Oxidation of a depleted uranium‐5 wt% molybdenum (U‐5Mo) alloy in UHV by AES and XPS

Abstract

Early stage oxidation of a dilute‐depleted uranium‐molybdenum alloy was analysed in situ under ultra‐high vacuum conditions by AES and XPS. At the equivalent of less than 300 ns at 1‐atm O2, U‐5Mo oxidizes to form stoichiometric UO2. No molybdenum oxidation is observed. After an oxygen dose of approximately 39 L, the oxide layer approached a limiting thickness of approximately 2.4 nm. The oxidation kinetics followed a logarithmic rate law, with the best fit to the experimental data for the oxide thickness, d, being given by d = 1.26 log(0.12t + 0.56). Changes in oxygen KLL and 1s peak positions associated with transformation from chemisorbed oxygen to metal oxide were observed at similar oxygen doses of 2.3 and 2.6 L O2 by AES and XPS, respectively, which opens up the possibility of using well‐characterized XPS chemical information to inform Auger peak shifts.

Quantitative XPS of plutonium: Evaluation of the Pu4f peak shape, relative sensitivity factors and estimated detection limits

“…It is of interest to contrast how reactive plutonium is compared with other actinide metals. Bacon et al have reported the oxidation of a uranium molybdenum alloy, U 0.95 Mo 0.05 , in the UHV environment of the XPS spectrometer and showed that it follows a logarithmic growth rate 64 . The intensity of the oxide and metal components of U4f region was extracted by curve fitting, and the oxide thickness was calculated as a function of time assuming a uniform film.…”

“…Comparison of the in situ oxidation in UHV of α‐Pu (blue), δ‐Pu(Ga) (red) and logarithmic growth under similar conditions of a U–Mo alloy (black line) 64 …”

“…The O1s:Pu4f oxide peak ratio is similar for both the α-Pu and δ-Pu(Ga) samples and shows a rapid increase during the initial oxidation followed by a constant value where the average stoichiometry is PuO 1.53 which is in the range reported for the cubic sesquioxide compound. 63 64 The intensity of the oxide and metal components of U4f region was extracted by curve fitting, and the oxide thickness was calculated as a function of time assuming a uniform film. Following this methodology, we have calculated the plutonium oxide growth during the in situ UHV oxidation reactions, and these are presented in Figure 9.…”

“…Comparison of the in situ oxidation in UHV of α-Pu (blue), δ-Pu(Ga) (red) and logarithmic growth under similar conditions of a U-Mo alloy (black line) 64 T A B L E 5 Estimated minimum detection limits for selected elements in α-Pu and δ-Pu(Ga) matrices helium in aged samples, and it is currently not possible to detect these elements in solids using conventional laboratory-based photoelectron spectrometers. 67…”