This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
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.
Figure 4936. Schematics of the electron optical column in a modern
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.
analytical electron microscope operated in STEM mode.
The collection angle for an energy loss can simply be optimized to an angle slightly larger than the relevant characteristic inelastic scattering angle (θE), 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.