Stanford Research Systems RGA100, Operating Manual And Programming Reference

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8-14 CDEM Handling and Care
SRS Residual Gas Analyzer
CDEM Handling and Care
Continuous Dynode Electron Multipliers (CDEM) have a history of high performance
and dependability in mass spectrometry applications. By following the simple
recommendations described below the user should achieve a long useful lifetime from
these detectors.
Handling and mounting
Handling and mounting of the CDEM should only be performed in a clean vacuum
fashion:
x
Work on a clean dust-free area. Avoid dust, lint and any kind of particulate matter.
x
Wear talc-free rubber gloves or finger cots.
x
Use properly degreased tools.
Avoid excessive shock, such as from dropping onto a hard surface (Remember that
CDEM’’s are made out of glass).
Operating pressure
The recommended operating pressure is 5
.
10
-6
or less. The lower the pressure during any
operation, the longer the lifetime of the CDEM.
Never apply voltage to a CDEM at pressures above 10
-4
Torr.
Operating temperature
The safe operating temperature of a CDEM is less than 100
q
C.
Bakeout temperature
The bakeout temperature must be less than 300
q
C with no voltage applied across the unit.
Operating voltage
The maximum voltage applied to any CDEM should never exceed 3500V. The SRS RGA
High Voltage power supply is limited to a 2500V output.
Typical operating range is 1100-2500V. In general,
the lower the average signal current
drawn from the detector and the lower the operating voltage, the longer the lifetime that
will be realized.
Initial pump-down
To best maintain the high gain and low dark current properties of electron multipliers it is
very important to follow the steps described in the
CDEM Pre-conditioning section of
this chapter during the initial pump-down of the electron multiplier. The procedure serves
to degas the detector and stabilize the gain. Failure to follow the recommended steps
before powering up the multiplier might result in gain degradation by as much as a factor
of ten.
Contamination
The active surface of the CDEM can be repeatedly exposed to air without degradation;
however, it should not be contaminated with dust, lint or other particles. Organic
substances present in the vacuum system tend to accumulate on the active surface of the