Final answer:
To emit electrons from its surface, the x-ray tube filament must be heated sufficiently to cause thermionic emission.
Step-by-step explanation:
X-ray tube filaments must be heated sufficiently to cause thermionic emission, which is the process by which electrons are emitted from a heated filament. Thermionic emission is the process by which electrons are emitted from a heated filament or cathode. When the filament is heated to a high temperature, some of the electrons gain enough energy to overcome the work function of the material, allowing them to escape and form a cloud of electrons near the filament.
This cloud of electrons is then accelerated toward the anode (target) in the x-ray tube, where it produces x-rays upon striking the target. The thermionic emission process is crucial for the generation of electrons necessary for the production of x-rays in medical and industrial applications.