AVS 71 Session AC+MI-FrM: Spectroscopy, Spectrometry, 5f Behavior and Forensics

Plutonium metal is highly reactive by immediately forming an oxide layer when exposed to air and quickly forming a hydride when exposed to hydrogen. The fundamental understanding of the impact of impurities and defects on the effect of oxidation and corrosion of Pu is limited in both experimental and theoretical studies. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) is a unique surface science technique that is highly sensitive to the first 1-2 monolayers of the surface (<1nm) and can detect all isotopes (including hydrogen) at parts-per-million levels, which gives a comprehensive survey of surface constituents. This technique also provides a structural and reactivity, chemisorption versus physisorption, information and complements other surface science techniques, such as X-ray photoelectron spectroscopy (XPS). In general, ToF-SIMS may provide a more in-depth analysis of surface constituents that otherwise might not be detected or deconvolute from a complex XPS spectra. A newly installed ToF-SIMS nanoToF 3 at LANL uses a 30 kV Bi₃⁺⁺ liquid metal ion gun as the primary ion source and has a mass resolution of 12,000 (Δm/m), thus providing a new level of mass resolution and sensitivity on Pu surfaces that was not previously achieved. I will show recently collected ToF-SIMS results of hydrogen and oxygen gas reactions on alpha-Pu and 2 at. % Ga stabilized δ-Pu surfaces and how they compare with other.

The advent of new, powerful, highly efficient, multi-component, X-ray monochromators used in the detection of tender x-rays has revolutionized spectroscopic investigations of the 5f electronic structure. All of the new experiments are, in essence, variants of X-ray Emission Spectroscopy (XES), where the improved monochromatized detection, applied to novel specific decay pathways, plays a key role. In HERFD (High Energy Resolution Fluorescence Detection) a type of Resonant Inelastic X-Ray Scattering (RIXS), the monochromatized XES detection allows the performance of a scattering experiment with vastly improved resolution.It is argued here that HERFD devolves into a higher resolution version of X-Ray Absorption Spectroscopy (XAS). It has been shown that the M₄ and M₅ spectra are essentially direct measurements of the j-specific (5f_5/2 and 5f_7/2) Unoccupied Density of States (UDOS), which can be directly correlated with the UDOS from Inverse Photoelectron Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS). [1-3] Furthermore, a remarkable level of agreement is achieved between a model based upon the UDOS of Th and a series of HERFD and IPES/BIS results with various 5f occupation levels. [4-6] Finally, the historical record of XAS will be examined, demonstrating the success of various resonant decay schemes as measures of the underlying XAS.

1. J.G. Tobin, H. Ramanantoanina, C. Daul, P. Roussel, S.-W. Yu, D. Sokaras and A. Kutepov, "Isolating Multiplet Structure in 5f Inverse Photoemission, " Solid State Sciences 160, 107779 (2025).
2. J. G. Tobin, H. Ramanantoanina, C. Daul, S.-W. Yu, P. Roussel, S. Nowak, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D. Sokaras, "The Unoccupied Electronic Structure of Actinide Dioxides," Phys. Rev. B 105, 125129 (2022)
3. J. G. Tobin, S. Nowak, C.H. Booth, E.D. Bauer, S.-W. Yu, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D.Sokaras, "Separate Measurement of the 5f5/2 and 5f7/2 Unoccupied Density of States of UO2," J. El. Spect. Rel. Phen. 232, 100 (2019).
4. J. G. Tobin, S. Nowak, S.-W. Yu, P. Roussel, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D. Sokaras, "The Underlying Simplicity of 5f Unoccupied Electronic Structure," J. Vac. Sci. Tech. A 39, 043205 (2021).
5. J. G. Tobin, S. Nowak, S.-W. Yu, P. Roussel, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D. Sokaras, "Comment on The Underlying Simplicity of 5f Unoccupied Electronic Structure," J. Vac. Sci. Tech. A 39, 066001 (2021).
6. J. G. Tobin, S. Nowak, S.-W. Yu, P. Roussel, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, D. Sokaras, "The Thorium Model and Weak 5f Delocalization," J. Vac. Sci. Tech. A 40, 033205 (2022).

