Instability in PMTs

Like afterpulsing, instability is probably eaused by ion feedback and possibly luminescence of dynodes. Therefore some manufaeturers specify not only a maximum operating voltage for their PMTs but also a maximum safe gain. 

A PMT can develop instability or permanently increased dark count rate after being heavily overloaded for a period of several seconds or longer. The reason is probably heating of the anode and the last dynodes which releases gas from these structures. 

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