as shown in Fig. 1(c). The precision of the particle relocation was less than 10 μm. Since the images in Fig. 1(a) and (c) are in mirror symmetry, it is expected that one uranium particle existed in the lower right position in Fig. 1(c). The backscattered electron image of the lower right position clearly indicated the presence of the particles containing heavy elements such as uranium as shown in Fig. 1(d). The secondary electron image with a high magnification of the particles and X-ray analysis (data are not shown) showed the presence of two uranium particles as shown in Fig. 1(e). One particle shown in Fig. 1(e) was then transferred onto a filament using a micromanipulator, as shown in Fig. 1(f), to avoid particle mixing. The micromanipulator has an ability to transfer particles with a diameter of around 0.5 μm.
A TIMS instrument (TRITON; Thermo Fisher Scientific, USA) and zone-refined rhenium double filaments were used for isotope ratio measurements. In the measurement, each uranium particle identified by APM screening was transferred onto the filament with a micromanipulator. Here, no treatment was carried out to fix the particle on the filament. The evaporation filament current was continuously increased during each measurement. The measurement procedure has been described in detail previously.18 The current was increased to 5000 mA at a rate of 100 mA min−1. The acquisition times of each cycle for 234U+, 235U+, 236U+, and 238U+ were 4, 4, 4, and 2 s, respectively. Mass fractionation factors were determined for each isotope ratio by performing measurements on a sample of the CRM U350 reference material. The uncertainties in the results were estimated considering measurement variability, the certified values for the reference material, and mass fractionation corrections, following the principles described in the Guide to the Expression of Uncertainty in Measurements (GUM).19 The analytical performance of the TIMS measurement for individual uranium particles was confirmed by using an NBL CRM U050 reference material in a previous study.8
Fig. 3(a) shows the 234U, 235U and 236U atomic ratios measured by APM-SIMS for the quality control sample. In the analysis of 12 individual particles, 7 natural uranium, 2 low 235U enriched uranium, and 2 highly 235U enriched uranium particles were found. Furthermore, one particle with a 235U atomic ratio of 1.1226(44) was observed, which suggested the mixing of some uranium particles with different atomic ratios. The 236U atomic ratios for natural uranium were slightly different from the reference value, probably due to the insufficient sensitivity. In the measurement by APM-TIMS, 4 natural, 6 low 235U enriched, and 2 highly 235U enriched uranium particles were detected as shown in Fig. 3(b). No particle mixing effects were observed because of the microsampling of individual uranium particles prior to the isotope ratio analysis by TIMS. No highly 235U enriched uranium particles were detected in SIMS without APM screening as shown in Fig. 3(c). These results indicated the excellent performance of APM screening for the detection of a smaller number of particles of interest.
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Fig. 7 A scanning electron microscopy image and an EDX spectrum of a uranium particle in the inspection sample C. This particle had a high 236U atomic ratio (0.0248(29)) in APM-TIMS as shown in Fig. 6(b). |
In the present study, an SG-SIMS instrument was used for the isotope ratio analysis of individual uranium particles. Ranebo et al. compared the analytical performance between SG-SIMS and LG-SIMS instruments.16 They reported that the LG-SIMS instrument produces excellent quality analytical data due to the resolution of almost all molecular ion interferences. Therefore, most of the molecular ion interferences observed in Fig. 4–6 could potentially be avoided if an LG-SIMS instrument was applied to the measurements. Ranebo et al. also compared the performance between LG-SIMS and TIMS instruments and reported that analyses using the LG-SIMS instrument had a limitation in the detection limit of 236U at higher enrichments due to the necessity for a hydride correction.16 The lack of a hydride correction requirement in TIMS analysis is a clear advantage compared to analysis with LG-SIMS instruments.
Particle mixing was observed in the APM-SIMS analysis of the quality control sample in Fig. 3(a). The process of microsampling each uranium particle under SEM observation (as shown in Fig. 1(e) and (f)) helped to avoid particle mixing in APM-TIMS. The distance between two uranium particles was only 3 μm in Fig. 1(e). It should be noted that it would be difficult to avoid particle mixing in this case even if a focused primary ion beam is used for the analysis using SG-SIMS or LG-SIMS instruments.
As mentioned above, reliable data from the analysis of individual uranium particles can be efficiently obtained by APM-TIMS. However, there was a disadvantage because additional work was required for the microsampling of individual particles under SEM observation. Typically, one day is the adequate time required to analyze the isotope ratios of 20 individual uranium particles by SIMS in the microprobe mode after APM screening. In APM-TIMS, one or two days are necessary to first identify the positions of each uranium particle and then to transfer the particles onto each filament after APM screening. Three additional days are then necessary to perform the isotope ratio analysis of 20 individual particles by TIMS. Therefore, the proposed method should be applied only to the analysis of samples containing a large number of uranium particles, which may have caused particle mixing. The analysis by APM-SIMS is effective for the samples containing a smaller number of uranium particles, which has a lower probability of particle mixing. The APM screening results can provide information on which method would be appropriate for each sample.
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