Reducing gain shifts in photomultiplier tubes

A device for reducing gain shift in a multiplier tube due to varying event count rates, the multiplier tube having dynodes arrayed in a cascaded series such that with appropriate biasing any dynode of the cascaded series is capable of being responsive to incident electrons directed thereto to emit secondary electrons at a rate amplified from the rate at which incident electrons are applied thereto, the secondary electrons emitted by one dynode being applied as incident electrons to the next consecutive dynode of the cascaded series, with the last dynode of the series being the output terminal, comprising:

• means coupled to the dynodes for limiting the number of consecutive active dynodes of the cascaded series to a predetermined number by rendering inactive at least one dynode of said cascaded series, with the last of the predetermined number of said consecutive active dynodes being the output terminal of the tube, and an amplifier coupled to said last consecutive active dynode and being responsive to the number of electrons incident on said last consecutive dynode to develop an output signal whose magnitude is proportional to and the amplification of the number of electrons incident on said last consecutive dynode.