Production of monodisperse uranium oxide particles and their characterization by scanning electron microscopy and secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) can be confidently used to measure uranium isotopic ratios in single particles. Dense particles of known isotopic composition and size allow the precision and the accuracy of the applied procedure to be estimated. These particles can be obtained by dissolving standard reference uranium materials, nebulizing the solution in droplets of proper diameter and collecting the particles after the desolvation and calcination of the droplets. A new instrumental set up, based on a commercial vibrating orifice aerosol generator to generate monodisperse droplets of the solutions from four uranium oxide reference materials, is described. The droplets were dried and calcined in a sequence of three furnaces. The morphology of the monodisperse uranium oxide particles was studied by scanning electron microscopy. It was observed that the particles were nearly spherical and consisted of dense material. Their diameter distribution evidenced the presence of two populations mainly, the first showing a narrow distribution with a maximum centered at approximately 1 μm. The first statistical moment ratios between the two populations remained practically constant at 1.24±0.01. This demonstrated that the second population was due to the formation of one particle from two droplets of solution (theoretical double mass≡diameter ratio of 23=1.26). Secondary ion mass spectrometry was used to verify the isotopic composition of the produced particles. Typical accuracies of better than 0.4% for 235U/238U and a few percent for the minor isotopes have been achieved. For the determination of the 236U content, the signal at mass M=239 (due to 238UH+) was used to correct the 235UH+ contribution to 236U at mass M=236, greatly improving the accuracy of the 236/238 ratio with increasing enrichment of the 235U isotope.