Which component of an X-ray tube emits electrons?

• A. Cathode (filament)
• B. Envelope/vacuum
• C. Be window and insulating oil
• D. Anode (target)

Answer: (A)

Electrons come from the heated filament in the cathode. When the filament is heated, electrons gain enough energy to overcome the material’s work function and are emitted into the vacuum, a process called thermionic emission. These free electrons are then pulled toward the positively charged anode, creating the electron beam that produces X-rays when it hits the target. The envelope keeps the tube in a vacuum so electrons can travel freely; the Be window and insulating oil mainly manage heat, insulation, and X-ray transmission, not electron production. The anode acts as the target where X-rays are generated, not where electrons originate. So the component that emits electrons is the cathode (filament).