Optimisation of the Positions and the Width of the Energy Windows for the Recording of EFTEM Elemental Maps

Abstract

Due to the small cross-sections for inner-shell excitations the detection limit in elemental maps is determined by the signal-to-noise ratio (SNR). Therefore it is vital to choose instrumental parameters leading to an optimized SNR. We have investigated the influence of the positions and widths of the energy windows for recording elemental maps following earlier works by Kothleitner [1] and Berger [2]. To be able to easily obtain reliable data for arbitrary energy windows, we acquired experimental electron energy loss spectra (EEL spectra) and determined the values for energy windows of larger widths by summing the spectral data over the corresponding energy losses. For the optimization procedure of the SNR we wrote a computer program to deal with an experimentally measured electron energy loss spectrum and varied the energy loss window positions in the pre-edge as well as in the ionization edge region, the energy slit width and the width of the pre-edge region, using one energy loss window above and at least three windows below the characteristic energy loss. The number of pre-edge windows can be selected by the user. The calculation performed by this program yields the energy loss window positions and the window width referring to the maximum SNR as well as the corresponding SNR value. The window positions and window width may then be applied to record an inelastically filtered image series from which an elemental map can be computed.