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 u ranium isotopic ratios in single particles. Dense particles of known isotop ic composition and size allow the precision and the accuracy of the applied procedure to be estimated. These particles can be obtained by dissolving s tandard reference uranium materials, nebulizing the solution in droplets of proper diameter and collecting the particles after the desolvation and cal cination of the droplets. A new instrumental set up, based on a commercial vibrating orifice aerosol generator to generate monodisperse droplets of th e solutions from four uranium oxide reference materials, is described. The droplets were dried and calcined in a sequence of three furnaces. The morph ology of the monodisperse uranium oxide particles was studied by scanning e lectron microscopy. It was observed that the particles were nearly spherica l and consisted of dense material. Their diameter distribution evidenced th e presence of two populations mainly, the first showing a narrow distributi on with a maximum centered at approximately 1 mu m. The first statistical m oment ratios between the two populations remained practically constant at 1 .24 +/- 0.01. This demonstrated that the second population was due to the f ormation of one particle from two droplets of solution (theoretical double mass = diameter ratio of 3 root 2 = 1.26). Secondary ion mass spectrometry was used to verify the isotopic composition of the produced particles. Typi cal accuracies of better than 0.4% for U-235/U-238 and a few percent for th e minor isotopes have been achieved. For the determination of the U-236 con tent. the signal at mass M = 239 (due to (UH+)-U-238) was used to correct t he (UH+)-U-235 contribution to U-236 at mass M= 236, greatly improving the accuracy of the 236/233 ratio with increasing enrichment of the U-235 isoto pe. (C) 2000 Elsevier Science B.V. All rights reserved.