A simple method is described for the gain stabilization of a photomultiplier in which the stabilization system does not interfere with the fast pulse spectrum to be measured. The functioning principle consists in sending a low fre.quency (77 Hz) and very low intensity light pulse, which gives a modulation of the photomultiplier thermionic nOise, on the photocathode together with the scintillation light. The modulation frequency is then selected on the anode by a s~ecial filter. The signal obtained in this way is used to trigger a feedback circuit, which makes the photomultiplier gain constant by suitably varying its voltage.

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