What is the main composition of the cathode filament in an X-ray tube?

The main composition of the cathode filament in an X-ray tube is tungsten. This choice is correct because tungsten has a high melting point, around 3422°C, which allows it to withstand the intense heat generated during the electron emission process. The filament operates by being heated to produce thermionic emission, where electrons are ejected from the surface of the filament.

Tungsten's durability and ability to maintain its structural integrity at high temperatures make it especially suitable for use in X-ray tubes. Additionally, tungsten has a high atomic number, which contributes to efficient X-ray production when the electrons strike the anode.

Other materials like thorium, gold, and nickel may have their specific applications in various aspects of the X-ray technology field, but they do not match tungsten's combination of thermal and electrical properties required for the filament in the cathode assembly. For instance, while thorium is known for its use in some types of electrodes due to its favorable ionization properties, it is not the primary material for cathode filaments in X-ray tubes. Gold and nickel, though valuable in electronics for their conductivity, do not provide the same thermal resilience as tungsten.