cantered at the sample.Its negative charge causes the electron beam to broaden. What can be prevented by charging the top and bottom plates of the CHA deflectors negative. What again causes a change in the deflection angle and has to be considered when designing the experiment. In the scattering process at the sample the electrons can lose energies from several 10−2 eV up to a few electron volt. The scattered electron beam which is of around 10−3 lower flux than the incident beam then enters, the analyzer, another CHA. The analyzer CHA again allows only electrons of certain energies to pass to the analyzing unit, a channel electron multiplier (CEM). For this analyzing CHA the same facts are valid as for the monochromator. Except that a higher resolution as in the monochromator is wanted. Hence the radial dimensions of this CHA are mostly bigger by like a factor 2. Due to aberrations of the lens systems the beam has also broadened. To sustain a high enough electron flux to the analyzer the apertures are also about a factor 2 bigger. To make the analysis more accurate, especially to reduce the background of in the deflector scattered electrons often two analyzers are used, or additional apertures are added behind the analyzers as scattered electrons of the wrong energy normally leave the CHAs under large angles. In this way energy losses of 10−2 eV to 10 eV can be detected with accuracies of about 10−2 eV. General problems of HREEL spectrometers Due to the electron flux the apertures can become negatively charged, which makes them effectively smaller for the passing electrons. This has to be considered when doing the design of the setup as it is anyway difficult to keep different potentials, of repeller, lenses, screening elements, and the reflector, constant. Unstable potentials on lenses or CHA deflectors would cause fluctuations in the measured signal. Similar problems are caused by external electric or magnetic fields, either they cause fluctuations in the signal, or add a constant offset. That is why the sample is normally shielded by equipotential, metal electrodes to keep the region of the sample field free so that neither the probe electrons nor the sample is affected by external electric fields. Further a cylinder of a material with a high magnetic permeability, e.g. Mu-metal, built around the whole spectrometer to keep magnetic fields or field inhomogeneities at the experiment down