Article pubs.acs.org/ac
Age Dating of Individual Micrometer-Sized Uranium Particles by Secondary Ion Mass Spectrometry: An Additional Fingerprint for Nuclear Safeguards Purposes Anne-Laure Fauré* and Thomas Dalger Commissariat à l’Energie Atomique DAM-DIF, 91297 Arpajon Cedex, France ABSTRACT: A direct and simultaneous analysis of the age and isotopic composition of nuclear material at the particle scale is described in this study. By comparison with other conventional techniques such as inductively-coupled plasma mass spectrometry or thermal-ionization mass spectrometry, secondary ion mass spectrometry enables one to determine the ages of individual particles in a mixture of nuclear materials. Having access to the purification date can give precious information on the history of a nuclear facility or nuclear material for safeguarding purposes. The high sensitivity of this technique combined with its imaging capabilities enables one to detect and to sort out all particles according to their isotopic composition in one analysis. The succession of two microbeam analyses on an individual particle allows the determination of the precise 235U abundance and the model age. The methodology was successfully applied to a mixture of uranium particles coming from certified reference material with a abundance ranging from 10% to 97%: the accuracy on the 235U abundance is greater than 0.5% and the accuracies on the abundance and the model age are better than 0.5% and 3%, respectively.
235
U U
by finding a technique that can detect, isolate, and date the different components of such sample. To that end, particle analysis was developed in the 1990s for Safeguards purposes and aimed to characterize particulate matter coming from nuclear facilities.5 These particles can be collected by wiping smooth surfaces at various locations inside the facility using square pieces of cotton cloths, referred to as “swipe samples”. Because particle isotopic and elementary compositions are representative of the original material, these particles can be considered as fingerprints of specific processes of the nuclear industry.6 Until now, particle analysis mainly focused on the determination of the isotopic composition of uranium.7−10 During the past 15 years, other applications were developed in order to obtain more information on these uranium particles, such as oxygen isotopic composition, which can be used as an identification tool of their origin.11 Also, fluorine measurements were developed, which combined with uranium isotopic measurement, is an efficient tool to detect a signature for the conversion process.12 Other studies have described attempts to determine the age of uranium or plutonium particles.13−16 However, the lack of a reference material in these studies makes it difficult to apply to uranium age dating. The purpose of this work is to develop a direct analysis that simultaneously gives access to the age of uranium particles (by means of 230Th−234U chronometer) and to the isotopic
A
ge determination of nuclear material is an important tool that is relevant for a nuclear forensics investigation or for nonproliferation studies. In the nuclear field, the “age” refers to the latest purification date. When nuclear matter is purified, the elements are separated from uranium. Two chronometers are of major interest to measure the age of uranium: 230Th/234U and 231 Pa/235U. The statement that the measured age corresponds to the purification date can be established under the following assumptions: the completeness of the purification of the uranium from its daughter at time zero and the absence of fractionation since production. Because of these assumptions, the “ages” measured by the use of such chronometers are called “model ages”. Inductively-coupled plasma mass spectrometry (ICP-MS) and thermal-ionization mass spectrometry (TIMS) are the reference techniques used to obtain “model ages”.1,2 Those two techniques efficiently measure the age of very young and low-enriched uranium matter.3 To obtain high performances, nuclear material must be dissolved and then purified to separate the different species (U and Th) by ion-exchange or extraction chromatography. All of these techniques require ponderable amounts of sample. Age dating of bulk uranium, in the tens-of-micrograms range, was successfully carried out in the context of a round-robin exercise.4 Nevertheless, the principal limitation of such techniques is the impossibility to determine the individual ages of a sample which consists of a mixture of nuclear materials of different ages. The measured age would be a mean of the ages of the different populations of nuclear materials, leading to a false value that would not match the real composition of the sample. This problem can be solved © 2017 American Chemical Society
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Received: March 10, 2017 Accepted: May 13, 2017 Published: May 13, 2017 6663
DOI: 10.1021/acs.analchem.7b00887 Anal. Chem. 2017, 89, 6663−6669
Article
Analytical Chemistry composition of these micrometric particles. To evaluate the accuracy of our method, particles coming from certified reference material and with a known purification date have been used. Secondary ion mass spectrometry (SIMS) is an adequate technique for such an analysis because of its high sensitivity and high lateral resolution. Moreover, SIMS is the only technique that can provide both particle location and isotopic measurements within one instrument. The automatic search for U particles by SIMS has been greatly improved by the development of the Automatic Particle Measurement software (APM) by CAMECA, which aimed at performing a two-dimensional image scan over a large surface to locate particles of interest at selected masses and to make a first assessment of their isotopic composition.17 The main goals of this study can be summarized as follows: (1) Determine the performance of a SIMS age-dating measurement on individual particles of a homogeneous material. The methodology has been developed on a smallgeometry SIMS (SG-SIMS). (2) Demonstrate the potential of SIMS for the determination of the ages of individual particles coming from a mixture of different nuclear materials. (3) Discuss the limitations of the methodology and compare the theoretical performances of SG-SIMS and large-geometry-SIMS (LG-SIMS). The latter is also a double-focusing mass spectrometer, but with a larger-radius magnetic sector. The implementation of this larger sector has two main consequences: a higher mass-resolution power without the loss of transmission and the possibility to equip the instrument with a multicollector device.
