Final answer:
Characteristic X-rays are emitted when an electron transition occurs from a higher energy level to fill a vacancy in the K shell of a tungsten atom, and require significant accelerating voltage due to tightly bound inner-shell electrons.
Step-by-step explanation:
The radiation produced at discrete energies when electrons interact with the K shell of the tungsten target atom is called characteristic X-rays. These X-rays are a form of electromagnetic (EM) radiation that is emitted when an inner-shell vacancy is filled, usually after an electron is knocked into a higher orbit or ionized, and another electron drops down to fill the vacancy. In the specific case mentioned in the question, it involves a transition from a higher energy level to the K shell (n = 1) of the tungsten atom which requires a significant accelerating voltage, often over 72.5 kV for tungsten, due to its tightly bound inner-shell electrons. The energy of these X-rays is characteristic of the element because it depends on the energies of electron states which are unique to each element.