Identification of the chemical forms of uranium compounds in micrometer-size particles by means of micro-Raman spectrometry and scanning electron microscope

We used a micro-Raman spectrometer with two different laser excitation sources (514 and 785 nm) and variable laser powers to identify some uranium chemical species contained in airborne particulate matter. In the first part of this paper, we demonstrate that characteristic Raman bands mentioned in the literature for several uranium compounds relevant in the nuclear industry (UO₂, UO₄·(4H₂O), U₃O₈, UO₂F₂ and UF₄) can be identified in particles in the few μm to 30 μm size range. In the second part of the paper, we describe a method to carry out Raman analysis on airborne uranium particles sampled along with a majority of other kinds of particles simply by dabbing adhesive carbon disks on dusty surfaces. This methodology involves an SEM equipped with an energy dispersive X-ray analyser and software for automated detection of particles specifically to locate uranium particles on the substrate before the Raman analysis. Then the sample holder is transferred to the micro-Raman spectrometer and particles are relocated using landmarks and simple geometric calculations. Raman analyses are carried out with the laser that gives the best signal to noise ratio. With such a method particles as small as 5 μm can be efficiently analysed, although most of the smaller particles cannot be analysed due to limited precision of the relocation process. This methodology was successfully applied to 20 particles collected in a nuclear facility.