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Depending on the polarity of the incident particles a positive high voltage must be applied to the output end of the CEM or a negative high voltage must be applied to the CEM input. It is important not to create a potential barrier to the charged incident particles which could then hinder their penetration into the CEM funnel. These considerations are of course not applicable to the detection of UV and X-ray photons. Often a CEM is part of an electro-optical system consisting of slits, ring electrodes or grids. The influence of their electric fields must also be taken into account. In addition to such desired effects as the repelling of secondary particles by means of a grid electrode or the increase of the effective area of the CEM by a ring electrode which may be necessary in order to prevent the loss of secondary electrons produced on the funnel edge unwanted effects may occur as well. If the distance between the electro-optical elements and the CEM is too small the applied voltage can lead to field electron emission and unwanted pulses. A ground potential in front of the CEM that operates with negative high voltage generates an acceleration voltage onto the secondary electrons in the CEM funnel and can reduce gain significantly. The simplest way to create a potential difference
between the end of the CEM and the anode is by using a resistor of
about 10 megohm that is in series with the CEM. Fig. 13 shows the
electrical connections for negative applied voltage; this was used
for the measurements in the chapter (Quality Assurance) (Fig. 15
- 22). Fig. 14 shows an example for the use of positive high voltage.
Here a capacitor must be used in the signal line preferably inside
the vacuum chamber. |