Large-geometry (LG)-SIMS instruments have been used to analyze actinides because of their simultaneous high transmission and high mass resolving power (MRP) 4– 8.

Uranium isotope and age dating measurements on small and/or young samples require the highest achievable ion yields coupled with low instrument background 4. This temporal information complements isotopic analyses for discriminating between past and present material processing by providing an activity timeline.

For the analysis of particles that have undergone chemical/material processing, the measured ratio of 230Th/ 234U in a particle will reflect its last purification date. Recently, the National Institute of Standards and Technology (NIST) developed an additional methodology for determining the ages of microscopic U particles by SIMS using the radioactive chronometer pair 234U: 230Th, which decays with a half-life of 245,500 ± 600 years 4. The distribution of U isotopic compositions from individual microscopic particles can help illustrate information about source material inputs and outputs for an associated set of particles from a specific process. Samples are collected on cotton swipes and then prepared for both microscopic inspection and isotopic analyses typically using thermal ionization mass spectrometry (TIMS) or secondary ion mass spectrometry (SIMS). Particulate nuclear material from environmental samples is routinely analyzed by both bulk and particle mass spectrometry techniques 1– 3. Particle measurements using O 3 − were improved compared to conventional O − beam analyses due to higher yields, smaller corrections, and reduced substrate effects. We found that O 3 − reduced instrumental mass fractionation and matrix/substrate effects relative to the other negative ion beams. 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 have determined that by using an O 3 − 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. Newer RF plasma ion sources make it possible to use O 2 − and O 3 − due to higher brightness and primary ion fluence, and the increased abundance of molecular species in the plasma relative to the duoplasmatron. 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. We have investigated the use of negative molecular oxygen primary ion beams (i.e., O 2 − and O 3 −) to determine the benefits of using such beams for uranium particle SIMS analyses.

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