Radiology Tech Physics Practice Test

Electrostatic shaping of the electron stream is the key idea. The molybdenum focusing cup around the cathode is held at a negative potential relative to the filament, creating an electrostatic field that repels electrons near the rim and redirects them toward the tube axis. This concentrates the emitted electrons into a narrow beam so they strike a small, well-defined focal spot on the anode, improving image sharpness and spatial resolution. Molybdenum is chosen for the cup because it withstands high temperatures, has a high melting point, and conducts heat effectively, helping manage the heat generated by thermionic emission. Its material properties also minimize unwanted interactions that could affect the beam. Shielding, cooling, and insulation are provided by other parts of the tube and housing, not by the focusing cup itself.