Which factor determines the maximum energy of x-ray photons produced during an exposure?

• A. The energy of the incident electron
• B. The binding energy of the k-shell
• C. The tube current
• D. The duration of exposure

Answer: (A)

The upper limit on x-ray photon energy is set by the energy the incoming electrons have when they hit the target. In an x-ray tube, electrons are accelerated by the tube voltage, giving each electron kinetic energy roughly equal to e times the kilovoltage. When a high-energy electron slows down in the target, it can emit a photon with energy up to its entire kinetic energy (bremsstrahlung). So the maximum photon energy directly tracks the incident electron energy controlled by the tube voltage.

The binding energy of the k-shell matters for characteristic x-rays, which are discrete lines corresponding to specific atomic transitions and are typically lower than the incident electron energy. They don’t set the maximum energy of all photons. Tube current and exposure duration affect how many photons are produced (quantity), not how energetic the photons can be.