Which material is used for both X-ray filament and target?

• A. Tungsten
• B. Copper
• C. Lead
• D. Aluminum

Answer: (A)

The material chosen must perform two demanding roles: it has to heat up enough to emit electrons for the filament, and it has to withstand and efficiently convert the energy of incoming electrons into X-rays at the target. Tungsten fits this balance surprisingly well. Its incredibly high melting point and low vapor pressure mean a filament can glow hot enough to trigger thermionic emission without evaporating or degrading. At the same time, tungsten’s high atomic number aids X-ray production when bombarded by electrons, and its excellent heat tolerance allows it to absorb and spread the intense energy at the anode, especially when used with cooling backing. This combination makes it ideal for both parts of the X-ray tube.

Other metals lack one or more of these critical traits. Copper conducts heat well but melts far too easily for a filament; lead has a high atomic number but is soft and toxic and not suitable for the filament or robust high-heat operation; aluminum melts at a relatively low temperature and doesn’t provide the needed emission or X-ray efficiency.