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Transpector MPS Operating Manual
3.5.2 The Electron Multiplier (EM) Detector
The Electron Multiplier (EM) acts as an in situ preamplifier for improved sensitivity.
Although there are several different types of EM, their operating principals are the
same. Incoming ions are accelerated into the input of the EM by a high negative
voltage (usually -1.0 kV or more). When an ion strikes the surface of the EM, one
or more secondary electrons are emitted. These electrons are accelerated to a
second surface which is at a more positive potential, where additional electrons are
generated.
This process repeats itself until a pulse of electrons emerges from the output of the
EM and is collected on a Faraday Cup. The result is that as many as a million
electrons or more can be produced by each incident ion. The current from a
Faraday detector is positive (for positive ions). An EM detector current is negative.
The ratio of the electron output current to the incident ion current is known as the
EM gain. The gain primarily depends on the EM type, the voltage applied to the EM
input, the voltage applied across the EM, the condition of the EM, and, to a lesser
extent, the mass and chemical nature of the incident ion. In general, the EM gain
decreases as the ion mass increases.
The advantage of the EM detector sensor is its high sensitivity (at least
500 amps/Torr), thus making it possible to measure partial pressures as low as
1.5 x 10
-14
Torr for Transpector MPS sensors. A typical Faraday Cup sensor would
have a sensitivity of only 3 x 10
-4
amps/Torr, resulting in a minimum detectable
partial pressure of 2.6 x 10
-12
Torr.