Table 1. Characteristics of the Uranium Reference Materials Used in This Studya
CRM
%at 234U (×10−2)
U100 U850 U900 U970
6.76 ± 0.02 64.37 ± 0.14 77.77 ± 0.15 166.53 ± 0.17
%at
235
U
10.190 ± 0.010 85.137 ± 0.0017 90.196 ± 0.011 97.663 ± 0.003
reference date20
estimated 230 Th amount per particle (ag)
8-Jan-1959 31-Dec-1957 24-Jan-1958 March 1965
0.2−10 −1.6−69 −2.0−85 −3.5−154
a Only 234U and 235U abundances are reported in this table. Reference date corresponds to the date when purification of the material was completed.20
using a SEM XL-30 (FEI, U.S.A.) with a U3O8 density (8.3 g · cm−3),21 assuming a porosity of 30% of the particle. The estimated porosity was determined by ablating some uranium particles by focused- ion beam-SEM. Within these parameters, the equivalent spherical diameters range from 0.8 to 2.8 μm. Because size distributions of the other reference materials were comparable, similar equivalent spherical diameters were used to estimate the amount of 230Th in individual U100, U900, and U970 particles. Secondary Ion Mass Spectrometry. SIMS measurements were performed with a double-focusing Cameca IMS 7f (Gennevillilers, France). This instrument is equipped with an oxygen ion source called a duoplasmatron. A primary ion beam of oxygen ions is typically used to enhance the production of electropositive ions such as U+. All measurements described in this paper were carried out with the O2+ primary beam accelerated to 15 keV. The positive secondary ions were accelerated through 5 keV. The energy band-pass was 75 eV. The contrast and field apertures were 400 and 1800 μm in diameter, respectively, and the field of view was 75 μm. The mass-resolving power was set to 400 to obtain flat-top peaks and a good sensitivity, which improves the accuracy and the reproducibility of the measurements. The ion species were recorded in a magnet-peak jump sequence using a singleelectron multiplier pulse-counting detector. Dead-time was experimentally determined to be 32 ns. SIMS Analytical Protocol. The first protocol corresponds to the analysis of carbon planchets containing particles coming from only one CRM. Uranium-bearing particles are located with a primary beam intensity of 150 nA and a raster of 500 × 500 μm at mass to charge ratio, m/z, corresponding to 238U+. Once a particle was located, the age measurement was performed using microbeam conditions, which consists of focusing the primary beam onto the particle and recording the ion intensities of 230Th+ and 234U+. When possible, the primary beam intensity was set to obtain a 234U+ signal close to 104 counts per second (cps) in order to obtain a 230Th+ signal above 1 cps. However, for U100 particles, such a 234U+ signal was never obtained. For this specific case, the primary beam intensity was set at 5 nA without any raster, which was found to be a good compromise between the secondary intensities and the duration of the analysis. The second protocol was used to analyze a carbon planchet containing particles coming from three different CRM. The first step consists of locating the particles and estimating their 235U abundance. This is carried out using the APM software which allows the recording of 235U+ and 238U+ images in a series of
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INSTRUMENT AND METHODS Materials and Sample Preparation. Model ages of several certified reference materials (CRM) coming from the New Brunswick Laboratory (DOE, U.S.A, formerly National Bureau of Standards) were measured by Williams et al. in 2009.18 Except for U005-A and U030-A, all other model ages agree with the purification dates. In order to measure the 230 Th+ signal on an individual particle, only the more-enriched CRM were used in this study: U100, U850, U900, and U970. For each CRM, particles were deposited on a cotton cloth (TX 303, Texwipe, NC, U.S.A.) by means of a plastic tip. The particles were then transferred onto a carbon planchet (grade FP2584 Schunk Electrographite SAS, Germany) inside a glovebag filled with nitrogen using the vacuum impactor technique.19 This method consists of impacting particles on a carbon planchet previously covered with