What is added to the filament to help prevent vaporization during operation?

The addition of thorium to the filament is beneficial because thorium improves the thermal stability and increases the melting point of the filament material, typically tungsten. This enhancement significantly reduces the rate of vaporization when the filament is heated during x-ray tube operation.

When the filament reaches high temperatures, the tungsten can evaporate, leading to a decrease in its lifespan and efficiency. The presence of thorium minimizes this effect by stabilizing the tungsten lattice, preventing the formation of volatile tungsten oxides, and therefore maintaining the integrity of the filament for a longer period.

Other elements, such as radium, rhenium, and chromium, do not offer the same advantages in terms of maintaining filament stability and minimizing vaporization. As such, thorium is the preferred additive in x-ray tube filaments due to its properties that enhance the overall performance and durability of the filament during operation.