Dynode

A dynode is an electrode used in electron multiplier devices, most notably in photomultiplier tubes (PMTs) and other vacuum electron tubes. Its primary function is to amplify an incoming beam of electrons through secondary-emission processes. In a typical arrangement, electrons emitted from a photocathode are accelerated toward the first dynode by a high voltage. When they strike the dynode surface, each incident electron releases several secondary electrons. These secondaries are then accelerated to the next dynode, where the process repeats across a chain of dynodes. The overall signal gain is the product of the gains at each stage, enabling the detection of very small light signals.

Dynode chains are designed with specific geometries and materials to optimize gain, linearity, timing, and efficiency. Dynodes are usually made from materials with high secondary-emission yields, such as certain oxides or coated metals, and may employ various surface treatments to maximize emission. Common dynode configurations include linear-focused, box-type, and venetian-blind designs, with typical counts ranging from about 8 to more than 20 stages. The resulting gains commonly fall in the range of 10^6 to 10^8, depending on the number of stages and operating voltage.

Applications of dynode-based systems include radiation detection, medical imaging, high-energy physics experiments, and spectroscopy. The performance can be influenced by factors such as vacuum quality, temperature, and the stability of dynode materials. In some detectors, alternative amplification technologies such as microchannel plate assemblies are used to achieve faster timing or different gain characteristics, either replacing or complementing dynode chains.