an organic compound, polyisobutylene in nonane, which acts as a sticky agent. Two kinds of samples were prepared. First, each carbon planchet contained particles coming from only one CRM to evaluate the performance of the methodology. Second, particles coming from three different CRM were deposited onto the same carbon planchet. The second type of sample was aimed at demonstrating that individual model ages can be measured from a mixture of different particles. Once the particles were deposited on the carbon disk, the disk was placed in a oven at 400 °C for 30 min in order to eliminate any organic residue that can interfere during SIMS analysis. The list of the samples used in this paper and their characteristics (isotopic composition and date of purification) are reported in Table 1. The amount of 230Th per particle was estimated from the equivalent spherical diameter measured on 100 particles coming from the reference material U850. This was measured 6664
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Analytical Chemistry Table 2. SIMS Operating Parameters operating conditions
protocol 1 age measurements
protocol 2 APM measurements
protocol 2 uranium isotopic measurements
primary ion current raster area analyzed isotope
2 nA − 5 nA 5 μm × 5 μm or no raster 230 Th [2s] 234U [1s]
150 nA 500 μm × 500 μm 235 U [5s] 238U[5s]
presputtering time
0s
6 s at 150 nA
250 pA No raster 235 U [2s] 238U [1s] for LEU 235 U [1s] 238U [2s] for HEU 0s
500 μm × 500 μm fields over the entire carbon planchet surface. Data treatment enables one to get the coordinates of these particles, and it gives an estimation of the 235U abundance. Particles can therefore be selected according to their isotopic composition. The second step consists of analyzing individual particles to get their precise uranium isotopic composition and their age. Two successive analyses in microbeam conditions are required. Isotopic composition is performed with a very low primary intensity and lasts less than 1 min, keeping enough signal to measure the age. The integration times of 235U and 238U isotopes were fitted to the estimated isotopic composition given by APM measurement. For low-enriched uranium (LEU, e.g. U100), 235U integration time is longer than it would be for 238U. Conversely, for very high- enriched uranium (HEU, e.g. U850, U900, and U970) this time is shorter. Age measurement was carried out in the same conditions as the ones previously described for the first protocol. Close attention must be paid when setting the instrument to focus the 250pA and 5 nA ion beams onto the very same point. All the SIMS operating parameters are described in Table 2. Calculation of the Model Age. The model age of uranium particle at time t was calculated using the Bateman eq 1: t=
λ 234U
Figure 1. Mass spectra of a CRM U900 particle (red line) and the corresponding background signal (blue line) under microbeam conditions.
in microbeam conditions in order to measure the 238U+ and 237+ signals. The abundance sensitivity at m − 1 was found to be (1.564 ± 0.080)·10−6. Assuming that this value can be applied to the ratio 234U+/235U+, the signal due to 235U+ abundance sensitivity would at best contribute to 0.02% of the 234U+ signal. Therefore, no correction of abundance sensitivity was applied. Internal Relative Sensitivity Factor. The relative sensitivity factor (RSF) is mainly used in SIMS as a quantification factor that enables one to relate measured secondary ion intensities to the concentration of an impurity in a matrix. The internal relative sensitivity factor (RSFi) that we calculated in this study was used to correct the final ratio of 230 Th/234U used for age determination according to the following eq 2:
⎛ λ − λ 234U ⎞ 1 ln⎜1 − R 230Th ⎟ − λ 230Th ⎝ λ 234U ⎠ 230
where R =
Tht
234
Ut
(1)
λ230Th and λ234U are the decay constants of 230Th and 234U.22 The initial amount of 230Th is assumed to be zero, and the amount of 234U is assumed to be constant during the elapsed time. More precisely, the decay of 234U induces a variation of 0.01% in the total number of 234U atoms over a period of 50 years, which has no effect on the age determination.
