Final answer:
Tungsten is used in X-ray targets because it has a high atomic number, requiring more energy to produce characteristic X-rays, and a high melting point to withstand the heat generated during X-ray production.
Step-by-step explanation:
Tungsten is the material of choice for x-ray targets due to its large photon energy that is a characteristic of X-rays from heavy elements. The energy of characteristic X-rays increases approximately as the square of the atomic number (Z²), which is substantial for heavier elements like tungsten. Additionally, a significant accelerating voltage, at least 72.5 kV, is required to create inner-shell vacancies in tungsten because its inner-shell electrons are tightly bound and other shells are filled. Tungsten's high melting point is critical because the impinging electrons are absorbed and raise its temperature substantially, necessitating a material that can withstand such conditions without melting.