Predicting material properties in f-block elements, especially actinides, is complicated by their complex electronic structures, such as multiconfigurational ground states and strong correlation effects. These structures arise from large electron degrees of freedom, posing challenges in modelling their behavior. A non-integer orbital occupancy representation describes the superposition mixing of multiple near-energy degenerate configurations. This representation generalizes by approximation to established ground states in elements with simpler electronic structures and enables an over-approximation of entropy for multiconfigurational ground state structures. A complementary combinatorial approach applies Hund's rule constraints to establish an under-approximation of entropy. Together, these methods bracket entropy limits, providing insights into electronic configurations that most significantly contribute to the multiconfigurational ground states of actinide elements to a low order approximation. Under an energy degeneracy assumption weighted by configuration permutations, calculations iteratively refine the contributing configurations, yielding low-order orbital occupancy estimates that align with experimental data and theoretical models. (LA-UR-25-22711)

Americium oxides are an integral part of the existing nuclear fuel cycle and are important considerations in future mixed-oxide (MOX) fuel cycles that involve the minor actinides for recycling. Knowledge of the chemical bonding and physical properties of the Am oxides is increasingly important for these envisioned future nuclear cycles. Synchrotron radiation soft x-ray spectroscopy complemented by theoretical calculations were utilized to characterize the electronic structure of americium dioxide (AmO₂) and americium sesquioxide (Am₂O₃). For the sesquioxide, this included x-ray absorption near-edge structure (XANES) spectroscopy studies at the Am O_4,5- and the N_4,5 -edges (Am 5d_5/2,3/2; Am 5d_5/2,3/2; respectively) and resonant inelastic x-ray scattering (RIXS) measurements at the Am O_4,5-edges. For the dioxide, XANES investigations conducted at the N_4,5-edges were compared to spectra obtained from the sesquioxide as well as a U_0.9Am_0.1O₂ specimen. Experiments were performed at beamlines of the Advanced Light Source at the Lawrence Berkeley National Laboratory and at MAXlab, (Lund, Sweden).

The results of the synchrotron radiation experiments were compared to theoretical calculations performed with several methods. These included the Anderson Impurity Model (AIM) with full multiplet structure to account for the 5f electrons, and progressively employing crystal-field multiplet theory when appropriate (Am₂O₃) starting with an atomic multiplet formulation. The results of the XANES and RIXS experiments combined with theory show that AmO₂ can be classified as a charge-transfer compound with a 5f occupation of 5.73 electrons with significant covalence in the Am 5f - O 2p bonds. Contrasting to this behavior, Am₂O₃ can be well-represented by a Mott-Hubbard system with a 5f occupation of 6.05 electrons. The RIXS result suggest that Am₂O₃ possesses weak Am 5f - O 2p hybridization. A recent development by Tobin et al. has utilized FEFF to identify the spectral shape on the higher energy side the Am N_4,5-white lines as arising scattering features.

References

S. M. Butorin and D. K. Shuh, "Chemical Bonding in the Americium Oxides: X-ray Spectroscopic View," Sci. Rep. 13, 11607 (2023).

S. M. Butorin and D. K. Shuh, "Electronic Structure in Americium Sesquioxide Probed by Resonant Inelastic X-ray Scattering," Phys. Rev. B 108, 195152 (2023).

J. G. Tobin, S.-W. Yu, D. K. Shuh, and S. M. Butorin, "A FEFF Analysis of Americium Oxides," J. Vac. Sc. Technol. A 42, 023209 (2024).

Motivated by a recent experimental study [1], we model the valence-to-core resonant inelastic x-ray scattering (RIXS) measured at the uranium M₅ edge in insulating compounds UO₂ and UF₄. We employ the Kramers–Heisenberg formula in conjunction with the Anderson impurity model extracted from the corresponding LDA+DMFT electronic-structure calculations [2], in which the double-counting correction is adjusted to best reproduce the experimental valence-band XPS spectra [3,4]. In our simulations, we find two sets of excited states. One group is formed by excitations of the 5f² shell that appear at energy losses ≲ 4 eV. These excitations are not well resolved in the experimental data [1] as they are largely obscured by the elastic peak. The other group of excited states is formed by the charge-transfer excitations corresponding to a transfer of an electron from the oxygen/fluor 2p states to the uranium 5f shell. We identify these excitations with the spectral feature experimentally observed at an energy loss of roughly 8–10 eV, in agreement with other closely related investigations [5,6]. Our model estimates the intensity, with which the charge-transfer excitations appear in the RIXS spectra, to be larger in UO₂ than in UF₄, just like it is observed in the experiment [1]. We analyze in some detail how this intensity depends on the strength of the metal-ligand hybridization and on other parameters of the model, such as the magnitude of the core-valence interaction acting in the intermediate state of the RIXS process.

