Monitoring the in-situ oxide growth on uranium by ultraviolet-visible reflectance spectroscopy

We demonstrate the in-situ monitoring of oxide growth on U-0.1 wt. %Cr by means of UV-visible reflectance spectroscopy in the thickness range of similar to 20-150 nm. Two different approaches are presented: In the "modeling approach," we employ a model for a metallic substrate covered by a dielectric layer, while taking into account the buildup of oxygen gradient and surface roughness. Then, we fit the simulated spectra to the experimental one. In the "extrema analysis," we derive an approximated analytical expression, which relates the oxide thickness to the position of the extrema in the reflectance spectra based on the condition for optical interference of the reflected light. Good agreement is found between the values extracted by the two procedures. Activation energy of similar to 21 kcal/mole was obtained by monitoring the oxide growth in the temperature range of 22-90 degrees C. The upper bound for the thickness determination is argued to be mostly dictated by cracking and detachment processes in the formed oxide. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759053]