In-SEM Raman microspectroscopy coupled with EDX--a case study of uranium reference particles.

Information about the molecular composition of airborne uranium-bearing particles may be useful as an additional tool for nuclear safeguards. In order to combine the detection of micrometer-sized particles with the analysis of their molecular forms, we used a hybrid system enabling Raman microanalysis in high vacuum inside a SEM chamber (SEM-SCA system). The first step involved an automatic scan of a sample to detect and save coordinates of uranium particles, along with X-ray microanalysis. In the second phase, the detected particles were relocated in a white light image and subjected to Raman microanalysis. The consecutive measurements by the two beams showed exceptional fragility of uranium particles, leading to their ultimate damage and change of uranium oxidation state. We used uranium reference particles prepared by hydrolysis of uranium hexafluoride to test the reliability of the Raman measurements inside the high vacuum. The results achieved by the hybrid system were verified by using a standalone Raman microspectrometer. When deposited on exceptionally smooth substrates, uranyl fluoride particles smaller than 1000 nm could successfully be analyzed with the SEM-SCA system.

Investigation of uranium isotopic signatures in real-life particles from a nuclear facility by thermal ionization mass spectrometry.

An improved method was recently developed for the isotopic analysis of single-reference uranium oxide particles for nuclear safeguards. This method is a combination of analytical tools including in situ SEM micromanipulation, filament carburization and multiple ion counting (MIC) detection, which is found to improve sensitivity for thermal ionization mass spectrometry (TIMS) isotope ratio analysis. The question was raised whether this method could be applied for the detection of nuclear signatures in real-life particles with unknown isotopic composition. Therefore, environmental dust was collected in different locations within a nuclear facility. The screening of the samples to find the uranium particles of interest was performed using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) detector. The comparison of the measurement results to reference data evaluated by international safeguards authorities was of key importance for data interpretation. For the majority of investigated particles, detection of uranium isotopic signatures provided information on current and past nuclear feed operations that compared well with facility declarations.