⎛ 230Th ⎞ ⎛ 230Th ⎞ = RSFi*⎜ 234 ⎟ ⎜ 234 ⎟ ⎝ U ⎠measured ⎝ U ⎠corrected
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RESULTS AND DISCUSSION Preliminary Studies. Prior to performing age measurements, background signal has been measured in microbeam conditions (see Table 2, protocol 1). The primary beam was focused on a particle-free area of the carbon planchet containing U900 particles, and the signal was integrated over 300 s. Five measurements were performed, and the average integrated 230Th signal was 3.5 counts, which corresponds to 0.02 cps. No background correction was further applied to age dating measurements. Typical mass spectra of the background and a U900 particle are illustrated in Figure 1. The abundance sensitivity of our SIMS instrument was also characterized. The abundance sensitivity is a measure of the contribution of the peak “tail” of a major isotope (in this work 235 + U ) to an adjacent m/z value (in this work 234U+) that can enhance the signal at a mass m ± 1 (in this work 234U+). To measure this factor, 30 natural uranium particles were analyzed
(2)
The determination of this factor requires a material with a 230 known 234Th ratio. CRM U900 was used as this reference U
material. The
230
Th U
234
ratio was calculated according to the
Bateman equation based on the purification date reported in the certificate of this material. Three analytical sessions on individual particles from U900 were made (Figure 2). Only particles that gave an integrated 230Th+ signal above 100 counts were selected in order to enhance counting statistics, thus keeping the individual relative uncertainties to approximately 10%. Finally the value of the RSFi was 1.313 ± 0.007 (1 SE, standard error of the mean). Analysis of Individual LEU and HEU Particles. To demonstrate the feasibility of determining the age of individual particles, each CRM was analyzed separately. Three to five analytical sessions were performed on each material, and the raw data were corrected with the RSFi. Results are given in 6665
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Application to a Mixture of LEU and HEU Particles. A preliminary study was carried out on U850 particles to evaluate the influence of the uranium isotopic analysis performed prior to age measurement on the integrated 230Th+ signal. During the same analytical session, particles were analyzed according the two protocols described in Table 2. The averages of the integrated 230Th+ signals were 147 ± 38 counts for protocol 1 (i.e., without uranium isotopic measurement) and 164 ± 95 counts for protocol 2 (i.e., with uranium isotopic measurement). Furthermore, the average model age obtained from protocol 2 is 57.1 ± 8.2 years (1 SD, standard deviation), which is in agreement with the average model age, 60.6 ± 5.1 y (1 SD), obtained from protocol 1. Both results indicate that uranium isotopic measurement has little to no consequence on the age-dating measurement. The final experiment of this study was carried out on a carbon planchet bearing U100, U850, and U970 particles in order to demonstrate the possibility of measuring individual ages on a mixture of different isotopic compositions. As described in the paragraph entitled “SIMS protocol”, APM was carried out in a first step in order to distinguish and locate uranium particles according their 235U abundance. The biggest particles of each population were then selected and microbeam analyses were performed. The first one consisted of determining the isotopic composition of the particles by measuring the isotopic ratio of 235U/238U. Because of the high density of particles at the surface of the planchet, only isolated particles were analyzed to prevent cross-contamination between the different isotopic compositions and ages. The comparison of the isotopic ratio estimated by APM and measured precisely by the microbeam is illustrated in Figure 4. Very good
Figure 2. Values of the internal RSF measured on individual particles of U900 during three analytical sessions (red circles). Uncertainties correspond to the standard deviation of each analytical session. Blue line corresponds to the mean (blue bar). Uncertainties correspond to the standard deviation of the mean of the different analytical sessions (dotted blue bars).
Figure 3. Only particles in which the integrated signal at 230Th mass was above 100 counts for HEU particles and 20 counts for
Figure 3. Model ages (red circle) of the different CRM particles measured by SIMS. Each point corresponds to an analytical session, in which 4 to 19 particles were analyzed. The blue squares represent the reference age. Uncertainties correspond to the standard deviation of each analytical session.
