Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 58 / 362

3-12 Ion Detector
SRS Residual Gas Analyzer
Ion Detector
Positive ions that successfully pass through the quadrupole are focused towards the
detector by an exit aperture held at ground potential. The detector measures the ion
currents directly (Faraday Cup) or, using an optional electron multiplier detector,
measures an electron current proportional to the ion current.
Description
The following figure describes the detector assembly including the electron multiplier
option.
Exit Plate
Faraday cup shield
CDEM
CDEM cone
Faraday cup
FC signal
CDEM signal
CDEM anode
ions
Probe flange
Figure 8 Detector Components
The Faraday Cup (FC) is a small stainless steel (type 304) metal bucket located on-axis
at the end of the quadrupole. It is shielded from the intense RF and DC fields of the
quadrupole by the grounded
exit plate , which also provides some focusing of the ions
into the detector. A cylindrical tube ( FC Shield ) encloses the FC, protecting it from the
strong electrodynamic potentials of the adjacent rods and from collecting ions originated
at sources other than the ionizer. The signal is carried to the electrometer through a
coaxial feedthru which extends into the ECU box and efficiently shields the small ion
currents. The FC is attached to the center feedthru with a push-on connector and can
easily be pulled out for cleaning or modification.
Notice that an electron suppressor electrode (commonly found in several commercially
available RGA’’s) is not part of this detection setup since the large aspect ratio (length-to-
diameter) of the FC assures efficient recapture of secondary electrons.
The electron multiplier is a state-of-the-art Macro Multi-Channel Continuous Dynode
Electron Multiplier (CDEM). It consists of a straight, four or six channel tube made out
of a special resistive glass (i.e. high secondary electron emission yield) with a cone of the
same material attached to the front end. It is placed upright, next to the FC, and away
from the axis of the analyzer (
off-axis configuration ). It is held safely in position by a
clamp and a clip. The clamp is connected to the neighboring rod that carries the voltage
from the High Voltage power supply, and is responsible for biasing the cone as well as
assuring its correct placement next to the FC. A hole on the side of the FC Shield allows
space for mounting the CDEM cone very close to the top of the FC. The clip anchors the
CDEM glass tube to the side of the FC Shield and holds the lower end of the tube at
ground. Chrome electrical coatings, deposited at both ends of the tube provide the