Stanford Research Systems RGA100, Operating Manual And Programming Reference

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7-14 Electron Multiplier Tuning Procedure
SRS Residual Gas Analyzer
Electron Multiplier Tuning Procedure
Accurate quantitative measurements with the electron multiplier detector require the
determination of the CDEM gain for all the ion peaks being measured. Frequent
recalibrations are recommended to correct against aging of the device.
The gain of the electron multiplier (CDEM) in the SRS RGA is defined relative to the
Faraday Cup output (which is assumed to be mass independent). It can be programmed
anywhere from 1 to 10
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adjusting the high voltage applied across the device, is highly
mass dependent, and changes with time due to aging.
The electron multiplier gain is easily calibrated in the SRS RGA since it is possible to
measure the same ion current with and without the CDEM. The common method of
calibrating the electron multiplier gain for a given mass peak is to measure the peak
intensity with the Faraday cup, and then repeat the same measurement with the electron
multiplier without changing anything else. The gain of the multiplier is the ratio of the
multiplier output current to the Faraday cup output current. (Note that there is no need to
change the sign of the electron multiplier signal prior to the division since the firmware
automatically reverses its sign before transmitting the value.)
The RGA Head can store a single set of [High Voltage, Gain] values for the electron
multiplier in its non-volatile memory. RGA Windows uses the voltage value to bias the
CDEM and the gain value to divide the ion currents when the CDEM is turned on. See
the HV, MV and MG commands in the RGA Command set for details on the command-
level implementation of this procedure.
RGA Windows can automatically program the gain of the electron multiplier for any
mass using the automatic Electron Multiplier Gain Adjustment function of the Electron
Multiplier command (Head Menu). Select a gain value between 10 and 1,000,000, choose
a mass value for partial pressure measurements , and the RGA program automatically
calculates the required high voltage setting when the Adjust button of the Electron
Multiplier Window is pressed. The HV and gain settings are saved into the RGA Head
and used every time the CDEM is turned on. All data acquired while the CDEM is on
gets divided by the gain automatically before it is displayed.