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Transpector MPH Operating Manual
3.5.3 The Continuous Dynode Electron Multiplier/Faraday Cup Detector
The Continuous Dynode Electron Multiplier/Faraday Cup (CDEM/FC), used for the
EM containing MPH models, offers the advantages of both the FC and EM
detectors combined in one unit.
Figure 3-6 A CEM/FC Detector
In CDEM/FC detectors, the EM cone amplifies the electron pulse, significantly
increasing the analyzer’s sensitivity.
The Transpector MPH CDEM/FC detector uses a continuous dynode element
made of a special type of glass, rather than discrete dynode EMs, which are made
from a copper-beryllium alloy. The principal advantage of the CDEM is that its
performance does not degrade when exposed to air. In order to prolong its useful
life, a copper-beryllium dynode multiplier must be stored under vacuum; exposure
to air for more than a brief period can result in a significant decrease in the
maximum available gain.
The CDEM has some disadvantages. The maximum operating temperature for the
CDEM is 150°C. (It can still be baked out at 300°C, provided that the high voltage
is off.) Also, the CDEM is slightly slower to recover after exposure to excessive
input or output currents, and it may take a bit longer to stabilize its gain after the
high voltage is changed.
The CDEM is operated at high voltages between -0.6 and -2.0 kV. A new CDEM
will typically have a gain of between 10 and 1,000 at -0.8 kV. The gain at -3.0 kV
typically exceeds 1 x 10
6
. Transpector MPH default EM voltage is -0.8 kV.
CEM Cone
Electron Collector
Signal Output
Deflector Shield
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