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

Source: OAI


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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    • "However, the possibility of surface oxidation by dissociation of the CO could increase the SEY. Exposure to background CO alone does increase the SEY [14]; however, there will be competition for the case of H + 2 /CO + ionsputtering vs. CO vacuum recontamination, as is the case for a circulating positron beam. "
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    ABSTRACT: In many accelerator storage rings running positively charged beams, multipactoring due to secondary electron emission (SEE) in the beam pipe will give rise to an electron cloud which can cause beam blow-up or loss of the circulating beam. A preventative measure that suppresses electron cloud formation is to ensure that the vacuum wall has a low secondary emission yield (SEY). The SEY of thin films of TiN, sputter deposited non-evaporable getters and a novel TiCN alloy were measured under a variety of conditions, including the effect of re-contamination from residual gas.
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    ABSTRACT: In the beam pipe of the positron Main Damping Ring (MDR) of the Next Linear Collider (NLC), ionization of residual gases and secondary electron emission give rise to an electron cloud which can cause the loss of the circulating beam. One path to avoid the electron cloud is to ensure that the vacuum wall has low secondary emission yield and, therefore, we need to know the secondary emission yield (SEY) for candidate wall coatings. We report on SEY measurements at SLAC on titanium nitride (TiN) and titanium-zirconium-vanadium (TiZrV) thin sputter-deposited films, as well as describe our experimental setup.
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