Thermionic emission takes place from the surface of metal because its easy for the free electrons to escape from the metal surface then from inside the metal where the attractive force is more as compared to the metal surface.

Where does thermionic emission occur?

Thermionic emission occurs in metals that are heated to a very high temperature. In other words, thermionic emission occurs, when large amount of external energy in the form of heat is supplied to the free electrons in the metals.

Where does thermionic emission occur in xray tube?

In the generation of X-ray production, a cathode filament machined in a cathode cup is activated, causing intense heating of the cathode filament. The heating of the filament leads to the release of electrons in a process called thermionic emission.