Final answer:
Thermionic emission occurs when high current heats a filament, releasing electrons, which are accelerated and strike a target to produce characteristic X-rays. The energy of these X-rays is determined by the atomic number (Z) and requires high melting point materials like tungsten for the target.
Step-by-step explanation:
When a high current passes through the filament, typically measured in amps, electrons from the outer shell are 'boiled off' and ejected. This process is known as thermionic emission. During operation of an X-ray tube, a significant current heats the filament, causing thermionic emission of electrons. These electrons are then accelerated towards a target material such as tungsten. When the electrons collide with the target, they can cause an inner-shell electron to be ejected. Later, when another electron fills this vacancy, the energy released in this transition results in the emission of an X-ray. The energy of these X-rays is characteristic of the atom and is dependent on the atomic number (Z), as the energy increases approximately as Z². Due to the large energies involved, high melting point materials like tungsten are used for the target in X-ray tubes.