Optical and Chemical Characterization of Uranium Dioxide (UO₂) and Uraninite Mineral: Calculation of the Fundamental Optical Constants

Abstract

Uranium dioxide (UO) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO, the mineralogical form of UO called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO specimens: synthetic single-crystal UO, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO under a variety of conditions.

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance

“…Growing crystals from such powders is difficult and time-consuming, and in most cases impossible. 12,13 Due to the lack of such crystals for most substances, alternative methods for extracting n / k from materials other than single crystals include in situ evaporation techniques 14 as well as pressing the powder form into a uniform pellet or disk for reflectance measurements. The salt (NH 4 ) 2 SO 4 was selected not only due to its importance in atmospheric chemistry, but because its n and k values have been previously reported, and the powder flows relatively easily to form pellets with measured densities close to its literature value.…”

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

“…45 The VNIR spectra were also measured, but those measurements are not discussed in this paper. 28,44 Briefly, two types of spectra are recorded for each sample as shown in Fig. 1, namely the diffuse-only reflectance spectra and the hemispherical (a.k.a.…”

“…Many HSI studies followed in the 1980s and 1990s 21 including, e.g., the first TIR imaging studies by Kahle et al 22 as well as several others. In many of these pioneering works, it was noted that minerals containing anion chemical moieties such as silicates, 23 carbonates, 24 phosphates, 25 sulfates, 26 or cations such as ammonium or uranyl 27,28 are all known to display characteristically strong emission/reflectance features in the LWIR between 7 and 13 μm, making them amenable to such TIR detection.…”

Practical model for improved classification of trace chemical residues on surfaces in active spectroscopic measurements

“…One of the more widely accepted approaches is to use single-angle reflectance spectroscopy followed by Kramers-Kronig transformation for estimating optical constants (ñ) for crystalline solids. [21][22][23][24] For solid minerals, DeVetter et al have shown that using carefully designed mask apertures with low reflectance is more optimal for solid minerals. 25 The optical constants used in this research were measured and provided by Pacific Northwest National Laboratory (PNNL) through IARPA's (Intelligence Advanced Research Projects Activity) SILMARILS (Standoff ILluminator for Measuring Absorbance and Reflectance Infrared Light Signatures) program.…”