Dynodes

Dynodes are electrodes in certain vacuum electron devices, notably photomultiplier tubes (PMTs), that amplify an incoming electron signal through secondary electron emission. In a PMT, a photocathode emits electrons when illuminated; these electrons are directed toward a series of dynodes held at progressively higher positive voltages. When an incoming electron strikes a dynode, it liberates several secondary electrons; those electrons are accelerated toward the next dynode, where the process repeats. The overall amplification, or gain, is the product of the secondary emission yields of each dynode stage, typically yielding gains of 10^6 to 10^7 for standard PMTs.

Dynodes are designed as a chain of around 10 to 20 stages, with voltages distributed to optimize collection efficiency and linearity. Materials for dynodes are chosen for high secondary emission yield; common materials include oxidized or coated metals and ceramics such as beryllium oxide (BeO), magnesium oxide (MgO), or copper coated with a suitable secondary emitter. The precise materials and geometry influence gain, timing, and aging characteristics.

Variants include box-type or linear focus dynodes, and microchannel plate (MCP) devices where a dense array of microchannels acts as many small dynode stages. High-voltage supplies for PMTs provide hundreds to thousands of volts across the chain, and modern designs use dc-dc converters with regulation to maintain gain and linearity. Applications span scintillation detectors in medical imaging, high-energy physics experiments, positron emission tomography, and photon counting.

Limitations include sensitivity to magnetic fields, aging of dynode materials reducing gain over time, and nonlinearity at high signal levels. Dynodes require vacuum environments and careful sealing.