AVS 70 Session AC+MI-ThA: Forensics, Disposal and Pu

Nuclear Forensic investigations rely on the analysis of the chemical and physical properties to determine the provenance of nuclear materials, found out of regulatory control. X-ray photoelectron spectroscopy (XPS) has been shown to be a powerful tool in supporting material assessment by analysis of the top few nanometres of the surface. With the onset of laboratory based hard x-ray photoelectron spectroscopy (HAXPES) instrumentation, this provides the opportunity to probe deeper into the bulk. Furthermore, HAXPES spectra of high Z elements will have multiple core level peaks occurring over a range of different depths and analysis of these can offer a unique non-destructive depth profile. 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. Survey quantification shows the changing stoichiometry, utilising a 9.25 KeV excitation range of HAXPES, from different average depths within the sample matrix. Peak fitting of high-resolution spectra allows identification of oxidation states as well inspection of secondary features, which provide insight into the material characteristics. Combined with XPS analysis, this shows different chemical and elemental states at the surface into the bulk sample, highlighting the usefulness of this approach. QUASES-Tougaard analysis was 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 dioxides for the first time for use in the application of a Wagner or chemical state plot. This study is the first in a series of analysis campaigns on uranium bearing materials.

The on-going development of improved characterization methods and signatures is crucial to ensure that nuclear safeguards and forensics activities remain effective. For these purposes, the development of X-ray techniques is particularly useful because of their elemental specificity and non-destructive nature that could enable a first-in capability. Tunable synchrotron radiation X-ray approaches provide a means to extend the scope of safeguards and forensics investigations in elemental, chemical, and structural analysis which can be done in imaging modes that in some cases, reaches to the nanoscale. The ability to use tunable, focused X-ray beams makes synchrotron radiation sources a potentially disruptive tool for addition into the array of characterization techniques currently employed, particularly for the investigation of particles or areas of interest in smaller specimens possessing bulk-like characteristics. However, the use of synchrotron radiation methods also presents some unique challenges for the implementation of safeguards and forensic efforts at the synchrotron radiation facility sites (ALS, APS, NSLS-2, and SSRL). Synchrotron radiation-based safeguards and forensics studies over the past several years conducted at nearly all of the U.S. Dept. of Energy synchrotron radiation user facilities throughout the X-ray spectrum have furthered this case, as well as clearly identifying areas that require improvement. At the same time, there have been significant performance improvements in fixed-energy laboratory-based X-ray instrumentation suitable for contemporary, spatially-resolved safeguards and forensics (primarily XRF) studies. Additionally, there have been continued efforts and strides made towards the realization of true laboratory-based X-ray light sources that could provide competitive X-ray flux and tunability, albeit in the future, that might provide the best combination of performance and characteristics necessary for safeguards and forensics characterization activities. A current assessment of the relative strengths and drawbacks of the respective X-ray approaches, as well as a performance comparison will be presented, leading into an overall the future outlook in terms of safeguards and forensics.

Nuclear forensics requires accurate identification of distinguishing material characteristics to delineate the material’s origin-to-interdiction information. Morphological and chemical variation in nuclear materials over large scales of observation can provide valuable information that can assist interpretation of the material’s varying provenance, process, and pathways. We will describe our efforts to strengthen operational and scientific methodologies to employ the focused ion beam-scanning electron microscopy (FIB-SEM) on uranium (U) and plutonium (Pu) materials for internal morphological analysis including site-specific 3D microscopy and spatially resolved characterizations of material features using transmission electron microscopy (TEM) and X-ray synchrotron spectromicroscopy. Both U and Pu materials show variations in the internal chemical composition and morphology from their production processes and storage environments. This information is of potential use in discriminating material signals to identify the origin and history of interdicted nuclear materials.

Janoschek, et al. described the ground state of plutonium (Pu) as being “governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations.” [Science Advances, 1, 6 (2015)]. By representing this ground state generally as

[Rn]7𝑠^{2+𝛾𝑠,𝑃𝑢}5𝑓^{6+𝛾𝑓,𝑃𝑢}6𝑑^{0+𝛾𝑑,𝑃𝑢}7𝑝^{0+𝛾𝑝,𝑃𝑢}

where γ_s,Pu,γ_f,Pu, γ_d,Pu, and γ_p,Pu represent potentially non-integer deviations from the integer occupancies of their respective orbitals, a combinatorial approach can be applied to the allowable permutations of quantum electronic configurations as a means of describing the electron degrees of freedom. This approach enables a classical statistical mechanics methodology to be applied to the discrete quantum states for conceptualizing and investigating the multiconfigurational ground state of Pu. Specifically, an over-approximation (with Hund’s rule relaxed) and an under-approximation (with Hund’s rule enforced) of electronic structure entropy has been established for varying ranges of potentially near energy degenerate occupancy configurations for plutonium. This bracketing of electronic structure entropy is compared with the known molar entropy of Pu to narrow down the set of possible localized and itinerant electronic configurations that can reasonably contribute to the quantum mechanical superposition of Pu’s multiconfigurational ground state, as well as to estimate the non-integer orbital occupancies of its 7s-, 5f-, 6d- and 7p- orbitals.

LA-UR-24-22282

The second derivative mode of peak analysis in electron energy loss spectroscopy (EELS) in a Transmission Electron Microscope (TEM) has been quantitatively evaluated in terms of the accuracy of the method.*This includes a demonstration of the importance of the second derivative peak width, the second order dependency of the accuracy upon that peak width and effect of high frequency noise in the spectra.It was found that, while EELS does converge to an X-ray Absorption Spectroscopy (XAS)-like limit (electron dipole transition) at high energies, there are significant issues with the 2^nd Derivative Model.

1. There is no fixed relationship between non-derivative peak intensities and 2^nd derivative peak intensities.Changing peak-shapes changes the ratio.
2. Assuming that peak-shapes remain constant, which may not be justified, there is a second order dependence of the ratio upon the peak width of the 2^nd Derivative peak.
3. The second order dependence upon the peak width manifests itself in two very distinct ways: with random errors and systematic errors.
   • The random error from high frequency noise places a limit on the number of significant digits in the BR result.
   • The systematic error has shown up in the BR predictions and 5f population estimates (n) for the localized systems Pu, PO₂ and UO₂.

The 2^nd Derivative Mode in EELS should be used with great caution in quantitative BR analyses of 5f populations.