Afterpulses in Photomultipliers

Abstract

Present fabrication and activation techniques have reduced afterpulses from most photomultipliers to a point where they are rarely important. Nevertheless, with some tubes in certain applications they may present a serious problem. More important, however, is the fact that the study of afterpulsing gives valuable clues to the nature of dark current in multipliers and also reveals a surprisingly high yield of electrons from alkali antimonide (cathode) surfaces per ion incident on the surface. It has been found that afterpulses are the result of ionization of gas by electrons in the volume between the cathode and first dynode, The positive ions formed strike the cathode and cause the release of secondary electrons. The sharp pulse characteristic of afterpulses is due to the fact that the field distribution between the cathode and first dynode is such that the time required by an ion of given mass and charge to reach the cathode is nearly independent of the point of origin Observation of afterpulses produced by single photoelectrons indicates that the secondary-emission ratio for ions in a cesium-containing photocathode is in the neighborhood of 3. The time of occurrence of afterpulses can be closely correlated with the atomic mass of the residual gas in the tube. The pulse-height distribution of afterpulses produced by single ions appears to be identical with the larger dark-current pulses, and it is quite certain that this component of dark current is the result of ion bombardment of the cathode.