Adsorption of Atmospheric Gases on Pu Surfaces
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Adsorption of Atmospheric Gases on Pu Surfaces A.J. Nelson1, K.S. Holliday1, J.A. Stanford1, W.K. Grant1, R.G. Erler1, P. Allen1, W. McLean1 and P. Roussel2 1 Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. 2 AWE Aldermaston, Reading, Berkshire RG7 4PR, U.K. ABSTRACT Surface adsorption represents a competition between collision and scattering processes that depend on surface energy, surface structure and temperature. The surface reactivity of the actinides can add additional complexity due to radiological dissociation of the gas and electronic structure. Here we elucidate the chemical bonding of gas molecules adsorbed on Pu metal and oxide surfaces. Atmospheric gas reactions were studied at 190 and 300 K using x-ray photoelectron spectroscopy. Evolution of the Pu 4f and O 1s core-level states were studied as a function of gas dose rates to generate a set of Langmuir isotherms. Results show that the initial gas dose forms Pu2O3 on the Pu metal surface followed by the formation of PuO2 resulting in a layered oxide structure. This work represents the first steps in determining the activation energy for adsorption of various atmospheric gases on Pu. INTRODUCTION Understanding the surface dynamics of oxidation and reduction reactions on actinide materials is important to the continued development of advanced nuclear fuel technologies. Knowledge of the surface reactions with various environmental and atmospheric agents, and the subsequent degradation processes, over a wide range of temperatures are vitally important. Past reviews of the oxidation of actinide elements [1-4] illuminate the ambient reaction mechanisms and growth kinetics at atmospheric and reduced pressures. The dynamics of gas-surface interactions depend on the energy of the gaseous species, the temperature, and the atomic structure of the solid surface. Radiological dissociation of the gaseous species and the electronic structure of actinide surfaces further enhance surface reactivity. [5] Here we investigate the chemical bonding of gas molecules adsorbed on Pu metal and oxide surfaces at 190K and 300K using X-ray photoelectron spectroscopy (XPS). The evolution of the Pu 4f and O 1s core-levels as a function of gas dose rates provides a comprehensive characterization of the oxide growth. EXPERIMENT Gas dosing of the electropolished gallium stabilized -phase Pu was performed in the UHV chamber at 1 x 10-6 torr after Ar-ion sputter etching to remove the native oxide. Sputter cleaning was accomplished with 4.5 kV Ar+ rastered over a broad 7 x 7 mm area giving a sputter etch rate of 0.1 nm/min. XPS experiments were subsequently performed using a Physical Electronics 5400 system with an Al K radiation (1486.6 eV) source for excitation and a hemispherical analyzer. XPS spectra were acquired in normal emission geometry. The pass
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energy was 22.35 eV giving an energy resolution of 0.5 eV that when combined with the natural Al K line width gives a resolvable XPS peak width of approximately 1.5 eV FWHM. Deconvolution of non-resolved peak
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