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 VIDEOS

Isotope Metrology Webinar Series – March 21, 2023

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  Fred MeisenkothenMaterials Measurement Science DivisionMaterial Measurement LaboratoryNational Institute of Standards and Technology[1]Gaithersburg, MD  20899Atom probe tomography is being employed in an ever-broadening range of applications to characterize nanometer-scale volumes of material. However, there has been an open question within the community about the reliability of chemical and isotopic analyses performed via the atom probe. While atom probe tomography can deliver near-atomic-resolution chemically- and isotopically-resolved 3-dimensional images of nanometer-scale volumes of material, the measurement science required for the technique to realize its full potential has been outpaced by the progress made in instrumentation.We have been exploring the accuracy of quantitative chemical and isotopic measurements via atom probe mass spectrometry to better understand the causes of some limited sources of bias. Since isotopic analysis is, in many ways, the simplest form of analysis we can do in the atom probe, these analyses provide a way of testing the basic assumptions used for quantitative analysis and for assessing the ultimate accuracy of the technique. The goal is to produce an accurate, repeatable, and mathematically defensible method for assigning ion counts (i.e., ranging) in the mass spectrum to specific ion species for quantification. At present, there are no consensus methods within the community for performing this task. We have been exploring a custom adaptive peak fitting measurement methodology, as an alternative approach to traditional ion ranging strategies. The precision provided by our analysis methodology suggests it may be possible to perform standards-based analysis in atom probe mass spectrometry, which is something not traditionally done within the field. Several natural materials and isotopic abundance reference materials were used as “known” isotopic abundance values against which our analysis results could be compared to permit a quantitative statement of accuracy. The methodology has been applied successfully to sulfur isotopic analysis in gold-bearing pyrites for fluid source fingerprinting.Richard EssexRadiation Physics DivisionPhysical Measurement LaboratoryNational Institute of Standards and Technology[1]Gaithersburg, MD  20899The Radioactivity Group is within the Radiation Physics Division of the Physical Measurement Laboratory. The focus of the Radioactivity Group is the realization of the becquerel (Bq) which is the SI unit for activity of the nuclide in inverse seconds. As such, much of the groups work centers on measurement of massic activity and refinement of decay data for specific radionuclides. The Radioactivity Group produces SRMs including single nuclide activity standards and complex matrix activity standards. In addition, the group performs a variety of measurement services including calibrating radioactive sources, calibrating instrumentation, and performing performance testing. Isotope metrology is not a core mission, but the Group does collaborate on isotope metrology projects and isotope ratio measurement data is often an important tool for radionuclide activity measurements. Examples include the production of Th-229 and Pa-231 isotopic and assay reference materials, determination of a Th-229 half-life, and validation of small solution mass measurement techniques using IDMS.  Future work will include using mass spectrometry to quantify the abundance of interfering Pu isotopes in a Pu-239 activity standard and an independent massic activity determination based on IDMS amount content of the Pu SRM.[1]Contribution of the National Institute of Standards and Technology.  Not subject to copyright.  Certain commercial equipment, instruments, or materials are identified to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are the best available for the purpose.

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