The optical properties of a polished uranium surface and its epitaxial oxide, and the rate of oxide growth determined by
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0893-JJ07-06.1
The optical properties of a polished uranium surface and its epitaxial oxide, and the rate of oxide growth determined by spectrophotometry. Wigbert J. Siekhaus, Art Nelson Dep. of Chemistry and Materials Sciences, Lawrence Livermore National Laboratory Livermore, CA 94550-9234 ABSTRACT Wide-band reflectrometry and ellipsometry have been used to determine the optical properties n and k of freshly polished uranium and of the epitaxial oxide layer, and also the rate of oxide growth in air. Results for uranium metal as well as for epitaxial oxide are compared with single wavelength ellipsometry literature values. INTRODUCTION The thickness of the oxide layer grown in air on the surface of uranium is of interest to the nuclear industry. To determine it, one must know the optical constants of the underlying uranium substrate and of the oxide layer. The optical constants n(refractive index) and k(extinction coefficient) at a fixed wavelength of 546.1 nm have been evaluated in a review of preceding work to be (n=3.1, k=3.9) for uranium, and (n=2.2, k=.5) for the oxide grown on a uranium substrate [1]. The dielectric constants (and hence n and k) of single crystal UO2 have been measured over a wide range of energies [2,3]. However, the surface oxide grown in air on a uranium substrate is not single crystal UO2 but shows Raman peaks of diverse oxide moieties [4] and hence the constants derived in [2,3] do not quite apply. The optical properties of the metal over a wide wavelength range can be determined accurately only by generating and holding the atomically clean sample surface in an ultra high vacuum, since uranium oxidizes rapidly. We use reflectrometry which can collect data quickly after polishing to collect data from a surface with a minimal oxide layer. Usually ellipsometry is used to measure properties of surface layers, here we apply both reflectrometry and ellipsometry to compare both techniques. EXPERIMENTAL DETAILS A uranium sample with a total weight impurity content of approximately 150 ppm was mechanically polished, finishing with 1µm SiO2 particles. The sample surface was analyzed a few seconds after polishing by a commercial reflectrometer (n&k 1700, n&k Technology Inc., Santa Clara, CA 95054) over a spectral range from 200 to 900 nm. The incident and reflected light were close to surface normal, and since data collection time was about a second, the effect of surface oxide on optical response was minimized. The same sample was analyzed subsequently by a commercial variable angle spectroscopic ellipsometer with a wide spectral range (193-2200 nm) (VASE, J.A. Woolam Co., Inc. Lincoln, Nebraska 68508) at 65, 70, and 75 degrees angle of incidence. The ellipsometry measurements require more time, and hence the first ellipsometry data contain a larger contribution of the surface oxide.
0893-JJ07-06.2
DISCUSSION Optical constants of Uranium and Uranium surface oxide Reflectance data for the metal were processed using the Cauchy dispersion relation and for the oxide by relying on Forouhi-Bloomer
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