Signal linearity in XPS counting systems

Abstract

Abstract Inadequate design, deterioration through ageing and poor setting-up procedures may cause the intensity measuring systems in XPS instruments, and very possibly in all simple particle counting systems, to show non-linearities. Even in correctly designed and used equipment smaller non-linearities occur but these, being predictable, may be corrected to a high level of accuracy. Non-linearities are shown for both the high and low count rate regimes for both single channel and multidetector systems. To monitor these defects for XPS systems three simple methods are described which require no additional equipment. The first two, for assessing the effects at high count rates, make use of `ratio plots' which involve the ratios of intensities measured at two X-ray source emission currents. The first of these methods uses the survey spectrum from a copper or stainless steel sample measured in either the constant ΔE/E or constant ΔE mode. The second method uses the Cu 2p3/2 peak to diagnose other effects. For very low count rates a signal generator simulating the detector pulses, but with pulses evenly spaced in time, may be used for checking the electronic part of the circuit. Using these methods, non-linearities of up to 50% are illustrated for commercial equipment for electron counting in common analytical use.