Application of automated particle screening for effective analysis of individual uranium particles by thermal ionization mass spectrometry

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 APMTIMS method eliminated molecular ion interferences in the uranium mass region in the analysis of real inspection samples, while higher and unreasonable U and U atomic ratios for some particles were obtained by APM-SIMS.


Introduction
][3] For the implementation of nuclear safeguards, environmental swipe samples are taken at nuclear facilities by inspectors of the International Atomic Energy Agency (IAEA) and the isotope ratios of uranium particles are measured to detect undeclared nuclear activities and facilities.Approximately 20 individual particles for one swipe sample are usually measured by mass spectrometry.
5][6][7][8] When the sample contains uranium particles, thermal neutron irradiation results in ssion tracks in a nuclear track detector.The positions of each uranium particle are identied by observing the ssion tracks.Furthermore, highly 235 U enriched uranium particles can be selectively sampled for subsequent isotope ratio analysis by TIMS because the number of ssion tracks depends on the number of 235 U atoms in each particle.The detection of highly 235 U enriched uranium particles is critical to detect undeclared nuclear activities for nuclear safeguards.One drawback in the ssion 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 ssion track-TIMS procedure. 80][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 soware developed by Hedberg and Peres et al. 12,13 signicantly improves the SIMS performance and enables the selective analysis of particles of interest. 14,15In APM screening, scanning ion images of isotopes such as 235 U and 238 U are measured over the sample using a rastered primary ion beam (e.g., 500 mm).As a result, the positions and approximate isotope ratios of each particle are identied 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, 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 ( 234 U and 236 U) 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 235 U 1 H + on 236 U + 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.

Experimental
Samples A quality control sample and multiple real inspection samples taken at nuclear facilities were used in this study.The quality control sample contained natural (0.72%), low 235 U enriched (3.8%) and highly 235 U enriched (61%) uranium particles with an approximate number ratio of 170 : 18 : 1.The particles in these samples were recovered onto glassy carbon planchets with a diameter of 25 mm (Hitachi Chemical Co.Ltd.) by a vacuum impactor technique 17 for APM-SIMS analysis.And then, particles were also recovered from the same samples onto other planchets for APM-TIMS analysis.Here, the surface of each planchet was coated with 5 mL of a 1 : 1 mixture of eicosane and nonadecane in hexane.Aer the particle recovery, the planchet was heated at 150 C for 90 seconds.The NBL CRM U350 reference material was used to determine the mass fractionation factors in the SIMS and TIMS measurements.

Particle screening and sample preparation
To select uranium particles to be analyzed, APM screening was performed using an SG-SIMS instrument (IMS-6F, CAMECA).A focused O 2 + primary ion beam with a current of 80 nA was rastered over an area of 500 Â 500 mm 2 .The 235 U + and 238 U + ion images were obtained for 20 Â 20 areas (i.e., 10 Â 10 mm 2 ).The measurement times of 235 U + and 238 U + were each set to 10 s.
Prior to the measurement, each area was pre-sputtered by a primary ion beam with a current of 500 nA for 3 s.The total acquisition time for one sample was approximately 240 min.Aer the measurement, the center of each of the areas (X, Y) ¼ (01, 01), (01, 20), (20, 01), and (20, 20) was marked with a primary ion beam with a current of 80 nA for 5 min.These marks were used to calculate the XY positions of each area in SEM instruments (JEOL, JSM-6700F and 7800F).Fig. 1(a) shows 235 U + and 238 U + ion images with a raster size of 500 mm in an analytical area obtained by APM screening for an inspection sample.The presence of one uranium particle in the lower le 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) ¼ (01, 01), (01, 20), (20, 01) and (20, 20).By this calculation, the analytical positions were easily identied in SEM observation as shown in Fig. 1(c).The precision of the particle relocation was less than 10 mm.Since the images in Fig. 1 presence of the particles containing heavy elements such as uranium as shown in Fig. 1(d).The secondary electron image with a high magnication 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 lament 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 mm.

