The time dependence of the sensitivity of photomultiplier tubes

A study has been made of the regular changes in sensitivity occurring in photomultiplier tubes under typical operating conditions. The particular phenomenon which takes place in the first minutes after a sudden alteration has been made in the illumination, was investigated in detail. An expression is given, capable of describing this change in the case of a sudden increase in the illumination to which the tube is submitted. Certain experimental results indicate that the dynodes are responsible for this effect. Finally, an electrical model of the behaviour of the dynodes is outlined, which functions according to the expression found for the time dependence of the sensitivity.