Dynode

A dynode is an electrode in a vacuum tube that serves as an electron multiplier through secondary emission. The first tube to incorporate a dynode was the dynatron, an ancestor of the magnetron, which used a single dynode.Photomultiplier and video camera tubes generally include a series of dynodes, each at a more positive electrical potential than its predecessor. Secondary emission occurs at the surface of each dynode. Such an arrangement is able to amplify the tiny current emitted by the photocathode, typically by a factor of one million.

Operation

The electrons emitted from the cathode are accelerated toward the first dynode, which is maintained 90 to 100 V positive with respect to the cathode. Each accelerated photoelectron that strikes the dynode surface produces several electrons, that are then accelerated to the second dynode, which is held 90 to 100 V more positive than the previous dynode. For conventional dynode materials, such as BeO and MgO, a multiplication factor of 10 can normally be achieved by each dynode stage. By the time this process has been repeated at each of the dynodes, 10⁵ to 10⁷ electrons have been produced for each incident photon, dependent on the number of dynodes.