Which component of the x-ray tube is the source of free electrons?

• A. Cathode
• B. Anode
• C. Filament
• D. Focusing cup

Answer: (A)

Electrons in an x-ray tube come from thermionic emission at the cathode. When the filament inside the cathode is heated, electrons gain enough energy to overcome the surface work function and are released into the tube as a free electron cloud. This emission originates from the cathode as a whole, which supplies the electrons that will be accelerated toward the anode to produce x-rays. The anode is just the target that stops the electrons to generate x-rays, while the focusing cup shapes and directs the beam by shaping the electric field. The filament is the heated element that provides the source of electrons, but the actual source term used in practice is the cathode.