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

Abstract

Uranium dioxide (UO₂) 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 UO₂, the mineralogical form of UO₂ 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 UO₂ specimens: synthetic single-crystal UO₂, 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^-1). 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 UO₂. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO₂ (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 UO₂ under a variety of conditions.