Specifications

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

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.