Collection Angles/apertures of EELS
- Practical Electron Microscopy and Database -
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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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Generally speaking, large collection angles of EELS will give high intensity but poor energy resolution. If it is in diffraction mode and the control collection angle (β) is controled by the entrance aperture, then a large aperture (high intensity, high β) will lower the resolution and vice versa. Furthermore, smaller collection angles also give a higher signal-to-noise ratio in the EELS spectrum.

The schematics in Figure 4936 shows the electron optical column in a modern analytical electron microscope operated in STEM mode, indicating the projector lens controlling detector collection angle.

Schematics of the electron optical column in a modern analytical electron microscope operated in STEM mode

Figure 4936. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.

For thick TEM specimens, e.g. ≥80 nm for Si, the surface-plasmon effects becomes negligible, while the beam spreading becomes significant because more electrons suffer inelastic collision in larger scattering angles. In this case, more electrons scatter outside the finite collection aperture.

The collection angle for an energy loss can simply be optimized to an angle slightly larger than the relevant characteristic inelastic scattering angleE), for instance, for 100-keV incident electrons, θE has a value of 1 mrad for a 200 eV energy loss, while 10 mrad for a 2 keV energy loss.

 

 

 

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