Isotope ratio analysis
The isotope ratio analysis of individual uranium particles was performed by SIMS and TIMS.In SIMS, each uranium particle identied by APM screening was measured with a focused O 2 + primary ion beam with a current between 0.5 and 5.0 nA and a raster size of 30 mm.The acquisition times of each cycle for 234 U + , 235 U + , 236 U + , 238 U + and 238 UH + were 4, 2, 4, 2 and 4 s, respectively.Here, the contributions of 235 U 1 H + ions to 236 U + ions at m/z 236 were corrected using 238 U 1 H + / 238 U + intensity ratios in SIMS, although no hydride corrections were made in TIMS.Mass fractionation factors were determined for each isotope ratio by performing measurements on a sample of the NBL CRM U350 reference material.
A TIMS instrument (TRITON; Thermo Fisher Scientic, USA) and zone-rened rhenium double laments were used for isotope ratio measurements.In the measurement, each uranium particle identied by APM screening was transferred onto the lament with a micromanipulator.Here, no treatment was carried out to x the particle on the lament.The evaporation lament current was continuously increased during each measurement.The measurement procedure has been described in detail previously. 18The current was increased to 5000 mA at a rate of 100 mA min À1 .The acquisition times of each cycle for 234 U + , 235 U + , 236 U + , and 238 U + 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 certied values for the reference material, and mass fractionation corrections, following the principles described in the Guide to the Expression of Uncertainty in Measurements (GUM). 19The analytical performance of the TIMS measurement for individual uranium particles was conrmed by using an NBL CRM U050 reference material in a previous study. 8

Analysis of a quality control sample
A quality control sample with a mixture of natural, low 235 U enriched, and highly 235 U enriched uranium particles was measured by SIMS, APM-SIMS and APM-TIMS.A result of APM screening is shown in Fig. 2. The 238 U signal intensities were plotted against the 235 U/ 238 U isotope ratios.Main populations of the 235 U/ 238 U isotope ratios at 0.0072 and 0.038 were observed for 904 uranium particles found.The maximum 235 U/ 238 U isotope ratio was 1.54, which corresponded to approximately 61% 235 U enriched uranium.
Fig. 3(a) shows the 234 U, 235 U and 236 U atomic ratios measured by APM-SIMS for the quality control sample.In the analysis of 12 individual particles, 7 natural uranium, 2 low 235 U enriched uranium, and 2 highly 235 U enriched uranium particles were found.Furthermore, one particle with a 235 U atomic ratio of 1.1226(44) was observed, which suggested the mixing of some uranium particles with different atomic ratios.The 236 U 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 235 U enriched, and 2 highly 235 U 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 235 U 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.

Analysis of inspection samples
Three inspection samples (A, B, and C) from nuclear facilities were measured by APM-SIMS and APM-TIMS.The 234 U and 236 U atomic ratios were plotted against the 235 U atomic ratios in Fig. 4-6.The average value of the 238 U 1 H + / 238 U + intensity ratios for these samples was 1.6 Â 10 À3 in APM-SIMS.The contribution of 235 UH + signals on signals at m/z 236 for each particle was estimated to be 55% as the average.An apparent correlation between the 234 U and 235 U atomic ratios in sample A was observed for both techniques (Fig. 4).The linear relationship between 234 U and 235 U is typical for samples taken at uranium enrichment facilities.However, one particle had relatively high 234 U (0.1196( 16)) and 236 U (0.0818(59)) atomic ratios based on the APM-SIMS results.This was presumably due to the spectral interferences by molecular ions, which would be produced from the elements in neighbouring particles.Such unreasonable ratios were not observed in APM-TIMS, indicating that this technique can provide data without molecular ion interferences.The highest 235 U atomic ratio in APM-TIMS was lower than that in APM-SIMS.This would be due to the sample preparation, because the particles for APM-SIMS measurement were rst recovered from the sample.The results obtained from sample B also showed an apparent correlation between the 234 U and 235 U abundance ratios for both techniques (Fig. 5).Similarly, some particles produced relatively high and unreasonable 234 U and 236 U atomic ratios in APM-SIMS.The 236 U atomic ratios obtained for sample C by APM-SIMS (Fig. 6) were relatively higher than the ratios obtained for sample A by APM-SIMS (Fig. 4).The SEM-EDX analysis conrmed that most of the uranium particles in sample C contained uranium, oxygen, and other impurities such as Na, Mg, Al, Si, S, Cl, Ca, Ti, Fe and Zn, whereas the uranium particles in samples A mainly comprised only uranium and oxygen.Therefore, the impurities in each uranium particle were considered to have produced molecular ions that resulted in higher 236 U atomic ratios in APM-SIMS.Most of the measured particles produced reasonably low 236 U atomic ratios in APM-TIMS, while one particle produced a 236 U abundance ratio (0.0248(29) in Fig. 6(b)).The reason for the high 236 U atomic ratio was unclear.Since the uranium particle mainly comprised uranium and oxygen (Fig. 7), the result suggested that the high atomic ratio was due to molecular ion interferences by organic compounds or impurity elements in the TIMS lament.
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. 16They 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 236 U at higher enrichments due to the necessity for a hydride correction. 16The 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 mm 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 aer APM screening.In APM-TIMS, one or two days are necessary to rst identify the positions of each uranium particle and then to transfer the particles onto each lament aer 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.

