Abstract
Atom probe tomography (APT)-based isotopic analyses are becoming increasingly attractive for analysis applications requiring small volumes of material and sub-micrometer length scales, such as isotope geochemistry, nuclear safety, and materials science. However, there is an open question within the atom probe community as to the reliability of atom probe isotopic and elemental analyses. Using our proposed analysis guidelines, in conjunction with an empirical calibration curve and a machine learning-based adaptive peak fitting algorithm, we demonstrate accurate and repeatable uranium isotopic analyses, via atom probe mass spectrometry, on U3O8 isotopic reference materials. By using isotopic reference materials, each measured isotopic abundance value could be directly compared to a known certified reference value to permit a quantitative statement of accuracy. The isotopic abundance measurements for 235U and 238U in each individual APT sample were consistently within ±1.5% relative to the known reference values. The accuracy and repeatability are approaching values consistent with measurements limited primarily by Poisson counting statistics, i.e., the number of uranium atoms recorded.
Highlights
Atom probe tomography (APT)-based isotopic analyses are becoming increasingly attractive for analysis applications requiring small volumes of material and sub-micrometer length scales, such as isotope geochemistry, nuclear safety, and materials science
The high detection efficiency of APT provides a significant advantage over other mass spectrometry techniques when analyzing small volumes of material, like the powdered reference material used in this study
APT is becoming increasingly attractive for analysis applications requiring small volumes of material and submicrometer length scales, such as geological materials and geochronology;[8−10] nuclear fuels and nuclear forensics,[6,11−16] semiconductor materials,[17−21] and meteoritic materials.[22−26] it has been demonstrated that quantitative analyses in the atom probe can be unreliable
Summary
Atom probe tomography (APT)-based isotopic analyses are becoming increasingly attractive for analysis applications requiring small volumes of material and sub-micrometer length scales, such as isotope geochemistry, nuclear safety, and materials science. NanoSIMS offers a high lateral spatial resolution but suffers from low count rates (due to the use of probe currents an order of magnitude smaller than those typically used for SIMS), low ionization efficiency, and sensitivity variations between different phases.[5,6] Likewise, laser ablation inductively coupled plasma mass spectrometry has been explored for measuring the isotopic composition of single uranium particles, but the useful ion yield when analyzing uranium is low, with values between 0.01% and 2.8%.7. The chemical composition or isotopic abundance must be known a priori, or additional information (e.g., correlative data from an independent analysis technique) is needed to lend confidence to the results obtained via the atom probe.[13,27−36] Further, no community-wide accepted practices for assigning regions of interest in the mass spectrum (or TOF spectrum) to specific ion species (e.g., UO22+, UO2+, or UO3+) have been defined. Using the following guidelines, we were able to demonstrate accurate isotopic analyses, even for data sets containing an exceedingly high fraction of multihit detection events: (a) analyze the corrected TOF spectrum (timing-signal-only based), using a bin width of 0.01 ns or less; (b) filter the data set to remove as many multihit detection events as possible; (c) treat each ion species as a separate measurement for isotopic abundance; (d) assume all isotopic variants of a given ion species have the same peak form; (e) use an optimization algorithm to determine an estimate of the common peak form shared by the isotopic variants of a given ion species; (f) pool the isotopic abundance measurements obtained from each ion species to report the isotopic abundance for the specimen (e.g., averaging).[37]
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