Off-Axis Conversion Dynodes

A different kind of discrete dynode electron multiplier detector using an off-axis conversion dynode is exem-phfied by the Daly detector (Daly 1960), developed in response to reduced hfetimes of conventional discrete 

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Continuous dynode multipliers with a channeltron . The ions are directed towards a collector whose entrance, in the form of a horn, is made of a lead doped glass with which acts as the conversion cathode. The ejected electrons are attracted towards a positive electrode (Figure 16.26) and their collisions against the internal walls give rise to multiplication, as with the separated dynodes. The assembly is usually mounted off-axis to avoid the impact of neutral species as well as photons emitted by the filament, equally susceptible to the removal of the electrons. 

Figure 7.5 Schematic diagram of an off-axis detector for a mass spectrometer. The conversion dynode is maintained at a high potential (up to 10-20 kV) thus accelerating the ions to high velocities to improve the secondary emission efficiency (the diagram is drawn to illustrate the airangement for positive ions). The secondary elections are then accelerated to the first dynode (maintained at 2 kV) and the SEM then amphfies the secondary electron current as usual. For negative ions the conversion electrode is maintained at a high positive potential and secondary positive ions are accelerated on to the first dynode. This example portrays a quadrupole analyzer with a discrete dynode SEM. Reproduced from Photomultiplier Tubes Basics and Applications (3rd Edn), Hamamatsu Corporation, with permission.

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