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Transpector SPS Operating Manual
5.2.3 The Ion Detector
The ion detector region of the sensor consists of the quadrupole exit lens, the
electron multiplier and the detector itself. The quadrupole exit aperture is biased
negatively with respect to the anode, focusing ions that have been transmitted
through the quadrupole into the electron multiplier and detector.
5.2.3.1 The Electron Multiplier (EM) Detector
The Electron Multiplier (EM) acts as an
in situ
preamplifier for improved sensitivity.
Incoming ions are accelerated into 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 output from
an EM detector is negative due to this pulse of electrons.
The ratio of the electron output current to the incident ion current is 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, EM gain decreases
as the ion mass increases.
The advantage of the EM detector sensor is it’s high sensitivity (as much as
500 A/Torr), making it possible to measure partial pressures as low as 1x10
-15
Torr
for a 100 AMU sensor.
