Application of FIB-ToF-SIMS to the Search for and Characterisation of Enriched Uranium Particles

A Focused Ion Beam - Scanning Electron Microscope (FIB-SEM) combined with a Time-of-Flight Secondary Ion Mass Spectrometer (ToF-SIMS) is one of the most powerful, multifunctional analytical instruments available as it enables simultaneous investigation by electron microscopy/microanalysis, ion beam sputtering and mass spectrometry. This unique combination of analytical capabilities enables the morphology, chemical and isotopic composition of materials to be studied at high resolution and high sensitivity in 2D or 3D. The key capabilities of the FIB-ToF-SIMS platform are outlined in Figure 1. This versatile analytical tool has been applied to the search for and characterisation of enriched uranium particles in dust swipes from environmental sampling of nuclear facilities. As part of an international effort to detect and monitor the use of uranium in nuclear facilities the International Atomic Energy Agency (IAEA) collects environmental samples from various countries and analyses them for the presence of uranium that has been enriched in the 235U isotope, which can be indicative of undeclared nuclear material or activities. Conventional environmental sample analyses involve using Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS) to scan many 1000’s of particles for the presence of uranium, then performing precise analyses to determine the isotopic composition. This process is successful, though complimentary information about particle morphology and associated elements is often missing or hard to correlate with the LG-SIMS data. Many analyses also result in the complete destruction of the particle of interest, which prevents further examination. A FIB-SEM with an Energy Dispersive X-ray Spectrometry (EDS) detector enables non-destructive, high spatial resolution analysis of particles for morphology and major element composition. Imaging with backscattered electrons (BSE) provides atomic number contrast and as such heavy element containing particles, such as uranium oxides, appear brighter than surrounding material. This method was used to search for and chemically identify uranium containing particles. In the swipe sample analysed 10 particles with >75 wt.% U + O were selected for detailed analysis. The detection of uranium bearing particles by SEM-EDS is insufficient to characterise them in the context of nuclear forensics. The relative abundance of the 235U isotope to the 238U isotope is indicative of provenance/synthesis method of the particle. The natural abundance of 235U is 0.72% [1]. Low enriched uranium, commonly used in nuclear reactors, has 1 to 20% 235U, whereas highly enriched uranium, used in nuclear weapons, can have 20 to >90% 235U. Isotopic analyses by FIB-ToF-SIMS were performed in-situ on the particles of interest to quantify the ratio of 235U to 238U and determine the level of enrichment. A Tescan Lyra3 FIB-SEM fitted with a Tofwerks Compact-Time of Flight detector was used in this study. FIB-ToF-SIMS in this configuration is capable of ≤50 nm lateral resolution SIMS maps at a mass resolving power of 800 [2]. A monoisotopic 69Ga+ primary beam with an energy of 30 kV and current of 75 pA was rastered in a 10 x 10 µm area over each particle. Positive ions up to a mass of 340 mass/charge were collected and the integrated peak area of the 235U16O and 238U16O ions were used for the isotopic analysis (Figure 2). An enrichment of up to 4.2% ± 1.1% was determined for the particles, which clearly indicated the presence of low enriched uranium in the swipe sample. Key capabilities of the FIB-ToF-SIMS platform. FIB-ToF-SIMS mass spectrum from a uranium oxide particle and a SEM image of a particle after the analysis.