Basic Properties of Electron Multiplier Ion Detection and Pulse Counting Methods in Mass Spectrometry

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Abstract
The critical role played by first stage gain in shaping the output pulse height distribution and detection efficiency of an electron multiplier used as an ion detector is studied theoretically and experimentally. In a well‐focused electron multiplier it is found that anode pulse height distributions observed for different ion species agree well with distributions calculated by using compound Poisson statistics. Models are developed to explain measurement bias in an electron multiplier which is used in either the dc or pulse counting modes and it is shown how these results can be applied to isotope ratio measurements. Design and operating characteristics are described for a 20 stage electron multiplier which was developed for high speed pulse counting and high detection efficiency. The surface of the first dynode is coated with a thin film of Al2O3 to provide a stable first stage yield of 7–9 secondary electrons/incident ion for monatomic alkali ions with 20 kV impact energy. The polyatomic ion Na2BO2+ yields about 16 secondary electrons/incident ion at 20 kV impact energy. Resolving time of the entire detector system is reduced to 10 nsec by using a specially designed solid state discriminator and pulse shaper. Also, a technique is described for the simultaneous detection and pulse counting of two ion beams.