Production: the Coolidge tube

The Coolidge tube is a high-vacuum glass tube containing:

• a hot filament (cathode) heated by a low-voltage current, which emits electrons by thermionic emission;
• a target (anode), made of a metal with a high melting point such as tungsten, set at a slope in front of the cathode;
• a high DC voltage (tens of kilovolts) between filament and target, which accelerates the electrons so that they hit the target with large kinetic energy (eV);
• a copper rod with cooling fins, which removes the heat produced (most of the electrons' energy, over 99 %, is turned to heat; a tiny part becomes X-rays).

When an electron collides with the tungsten target, part or all of its kinetic energy is converted to X-ray photons.