Production and Characterization of Monodisperse Plutonium, Uranium, and Mixed Uranium-Plutonium Particles for Nuclear Safeguard Applications

In order to prevent nuclear proliferation, the isotopic analysis of uranium and plutonium microparticles has strengthened the means in international safeguards for detecting undeclared nuclear activities. In order to ensure accuracy and precision in the analytical methodologies used, the instrumental techniques need to be calibrated. The objective of this study was to produce and characterize particles consisting of U, Pu, and mixed U-Pu, suitable for such reliability verifications. A TSI vibrating orifice aerosol generator in connection with a furnace system was used to produce micrometer sized, monodispersed particles from reference U and Pu materials in solution. The particle masses (in the range of 3-6 pg) and sizes (similar to 1.5 mu m) were controlled by the experimental conditions and the parameters for the aerosol generator. Size distributions were obtained from scanning electron microscopy, and energy-dispersive X-ray analysis confirmed that the particle composition agreed with the starting material used. A secondary ion mass spectrometer (SIMS) was used to characterize the isotopic composition of the particles. Isobaric and polyatomic interference in the SIMS spectra was identified. In order to obtain accurate estimates of the interference, a batch of Pu particles were produced of mainly Pu-242. These were used for SIMS analysis to characterize the relative ionization of Pu and U hydride ions and to determine the SIMS useful yields of U and Pu. It was found that U had a higher propensity to form the hydride than Pu. Useful yields were determined at a mass resolution of 450 for U-Pu particles: (1.71 +/- 0.15) % for Pu and (0.72 +/- 0.06) % for U. For Pu particles: (1.65 +/- 0.14) % for Pu. This gave a relative sensitivity factor between U and Pu (RSKJA,) of 2.4 +/- 0.2. However, the RSFU, Pu showed large fluctuations during the sputtering time for each analyses of the mixed U Pu particles, in the range of 1.9-3.4.