The FOCUSING CUP in the cathode assembly of a dental X-ray tube is made of which material?

Correct answer: C. Molybdenum

• A. TUNGSTEN is used for the FILAMENT itself (thermionic emission of electrons) and for the ANODE TARGET, where the electrons are stopped and X-rays are produced. Tungsten is chosen there for its very high atomic number and melting point.
• B. LEAD forms the protective HOUSING and collimator, absorbing stray radiation and shaping the useful beam.
• C. CORRECT. The FOCUSING CUP is a negatively charged, cup-shaped MOLYBDENUM holder surrounding the tungsten filament. Molybdenum is chosen because it has a high melting point yet does not readily emit electrons itself. Its negative charge electrostatically repels the electron cloud produced by the filament, focusing that diverging cloud into a narrow beam aimed at the focal spot on the anode.
• D. COPPER forms the ANODE STEM that conducts heat away from the tungsten target — a heat-dissipation role, not focusing.

Note: Trace the components in sequence: the tungsten FILAMENT boils off electrons (thermionic emission, controlled by mA); the MOLYBDENUM focusing cup narrows that cloud; the kVp accelerates the electrons across to the ANODE; and at the tungsten TARGET, about 99% of the energy becomes heat and only about 1% becomes X-rays.