X-ray photoelectron spectroscopy on uranium oxides: a comparison between bulk and thin layers

The use of sputter deposited thin layers of UO2 as a model system for the investigation of fuel–fission product interactions is presented. The representativity of the layers for the bulk system will be validated and it will be shown, both on theoretical and experimental grounds, that layers of stoichiometric UO2 can be produced by this method. A comparison will be made between X-ray photoelectron spectroscopic (XPS) results on bulk UO2 and on the deposited layers. The films deposited can easily be doped with other elements, such as fission products, by codepositing these elements with the UO2. This codeposition technique has subsequently been used to produce layers of UO2 containing cesium. It will be demonstrated that the codeposition with cesium produces uranium in higher valence states (up to UVI), while without cesium, no higher uranium valencies can be obtained.