Improved uranium particle analysis by SIMS using O3− primary ions

Abstract

We have investigated the use of negative molecular oxygen primary ion beams (i.e., O₂⁻ and O₃⁻) to determine the benefits of using such beams for uranium particle SIMS analyses. Typically, O⁻ is the most practical negative primary ion species from the conventional duoplasmatron ion source for both age dating and uranium isotopic analysis of particles. Newer RF plasma ion sources make it possible to use O₂⁻ and O₃⁻ due to higher brightness and primary ion fluence, and the increased abundance of molecular species in the plasma relative to the duoplasmatron. We have determined that by using an O₃⁻ beam, the ionization yield can be increased by a factor of approximately two over an O⁻ beam, up to 4.7%, a substantial improvement that positively impacts measurement precision and detection limits. We also investigated the effect of the molecular oxygen beams on uranium isotope mass fractionation and the Th/U relative sensitivity factor for SIMS analyses in comparison to O⁻ beams. We found that O₃⁻ reduced instrumental mass fractionation and matrix/substrate effects relative to the other negative ion beams. Particle measurements using O₃⁻ were improved compared to conventional O⁻ beam analyses due to higher yields, smaller corrections, and reduced substrate effects.