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4.1.2.3 Detectors<br />
The ions that are separated in the rod system on the basis of their mass-to-charge ratio can<br />
be electrically detected by means of various types of detectors:<br />
By means of a Faraday cup for direct measurement of the ion current using<br />
an electrometer amplifier<br />
Using a secondary electron multiplier (SEM) of discrete design with individual dynodes<br />
By means of a continuous secondary electron multiplier (C-SEM)<br />
Detector selection will primarily be based upon requirements that relate to detection sensitivity,<br />
detection speed and signal-to-noise ratio. <strong>How</strong>ever it will also be governed by other<br />
application-specific requirements that relate to stability, thermal and chemical resistance, as<br />
well as space requirements.<br />
Faraday cup<br />
In the simplest case, the ions strike a Faraday collector (Faraday cup), where they emit their<br />
electrical charge.<br />
= Selected positively charged ions<br />
Figure 4.15: Operating principle of a Faraday Cup<br />
The resulting current is converted to a voltage that is proportional to the ion current by means<br />
of a sensitive current / voltage inverter (electrometer amplifier). Because it is necessary for the<br />
input resistance R of the current amplifier to be extremely high, time constants � = R . C<br />
where 0.1 s < � < 100 s occur together with the capacities C of the measurement lead.<br />
Depending upon the time constant, the measurement limit is between 1 . 10 -16 and 1 . 10 -14 A.<br />
In addition to its simple, robust design, a Faraday detector is characterized by its long-term<br />
stability and its ability to withstand high temperatures. To keep the time constants small<br />
and to avoid other interfering effects, the electrometer amplifier is connected directly to the<br />
analyzer and its output signal is supplied directly to the data analysis system. This is why the<br />
Faraday Cup is also present in all Pfeiffer <strong>Vacuum</strong> mass spectrometers. It is only suitable<br />
for detecting positive ions.<br />
If extremely small ion currents are to be measured or if an extremely high measuring speed is<br />
required, physical pre-amplifiers, so-called secondary electron multipliers, are used.<br />
Page 101<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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