What is thermionic emission?

Thermionic emission refers specifically to the phenomenon where electrons are emitted from a material, typically a heated filament, due to thermal energy. When the filament in an x-ray tube is heated, typically by passing an electric current through it, the thermal energy causes some of the electrons within the material to gain sufficient energy to overcome the attractive forces of the atomic structure and be released into the surrounding space. This is a crucial step in the generation of x-rays, as these emitted electrons are later accelerated towards the anode, where they will interact and produce x-rays.

The process is fundamental to the functioning of x-ray tubes and is distinguished from other related phenomena, such as the movement or acceleration of electrons. In the context of the options provided, the emphasis on heating the filament is key, making it clear why the release of electrons due to this heating is the correct definition of thermionic emission.