Conclusions
The screening by APM was performed prior to the isotope ratio analysis of individual uranium particles by TIMS.The results indicated that a few uranium particles of interest among many uranium particles were able to be selected prior to the analysis by TIMS.Furthermore, molecular ion interferences were almost completely avoided using a TIMS instrument instead of an SG-SIMS instrument.Therefore, the results showed that APM-TIMS was effective for the isotope ratio analysis of individual particles.Since the screening by APM required no thermal neutron irradiation in a nuclear reactor, particles that contained no ssile materials can be identied.Therefore, APM-TIMS as well as APM-SIMS can be applied to the analysis of various elements in individual particles in the environment, which will open up new research elds in environmental sciences.
Fig. 1 (a) The 235 U + and 238 U + ion images in an analytical area obtained by APM screening for an inspection sample.(b) A CCD and (c) secondary electron image of the area.(d) A backscattered electron image of the area noted as a small square in (c), and (e) a secondary electron image of two uranium particles in the area.(f) Transfer of the large uranium particle in (e) onto a filament for isotope ratio analysis by TIMS.

Fig. 2
Fig. 2 An APM screening result measured for a quality control sample with a mixture of natural, low 235 U enriched, and highly 235 U enriched uranium particles.The 238 U signal intensities of 904 uranium particles were plotted against the 235 U/ 238 U isotope ratios.The data with 235 U signal intensities below 1 count per s were filtered.

Fig. 3
Fig. 3 Uranium atomic ratios of individual particles in a quality control sample with a mixture of natural, low 235 U enriched, and highly 235 U enriched uranium particles measured by (a) APM-SIMS, (b) APM-TIMS and (c) SIMS without APM screening.The intersections of the crossed lines represent the reference values of each particle.The error bars represent the expanded uncertainties with a coverage factor of k ¼ 2.

Fig. 4
Fig. 4 The uranium atomic ratios of individual particles in the inspection sample A measured by APM-SIMS and APM-TIMS.The (a) 234 U and (b) 236 U atomic ratios are plotted against the 235 U atomic ratios.The error bars represent the expanded uncertainties with a coverage factor of k ¼ 2.

Fig. 5
Fig. 5 The uranium atomic ratios of individual particles in the inspection sample B measured by APM-SIMS and APM-TIMS.The (a) 234 U and (b) 236 U atomic ratios are plotted against the 235 U atomic ratios.The error bars represent the expanded uncertainties with a coverage factor of k ¼ 2.

Fig. 6
Fig. 6 The uranium atomic ratios of individual particles in the inspection sample C measured by APM-SIMS and APM-TIMS.The (a) 234 U and (b) 236 U atomic ratios are plotted against the 235 U atomic ratios.The error bars represent the expanded uncertainties with a coverage factor of k ¼ 2.

Fig. 7 A
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 236 U atomic ratio (0.0248(29)) in APM-TIMS as shown in Fig. 6(b).