Practical Electron Microscopy and Database

A single-channel electron detector behind the slit of an energy filter produces an electrical signal proportional to the electron intensity at each energy loss. This system normally consists of a scintillator and a photomultiplier tube. This simple system works well but provides noisy data at high-energy loss, where the electron intensity is very weak. The reason lies not in the photomultiplier tube, which is a low noise detector, but in the fact that only a small range of energy loss is sampled at any event, resulting in pronounced noise due to the limited number of electrons being recorded. On the other hand, the spectrometer background originates mainly from backscattering of the zero-loss beam from a beam-trap aperture located in front of the detector, also resulting in most noticeable at high energy loss ( > 1 kV).

To facilitate recording higher energy losses, parallel EELS detectors were developed. They contain no energy selecting slit, so no electrons are wasted. Practically, larger pre-edge intervals (in three window method) can be chosen at higher energy losses due to less likely edge overlappings and can also allow for wider post-edge windows.