PMT Voltage Dividers

The width of the transit time spread is proportional to the reciprocal square root of the voltage between the cathode and the first dynode. Increasing this voltage improves the IRF noticeably. It is, however, unknown how far the voltage can be increased without dielectric breakdown in the tube or the socket. 

The voltage divider can dissipate a considerable amount of heat. The heat should be kept away from the tube in order to keep the dark count rate low. In addition, good electrical shielding of the tube and the voltage divider is essential to obtain a good time resolution, see Sect. 7.5.4, page 311. 

The eommonly used MCPs and PMTs deliver single-electron pulses of 20 to 50 mV when operated at maximum gain. Although these pulses can be detected by the input diseriminators of most TCSPC modules, a preamplifier is recommended for several reasons. The most obvious one is that a good preamplifier, if it is con-neeted elose to the detector output, improves the noise immunity of the system. Moreover, with the amplifier the CFD ean be operated at a higher discriminator threshold, which improves the timing and the threshold stability. 

The gain of the preamplifier allows the PMT to be operated at a correspondingly lower gain. The reduction of the average output eurrent at a given eount rate improves the timing stability and inereases the lifetime of MCP PMTs. 

The strongest argument is that a properly designed preamplifier ean be used to protect the detector against overload. The principle is shown in Fig. 7.38. 

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The used PMT, Voltage divider. High Voltage and related electronics should have accurate enough performances not to degrade the detector one, especially for in beam experiment which could last weeks with high and non constant counting rates. 

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