Fixed-angle reflectance from uranium compounds and U-bearing minerals: experimental challenges to determine the optical constants n and k

Brent M. DeVetter; C. Tom Resch; Tanya L. Myers; Jordan F. Corbey; Molly Rose K. Kelly-Gorham; Bret D. Cannon; Nicole K. Scharko; Timothy J. Johnson

doi:10.1117/12.2519504

17 May 2019 Fixed-angle reflectance from uranium compounds and U-bearing minerals: experimental challenges to determine the optical constants n and k

Brent M. DeVetter, C. Tom Resch, Tanya L. Myers, Jordan F. Corbey, Molly Rose K. Kelly-Gorham, Bret D. Cannon, Nicole K. Scharko, Timothy J. Johnson

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Proceedings Volume 11010, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XX; 110100S (2019) https://doi.org/10.1117/12.2519504
Event: SPIE Defense + Commercial Sensing, 2019, Baltimore, MD, United States

Abstract

Fixed-angle reflectance spectroscopy using a commercial Fourier transform infrared (FTIR) spectrometer was employed to derive the optical constants n and k of several uranium compounds. This technique relies upon measurement of the quantitative reflectance R(ν) spectra from a polished surface across a broad spectral range (in this case, the mid- and far-IR covering ca. 7500 to 50 cm^-1 ) followed by application of the Kramers-Kronig transformation (KKT). Near-normal fixed-angle measurements as used in this technique require continuous reflectance spectra to as low a wavenumber value as possible. Here, we discuss some of the many challenges in measuring the far-IR and very far-IR (terahertz) spectra using an interferometric instrument, particularly those stemming from small sample sizes, typically just millimeters on a face for crystalline samples, as well as limitations due to optical components and diffraction. We apply this method to single-crystal UO₂ and its mineralogical form uraninite, as well as other Ubearing minerals such as autunite [Ca(UO₂)₂(PO₄)₂·8-12H₂O] and the dehydrated form of autunite, meta-autunite. In addition to the specular reflectance spectra, x-ray diffractometry was used as a confirmatory technique to analyze the surface composition of the species. Deriving the infrared optical constants for such U-bearing species (as well as other solids) will enable nondestructive detection under a variety of environmental and compositional conditions.

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Brent M. DeVetter, C. Tom Resch, Tanya L. Myers, Jordan F. Corbey, Molly Rose K. Kelly-Gorham, Bret D. Cannon, Nicole K. Scharko, and Timothy J. Johnson "Fixed-angle reflectance from uranium compounds and U-bearing minerals: experimental challenges to determine the optical constants n and k", Proc. SPIE 11010, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XX, 110100S (17 May 2019); https://doi.org/10.1117/12.2519504

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KEYWORDS

Reflectivity

Minerals

Crystals

Uranium

Infrared radiation

Reflectance spectroscopy

Solids

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