A New Approach for Nuclear Forensics Investigations of Uranium Dioxide: Application of Laboratory-Based Photoelectron Spectroscopy with Hard and Soft X-ray Sources

Nuclear Forensic investigations rely on the analysis of the chemical and physical properties of nuclear materials. X-ray photoelectron spectroscopy (XPS) is a powerful tool that supports material assessment, typically analyzing the top few nanometers of the material. The onset of laboratory-based hard X-ray photoelectron spectroscopy (HAXPES) instrumentation provides the opportunity to probe deeper into the material’s bulk. The work presented in this study demonstrates the utility of a combined XPS and HAXPES analysis to isolate forensic signatures on the surface and into the bulk of uranium dioxide. A non-destructive depth profile, using the transitions observable with a 9.25 keV excitation source, highlighted an oxidized overlayer deeper than the XPS sampling depth. Peak fitting of high-resolution spectra allows identification of uranium oxidation states as well as inspection of secondary features, which provide insight into the material characteristics with an evolving chemistry from the surface to a more bulk like composition. Inelastic background analysis is performed to determine the in-depth distribution of atoms, developing a consistent model to describe the surface overlayer, correlated to the chemical and stoichiometric differences over the excitation range. Finally, the MNN X-ray excited Auger electron spectra are acquired from uranium dioxide for the first time for future use in the application of a Wagner chemical state plot to support nuclear forensics investigations.