What serves as the source of free electrons in an x-ray tube?

In an x-ray tube, the source of free electrons is the heated tungsten filament. When the filament is heated, it undergoes a process called thermionic emission. This process allows the filament to emit electrons into the surrounding vacuum of the tube.

Tungsten is used because it has a high melting point (around 3,400 degrees Celsius) and can withstand the high temperatures needed for this emission without melting. Once the electrons are emitted from the filament, they are attracted towards the anode due to a high-voltage potential difference between the cathode (where the filament is located) and the anode.

This flow of electrons is crucial for generating x-rays, as when these fast-moving electrons collide with the anode target, x-ray photons are produced. The other options do not serve as a source of free electrons: electric pulses are responsible for accelerating the electrons, the anode target is where the x-rays are produced when electrons strike it, and the glass housing provides structural support and maintains the vacuum environment necessary for optimal electron flow.