An EELS detector is an electron spectrometer unit and detection unit. The main feature of an electron spectrometer is that the intensity of electrons at different energies is obtained by dispersing the electrons with a magnetic prism. Over the years, there are several types of common electron spectrometers:
i) Sector magnet spectrometer (post-column filter).
ii) Ω-filter (in-column
iii) Castaing-Henry prism.
iv) Mollenstedt analyser.
The first two types of spectrometers are commercially manufactured. However, the most common electron spectrometers today, especially in STEM instruments, are based on magnetic sector prism, such as Gatan GIF systems. An important reason for its popularity is that they are not designed to be in-column, but can be attached as a peripheral to an existing EM column.
EELS can be recorded simultaneously by a photodiode or charge coupled diode (CCD) array. The schematics in Figure 4529 shows the electron optical column in a modern analytical electron microscope operated in STEM mode, indicating the projector lens controlling detector collection angle.
Figure 4529. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.
A EELS spectrometer consists of a couple of lenses. For instance, Enﬁna ER has 2 hexapoles and 5 electromagnetic round lenses, 7 dipoles for alignment, and 1 quadrupole and 1 additional hexapole for astigmatism correction.
Note that occurrence of degradation in the energy spread can be due to the substantial broadening in the detector for EELS if there is cross-talking between neighbouring channels.