Final answer:
Mammography x-ray tubes commonly use tungsten or molybdenum targets because of their high atomic numbers and ability to withstand high temperatures. These targets produce high K characteristic x-ray energies due to the filling of inner-shell vacancies, with energy levels increasing as Z². A minimum of 72.5 kV is required to create inner-shell vacancies in tungsten, necessary for generating these x-rays.
Step-by-step explanation:
Characteristic X-ray Energies and Targets in Mammography
Mammography x-ray tubes usually have tungsten or molybdenum targets, which are materials with a high atomic number (high Z) and consequently exhibit high K characteristic x-ray energies. These materials are chosen because they can withstand the significant temperatures resulting from the absorption of energy by the impinging electrons. The characteristic x-rays are produced when an inner-shell vacancy is filled, and since inner-shell electrons are tightly bound, especially in heavier elements, the emitted x-ray photons have large energies.
Characteristic x-ray energies increase approximatedly as Z2, meaning heavier elements produce higher energy x-rays. The acceleration voltage required to create inner-shell vacancies is at least 72.5 kV for tungsten, this is necessary to remove an electron completely from the tightly bound inner shell. Tungsten's high melting point makes it suitable for x-ray tube anodes, as it can endure the high temperatures without melting.
Additionally, the applications of x-rays, including medical imaging like mammography, rely on the penetrative ability of the photons. This ability is due to their high energies, which allow them to penetrate materials that are opaque to visible light, such as human tissue and bone, making them valuable for imaging in healthcare.