Final answer:
Tungsten is ideal for X-ray photon generation due its high photon energy emission, necessity for significant accelerating voltage to create inner-shell vacancies, and its high melting point that allows it to withstand the high temperature from absorbed energy.
Step-by-step explanation:
Tungsten is the metal of choice for the source of X-ray photons for three reasons:
- High photon energy: Tungsten produces characteristic X-rays with large photon energy, which is typical for heavy elements. These energies are larger than other atomic emissions due to the emission occurring when an inner-shell vacancy is filled. These vacancies involve tightly bound electrons and the energy of the characteristic X-rays increases approximately as Z² (atomic number squared).
- Significant accelerating voltage: To produce these inner-shell vacancies, a substantial accelerating voltage is necessary. For tungsten, this voltage is at least 72.5 kV. This is because the electron shells are filled and an electron cannot be simply bumped to a higher filled shell; it must be removed completely.
- High-melting-point: Since so much of the energy of the impinging electrons is absorbed, significantly raising its temperature, a material with a high melting point is required. Tungsten's high melting point makes it suitable as an anode material in X-ray tubes, able to withstand the heat without melting.