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Electron source
The electron source is the RHEED- gun RG15.
Detector and analyser
The figure shows the outline of the detector and analyser.
Outline of the SPA-RHEED-System with energy analyser. The RHEED-gun RG15 is not shown.
The octopol’s front flange has a inner diameter of 33mm. Thus for the flange to sample distance shown the octopole has a angle acceptance of ± 6.8°. The next figure shows a scheme of the electronoptical active parts of the detector.
A possible electron trajectory is shown in the region of the double octopole deflector. After passing the aperture
the electrons either are counted by the "in-line" channeltron or deflected onto the analyser.
Coordinates for momentum transfer (k-space scan)
The double octopoles of the gun and of the detector enable the choice of the momentum transfer at the sample in
all three directions. Fig. 3.3 shows the coordinate systems of gun, sample and detector. The components of the initial momentum kiand of the final momentum ka
are a function of the angles f , q , b and g :
  
  
with  and 
With these formula the momentum transfer in all three space directions can be calculated.
The coordinate systems of gun may be rotateted to the sample’s system, depending on the sample position or
the guns mounting. As well the detector system may be rotated. This is measured by the angles a and d as shown in the figure.
Octopole deflector
The double octopole deflector deflects an electron beam starting with an definite angle onto the aperture. All
angles are deflected in a way that the electrons reach the aperture parallel to the detector’s axis
Suppressor electrode with selectable apertures
The suppressor electrode is used to repell low energetic electrons during spot analysis. The electron optics is
designed to give a sharp cutoff to negative suppressor voltages which enables one to use this as a high-pass
energy filter. The derivative of this high-pass electron spectrum gives a true electron energy spectrum. The energy
resolution of the suppressor varies with the aperture size. With the smallest aperture an energy resolution down to 1.5eV could be achieved whereas the 100m m aperture gives a cutoff with an energy width of about 4eV.
Aperture wheel / aperture change
The suppressor optics features five selectable apertures of different sizes that can be choosen by rotating the
aperture wheel. The five apertures are: Ø2mm, 300m m, 100m m, 30m m and 10m .
Analyser CSA200HV and 90°-Deflector
The CSA200HV is an electrostatic Cylinder Sector Analyser with a slit-to-slit distance of 200mm. Its inner
isolation and feedthroughs are prepared for energy analysis of electrons with an energy of up to 15keV. For further details on the analyser see CSA200 / 300.
The 90°-deflector employs a spherical electrostatic field for deflection that is simulated by cylindrical electrodes.
An electron beam that is focused onto the entrance slit will form a parallel beam at the entrance slit. This facilitates the transfer to the analyser.
The entrance element of the deflector is designed to cause optimal focussing of the electrons onto the entrance
slit together with the geometry of the suppressor electrode. The 90°-deflector is also mounted and equipped with
feedthroughs to deflect electrons of an energy of up to 15keV. When operating with the power supply Analyser
Control the maximum electron energy accessible is 3276eV. A power supply for higher energies is in preparation.
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