U100 particles were selected in order to limit the uncertainties to around 10% for HEU particles and around 20% for U100 particles. It is recommended to select particles that are at least 100 μm away from another U particle to prevent background enhancement due to redeposition of previously sputtered particles. Special attention should be paid to U100 analysis. Because of the very low 230Th+ signal in such particles, it is recommended to perform mass calibration on HEU particles prior to U100 measurements. This study on individual CRM demonstrates the potential of SIMS to determine the age of micrometer-sized particles. Nevertheless, the uncertainties on the model age measured on an individual particle are approximately 10% for HEU CRM and approximately 20% for U100. In all, this is not yet compatible with the requirements for nuclear forensics investigations or for nonproliferation studies. The major source of uncertainties lies in the counting statistic for 230Th, because of its extremely low micrometer-sized particle concentration. However, by measuring more than 30 particles per sample, the uncertainties on the average model age can be reduced to 1−3 %.
Figure 4. 235U/238U Ratio estimated by APM vs 235U/238U ratio measured by microbeam in a carbon planchet containing particles coming from 3 different CRMs.
agreement (R2 = 0.93) is obtained, which confirms the ability of SIMS, and more precisely APM analysis, to determine the different components of a mixture of uranium particles. In terms of microbeam results, accuracy on the 235U abundance is better than 0.5%, even if very short analyses (less than 1 min) are performed: − 0.24% for U970 particles, − 0.08% for U850 particles and −0.02% for U100 particles. Following this isotopic characterization, age-dating measurements were performed on the same particles. A combination of isotopic measurement and age determination is illustrated in Figure 5. These results are in agreement with the results obtained for individual CRM particles (cf. Table 3). 6666
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switching time and the integration time of the different isotopes. In the methodology described here, the 230Th detection yield is 42%. Based on these two assumptions, the theoretical detection limit is 2.7 ag for SG-SIMS and 0.8 ag for LG-SIMS, corresponding to a gain of a factor of 3.4. This implies that for particles of similar size, LG-SIMS will enable one to characterize younger particles than SG-SIMS or, conversely, for particles of the same age, LG-SIMS will enable one to characterize smaller particles. This result is illustrated in Figure 6, which shows the evolution of the capacity of LG-
Figure 5. Comparison of the model ages and the isotopic composition determined by SIMS at the particle scale (full red circles) and the CRM certified data (blue bars). Uncertainties correspond to one standard deviation. Estimated bulk results were also represented as dotted red circles. Uncertainties correspond to Poisson statistics.
Table 3. Model Ages of the 4 Different CRM U100, U850, U900, and U970 Measured by SIMSa CRM U100 U850 U900 U970
model age measured by SIMS
deviation from the reference ages (years)
± ± ± ±
1.5 -0.34 -0.06 -0.34
58.5 58.17 58.4 50.9
1.9 0.69 1.1 1.0
a
Figure 6. Plotted detection limits in terms of age or size of a particle containing 2.7 ag of 230Th (for SG-SIMS) and 0.8 ag of 230Th (for LGSIMS). Comparison of the performances of LG-SIMS and SG-SIMS on uranium particles with a 235U abundance of 20%.
By comparison, we also calculated the model age and uranium isotopic ratio that would be obtained with a bulk methodology. To that end, the integrated 235U+, 238U+, 230Th+, and 234U+ signals obtained for each particle were summed (Figure 5). The calculated model age by this bulk method is 53.1 ± 1.6 years, and the calculated isotopic ratio is 16.458 ± 0.008, which does not correspond to the actual composition of the sample. Consequently, coupling uranium isotopic measurements and age dating at the particle scale is currently the only way to identify the three components of this mixture of materials. Limitations of the Method. Theoretical performances of SG-SIMS and LG-SIMS have been compared on the basis of the detection limit of these instruments. The detection limit refers to the amount of 230Th required to detect a signal of 20 counts at the mass corresponding to 230Th+. A threshold at 20 integrated counts has been fixed because this will lead to a relative uncertainty in the model age of approximately 20%. To determine this detection limit, two parameters must be known: the useful ionization yield and the detection yield of 230Th. The first parameter refers to the proportion of detected ions relative to the amount of sputtered atoms from the particle. Ranebo et al. determined the useful yield for uranium both for IMS 6F (SG-SIMS) and IMS 1270 (LG-SIMS).23 The IMS 6F results described in that paper can be applied to the instrument used in this study. The second parameter depends on the detection pattern. With a multicollection instrument, such as LG-SIMS, all the isotopes are detected simultaneously which leads to a 230 Th detection yield of 100%. With a SG-SIMS equipped with only one detector, the 230Th detection yield is below 100%, because the acquisition time is shared between the magnet-peak
