Dynode
Definition and meaning of Dynode in chemistry.
Dynode refers to one of a series of electrodes inside a photomultiplier tube (or electron multiplier) that emits multiple secondary electrons when struck by an incoming electron, amplifying a weak electrical signal.
In more detail
Each dynode is coated with a material of high secondary-emission yield, such as beryllium oxide or gallium phosphide, and held at a progressively higher positive voltage than the one before it. An electron striking a dynode's surface ejects several new electrons, which accelerate toward the next dynode and repeat the process, producing an exponential cascade. A typical photomultiplier tube contains 8 to 14 dynode stages, converting a single photoelectron into a detectable current pulse containing millions of electrons.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Typical gain per stage | 3 to 6 secondary electrons |
| Common coating materials | BeO, GaP, Cs-Sb alloys |
| Typical number of stages | 8 to 14 dynodes per tube |
In a scintillation counter, a photon striking the photocathode releases one photoelectron, which is accelerated into the first dynode; the resulting cascade through successive dynodes yields roughly 10^6 to 10^7 electrons collected at the anode, generating a measurable output pulse.
Frequently asked questions
What is the difference between a dynode and a photocathode?
A photocathode converts incoming photons into a single photoelectron via the photoelectric effect, while a dynode amplifies that electron signal through secondary electron emission at each successive stage.
Why are dynodes held at increasing voltages?
The rising positive potential between successive dynodes accelerates electrons so they strike the next surface with enough energy to eject several secondary electrons, sustaining the amplification cascade.