1. J. G. Tobin et al., J. Phys.: Condens. Matter 34, 505601 (2022).

2. J. Kolorenč, A. Shick, A. Lichtenstein, Phys. Rev. B 92, 085125 (2015).

3. Y. Baer, J. Schoenes, Solid State Comm. 33, 885 (1980).

4. A. Yu Teterin et al., Phys. Rev B 74, 045101 (2006).

5. K. Kvashnina et al., Chem. Commun. 54, 9757 (2018).

6. K. Kvashnina et al., Phys. Rev. B 95, 245103 (2017).

Plutonium (Pu) is a complex element with an interesting electronic structure, and it is also a material of great importance for both nuclear energy and security. To better understand its interaction with gases, surface analysis of the alpha (ɑ) variant provides valuable insight when coupled with a technique such as X-ray photoelectron spectroscopy (XPS). Different core electron orbitals may be probed, and binding energies from emitted electrons provide information on the local chemical state, i.e., degree of oxidation, reduction, or carbonization within the ɑ-Pu.

Here we investigated the effect of hydrogen (H₂) gas dosing of ɑ-Pu surfaces, which reacts and forms plutonium hydride (PuH₂) at temperatures >100 °C. By slowing the kinetics at room temperature, we may witness H₂ dynamics on native ɑ-Pu surfaces, and view how Pu materials such as oxidized and carbonized forms evolve with H₂ exposure. In addition, we present our findings from density functional theory (DFT) validating experimental observation. To provide an example, Fig. 1 shows a plot of various Pu 4f spectra. In red, metal ɑ-Pu is observed after having been sputtered to remove both surface contaminants and the native oxide layer. The defining metal feature in the 4f_7/2 peak is seen at ~422.2 eV. Next, the sample was dosed with H₂ gas for 198 Langmuir (L) (blue line), and then the exposure was increased (green line) until reaching 396 L. A clear reduction in the signal’s intensity is seen in both the 5/2 and 7/2 metal peaks. Secondly, the 7/2 satellite shows an increase in signal, which is indicative of surface passivation. Clearly, more is needed to know what these H₂ induced changes signify, and this presentation will show additional spectra from the O 1s, C 1s, and the Pu valence band, along with DFT to contextualize the on-going mechanisms of H₂ with the ɑ-Pu surface.

Fission track analysis is a technique employed in nuclear forensics to identify and examine fission isotopes. This technique is specific for small samples in the range of a few picograms or to analyze bigger samples and check for homogeneity.

In the old Lexan detector, the tracks are pretty close, and that limits much the ability to count the tracks and analyze the length of the tracks. The main target of the fission track is to locate the fission ions in between a lot of other isotopes. The located fission ions could be transferred to other techniques like ICP-MS for further analysis. Better separation between tracks and analysis could lead to showing the yield of fission products, which is specific to every fission isotope. The yield fission products are two humps on the graph that are equal in area. One hump is around A=95, 135; in the length of the track histogram, the two humps look different due to the difference in dE/dx of the different energies. The light elements hump looks narrow, and the heavy elements hump looks wide; still, the area of those humps is equal. We created a novel detector for fission track analysis with the Lexan-modified detector.
This innovative detector exhibits more dispersion of fission tracks. In this innovative approach, we adhered aerogel to the Lexan. The aerogel has a low absorption coefficient; hence, it does not substantially obstruct the fission products in the detector. The incorporation of aerogel modifies the geometric configuration, enlarges the dimensions of the fission track stars, and increases the separation between individual tracks, as seen in Fig. 1 in the supplement. A fission track star of a size of 150 microns can reach 350 microns with the aerogel configuration. Given that the fission products are distributed isotopically while the aerogel is two-dimensional, it is necessary to employ stereoscopic projection to facilitate their integration. An illustration of this enhancement of the fission track star is seen in Fig. 1, where the dimensions of the fission track star are greater and the tracks are widely spread. The newly developed analytical program, Finder, may utilize a 2D representation of the fission track star. Whether an actual star or a simulated star, of a fission track to conduct analysis and provide 3D evaluations, therefore illustrating the fission yield of the fission isotope. The analysis of the fission track star is shown in Fig. 2, supp. The fission track analysis of ²³⁵U star in that software is depicted in Fig. 3 supp.

Fission track length before the detector and in it are shown in that figure of the fission track analysis of ²³⁵U star.