SRS Labs QMS 100 Series, Руководство пользователя

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Ion Detector 2-19
Technical Reference
of the electron multiplier currents is reversed before the current value is sent out over RS232 so that the
computer does not need to do any sign flipping on the currents received when the CDEM is activated.
The gain of the electron multiplier in the RGA is a function of the bias voltage and is measured relative to
the FC signal. The following figure shows a typical
“gain characteristic ” (i.e. gain vs bias voltage)
curve obtained for H
2
O
+
ions at 18 amu.
Figure 9 Typical CDEM Gain Characteristic Curve
Very high gain values can be obtained and controlled with voltages smaller than -2500 Volts. The
sensitivity of the spectrometer increases with the gain of the electron multiplier . Higher
sensitivities provide lower minimum detectable partial pressures and faster spectral scans. Minimum
detectable partial pressure limits as low as 5
.
10
-14
Torr are commonly achieved.
However, the increase in sensitivity is obtained at the price of limited dynamic range, mass discrimination
effects, gain instabilities and finite lifetime of the device. A good understanding of these limitations is very
important to assure accurate quantitative measurements.
The dynamic range of electron multipliers is determined by their dark current at the low end, and by
the bias current value at the high end.
The bias current is established by the external voltage drop along the resistive glass tube and flows