SIMS and SG-SIMS to characterize uranium containing 230Th at the mass detection limit according to the age or the size of the particle with a 235U abundance of 20%. This capacity is a great interest for safeguarding purposes because a 235U enrichment of 20% is considered the threshold for civilian exposure to uranium. It is also interesting to compare the performances of these two instruments for particles of different 235U abundances. In Figure 7a, the mass of 230Th is plotted versus the particle equivalent diameter for a 15 year-old particle with three different 235U abundances: natural uranium, 20% and 97%. An age of 15 years has been selected because most of the nuclear proliferation cases appeared in the beginning of the 21st century. It is apparent that the capacities of detection of HEU particles are almost equivalent for both instruments: for 97% enriched particles, LG-SIMS can characterize submicrometric particles, whereas SG-SIMS analysis requires 1 μm-sized particles. For 20% enriched particles, performances are still comparable, with a minimum size of 1.8 and 2.5 μm for LGSIMS and SG-SIMS, respectively. These particle sizes can often be found in on-field environmental samples. On the contrary, age dating of natural uranium particles is limited either by LGSIMS or SG-SIMS because minimal particle sizes are 5 and 7 μm, respectively. Such large particles are rarely found in the environmental samples that are routinely analyzed by the Network of Analytical Laboratories of the International Atomic Energy Agency. An internal study of the particle size detected in environmental samples analyzed by SIMS in our laboratory since 2012 shows that the particle size ranges between 0.5 and 3 μm. If the amount of 230Th is plotted versus the age of a particle whose diameter ranges from 0.5 to 3 μm, it confirms that SIMS (LG-SIMS or SG-SIMS) is not an adequate technique for natural uranium particle age dating even if they
Uncertainties correspond to the standard deviation of the mean of the different analytical sessions. Deviations from the reference age were also reported.
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particles from the same source in order to improve the accuracy. Particle analysis is of great interest to determine the homogeneity of the material and to carry out age dating on the different populations. Even if the performances will never reach those of ICP-MS or TIMS, SIMS analysis of about 30 particles enables one to characterize heterogeneous material with accuracy below 5% on the age and below 0.5% on the isotopic composition. However, this study highlights the limitation of age dating using a single collector SIMS on particles whose size is below 3 μm. The adaptation of the methodology on LG-SIMS, with a multicollection device and a better ionization yield, could increase the integrated 230Th+ signal by a factor of 3. Nevertheless, because of size and particle number limitations, age-dating application on environmental samples appears to be limited. On the contrary, particles up to 10 μm, and in greater numbers, can be sampled at the surface of a bulk or pulverulent material. Further studies must now focus on real sample analyses in which other material than U can be found and lead to isobaric interferences.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Anne-Laure Fauré: 0000-0002-6381-5325 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to warmly thank the CEA Saclay (Gif-sur-Yvette, France) for providing us some particles coming from CRM that were missing in our library.
Figure 7. Comparison of the performances of SG-SIMS and LG-SIMS for measuring the age of 15 year-old uranium particles of different 235U abundances (panel a) and for measuring the age of uranium particles of different 235U abundances with an equivalent diameter ranging from 1 μm (full line) to 6 μm (dotted line) (panel b).
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REFERENCES
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are 60 years old (See Figure 7b). Age dating of relatively young (i.e, < 20 years) particles with a 235U abundance of 20% implies that the particle size is above 2.5 or 1.5 μm, for SG-SIMS and LG-SIMS analyses, respectively. Regarding submicrometric particles, which are more commonly found, performances are very limited: only LG-SIMS can characterize such small particles, but only for HEU particles (above 20%). Age dating at the particle appears to have strong limitations in the environmental field due to particle size. Nevertheless, in the case of bulk material or powder characterization, bigger particles can be sampled (>10 μm) and thus analyzed by SG or LG-SIMS. However, all these calculations are made on pure uranium particles. In real environmental samples or in the case of smuggled material, such uranium particles are often mixed with other materials (dust, industrial particles) that can lead to isobaric interferences.
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CONCLUSIONS The methodology described in this study illustrates the potential of SIMS for the age dating of uranium material at the particle scale and for the characterization of mixtures of nuclear materials by combining age dating and isotopic composition. Because of the low amount of 230Th in a single particle (below 1 fg), uncertainties are quite large. Therefore, a sample characterization requires the analysis of numerous 6668
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