Secondary electron

A secondary electron is emitted from a material after excitation by incident radiation or particles, rather than simply being a high-energy primary electron redirected back out. Many detected secondaries have low enough kinetic energy that escape strongly favors regions near a surface.

This makes secondary-electron collection useful for secondary-electron topographic contrast in a scanning electron microscope. Surface angle, local electric fields and collector bias affect the signal as well as emission itself. Backscattered primaries can also excite secondary electrons away from the probe's immediate landing position, so every detected secondary need not originate directly beneath the nominal probe.^[ref]

An electron multiplier amplifies a signal by accelerating incoming electrons into surfaces that emit additional secondary electrons. Repeating this process at successive stages produces a measurable charge packet from a small initial signal.

A photomultiplier adds a photosensitive input that converts incident light into a photoelectron; its dynodes then perform multiplication. In a simple equal-stage model with mean secondary yield $\delta$, $m$ stages give mean gain ${\delta }^m$. The output fluctuates because emission is stochastic, and saturation can invalidate constant gain. Multiplication increases the signal scale but cannot recreate input events that were never detected.