In serial EELS detectors, the spectrum is acquired by scanning the signal across a single detector with one channel at a time. In those systems, a narrow slit is located at the spectrometer image
plane, in front of a single-channel electron detector (scintillator and photomultiplier tube). In this case, spectrum is recorded by varying the strength of the magnetic field, thus sweeping the spectrum over the slit and acquiring the spectrum serially.
Electrons passing through this
slit can be detected in various ways but the most common system uses a scintillator (to convert the electrons to visible light) followed by a photomultiplier (PM) tube
to produce an electrical signal as shown in Figure 4881. Electron current from the PM tube can be digitized, using an analog-to-digital or a voltage-to-
frequency converter, to send to multichannel
analyzer (MCA) (or called as controller) and to store in a computer.
Figure 4881. Schematic of Serial EELS system.
The controller sends a ramp signal to the spectrometer and thus,
the magnetic field is slightly changed. In this case, the spectrum is scanned across the detector slit and a
time-dependent energy loss spectrum is recorded in successive
memory locations (channels) and stored in the computer.
To avoid recording and storage problems, the spectrum can be broken up into two or more ranges,
separated by a gain change (typically a factor of 100–1000) by varying the
sensitivity of the photomultiplier, either by changing the photocathode voltage or by
changing from analogue detection to electron counting. The electron slit and its surroundings must be carefully designed so as to minimize
the spectrometer background which mainly arises from high-energy electrons
which are backscattered from the slit blades and which, through multiple scattering,
eventually reach the scintillator. For accurate measurements, it is necessary to subtract this background from the acquired data.
It is important to make the slit width variable. Gatan EELS systems normally have two slits at different widths. The narrow slit limites spectrometer aberrations and the energy width of
the electron source ( typically 1 – 2 eV) and thus, provides the better
energy resolution, but gives the lower signal counts, therefore, is
more suitable for recording low energy losses. To record inner-shell edges, the wider
slit is needed to obtain data which are not dominated by shot noise.
The development of parallel EELS detectors, specifically multichannel arrays, has allowed a large gain in the collection efficiency, typically by a factor of 500, compared with earlier serial EELS spectrometers.