Secondary Electron Multiplier as a Detector of Very Feeble Ion Current down to 10-19 Amperes

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Abstract
A 12-stage Cu–Be electron multiplier was constructed for detecting very feeble ion current. General characteristics of the multiplier coupled with a vibrating reed electro-meter were investigated by measuring the output of a high vacuum mass spectrometer. The stability of the multiplier gain was sufficient for the measurement of isotope ratios. For example, the 20Ne/22Ne ratio was found to be 10.563±0.006 (un-corrected for mass discrimination). At an ultimate pressure of 10-8 Torr in this mass spectrometer, background mass spectra up to M/e 150 were recorded, and some hydrocarbon fragment peaks corresponding to 10-16–10-18 amperes were found in C2–C7 groups. Mass peaks below 10-16 amperes fluctuate considerably, but such statistical fluctuations may be smoothed out by using an amplifying system with a sufficiently long time constant and a low scanning speed. Under such conditions, an ion current down to 1×10-19 amperes was distinguishable over the noise level of the amplifying system.

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