Final answer:
Specialty X-ray tubes for mammography use molybdenum or rhodium targets due to their suitable atomic numbers and optimal K-characteristic x-ray energies for imaging soft tissues.
Step-by-step explanation:
Specialty x-ray tubes for mammography have molybdenum or rhodium targets because of their low to intermediate atomic number and optimal K-characteristic x-ray energy. The use of molybdenum and rhodium is due to their ability to produce X-rays with energies that are suitable for imaging soft tissues like breast tissue. The K-characteristic X-rays are a result of transitions from higher energy shells to the K-shell in the atom, which occurs when inner-shell electrons, tightly bound, fill a vacancy.
Characteristic X-ray energies increase approximately as the square of the atomic number (Z²) of the element, thus heavier elements like tungsten require higher voltages to create these vacancies. However, in mammography, the relatively lower atomic number elements, such as molybdenum and rhodium, offer the advantage of producing X-rays with lower energy which are more appropriate for the soft tissues and provide better contrast in the resulting images than heavier elements like tungsten could provide.