EELS Signal/intensity Affected by Collection
& Convergence Angles & Apertures
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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There is a trade-off between the EELS signal strength and energy resolution in the selection of collector aperture (also called entrance aperture) sizes, because of the following reasons:
         i) The aberrations are worse, and thus the energy resolution is degraded as the width of the electron beam entering the prism increases.
         ii) The collector aperture limits the number of electrons entering the prism more significantly, and thus the efficiency of the spectrum detection decreases as the width of the electron beam entering the prism decreases.

Aperture sizes in the range of 0.5 to 5 m are typically provided by the manufacturers and applied in various analyses.

Two of the important factors for elemental quantities using EELS (Electron Energy Loss Spectroscopy) are the convergence angle, β, and the collection angle, α, because they enter into the formulae for the partial inelastic scattering cross-sections.

In many cases when the EELS signal is very weak and if you do not need to care about the spatial resolution, no condense aperture and a big EELS entrance aperture (e.g. 5 mm) can be used to increase the signal. For instance, no condense aperture was used to capture ELNES spectra at the Al K-edge for the Al2O3/Al nano samples [1]. In these measurements, the operator has to pay attention to the electron intensity during alignment of zero-loss peak in order to avoid damaging the EELS detector.

 

 

 

 

 

[1] J. C. Sánchez-López, A. Caballero and A. Fernández, Characterisation of Passivated Aluminium Nanopowders: An XPS and TEM/EELS Study, Journal of the European Ceramic Society 18 (1998) 1195 - 1200.

 

 

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