Which term describes electron emission due to heating of the cathode in an x-ray tube?

• A. Field emission
• B. Thermionic emission
• C. Secondary emission
• D. Photoelectric emission

Answer: (B)

Thermionic emission. Heating the cathode raises the filament’s temperature so electrons gain enough thermal energy to overcome the metal’s work function and escape from the surface. In an x‑ray tube this creates a cloud of electrons near the cathode that are then accelerated toward the anode to produce x-rays; the hotter the filament, the more electrons are emitted. This distinguishes it from field emission, which is driven by a strong electric field and tunneling rather than heat; secondary emission, which occurs when incoming particles cause additional electrons to be ejected after impact; and photoelectric emission, which requires photon absorption to eject electrons.