Why are tungsten filaments common in X-ray tubes and how does thorium alloying affect emission?

• A. Tungsten has a low melting point; thorium alloying reduces emission.
• B. Tungsten provides chemical inertness; thorium improves durability.
• C. Tungsten has a high melting point and stability; thorium alloying lowers work function and enhances thermionic emission.
• D. Tungsten is cheap; thorium increases X-ray energy.

Answer: (C)

In X-ray tubes, the cathode filament must emit a large number of electrons when heated, without melting or deteriorating. Tungsten is ideal for this role because it has an extremely high melting point and excellent stability in vacuum, so the filament can run at high temperatures with minimal evaporation and deformation.

However, tungsten’s surface work function is relatively high, which would require very high temperatures for strong thermionic emission. Alloying tungsten with thorium changes that. Thorium lowers the work function of the filament surface, so thermionic emission increases at a given temperature. This means you get more electron flow without pushing the filament to hotter, more damaging temperatures, improving emission efficiency and filament life.

In short, tungsten provides the necessary high-temperature stability, and thorium alloying enhances emission by reducing the energy barrier for electron emission.