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The Activation of Non-evaporable Getters Monitored by AES, XPS, SSIMS and Secondary Electron Yield Measurements

Source: OAI

ABSTRACT In this thesis the potential of the three classical surface analysis techniques Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS) for the characterisation of non-evaporable getter (NEG) materials is assessed and artefacts are described. The various NEG samples have been analysed in the context of the development of NEG thin film coatings for use in accelerator ultra high vacuum (UHV) systems. The secondary electron yield (SEY), which is a functional surface property of great importance for the application of NEG to accelerators, has been measured. The maximum SEY of an air exposed TiZr and TiZrV coating can be reduced from above 2.0 to below 1.1 during a 2 h heat treatment at 250 and 200 °C, respectively. Saturating an activated TiZrV surface in UHV increases the maximum SEY by about 0.1. Thus, in UHV the SEY of an activated NEG coating does not exceed the threshold value of 1.35, above which multipacting is predicted to occur in the LHC beam vacuum system. The influence of air exposures on the SEY of metals and the effect of thermal treatments in general are discussed. Electron beam induced surface modifications on technological metal surfaces in the electron dose range 10-6-1 C mm-2 have been studied in the context of the surface conditioning in accelerator UHV systems. Electron irradiation causes a surface cleaning through electron stimulated desorption (ESD) and simultaneously the deposition of a carbonaceous surface layer. Both processes reduce the SEY. On a saturated NEG electron stimulated carbon adsorption from gas phase CO and CO2 takes place while the CH4 pressure has no influence on the carbon deposition rate. The mechanism and the kinetics of the different processes are discussed.

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