Final answer:
X-rays of sufficient energy for diagnostic radiography are produced from the inner-shell of heavy elements, with energies increasing with the atomic number squared, requiring high accelerating voltage such as at least 72.5 kV for tungsten.
Step-by-step explanation:
Only x-rays produced at the inner-shell, or specifically from heavily bound electrons in the inner orbitals of heavy elements such as tungsten, are of sufficient energy to be used for diagnostic radiography. Characteristic x-ray energies are significant due to energy released when an inner-shell vacancy is filled, and these energies increase approximately as Z², with Z being the atomic number. To generate x-rays with adequate energy, significant accelerating voltage is needed to create inner-shell vacancies, and for an element like tungsten, which is commonly used in x-ray tubes, at least 72.5 kV is required.