A student asks why raising kVp hardens the beam and creates a different spectrum than increasing mA during a morning tube-physics review with no patient present. Which target-interaction explanation should the instructor give to correctly contrast bremsstrahlung and characteristic radiation?

The correct explanation distinguishes braking near the nucleus producing continuous bremsstrahlung with variable energy loss from K-shell ejection and refill producing discrete characteristic lines, so raising kVp increases maximum and average photon energy, hardens the beam, and once incident energy exceeds the tungsten K-edge near 69.5 keV enables K-lines, while mA only changes electron number and photon quantity. The braking plus outer-shell refill controlled by mA claim fails by shell and parameter concept because characteristic photons require a K-shell vacancy created by threshold kVp, not outer-shell transitions driven by more current. The swapped-origin claim fails by mechanism concept because it assigns continuous nuclear braking to characteristic and discrete ejection-refill to bremsstrahlung, reversing their physics. The mA-hardens claim fails by exposure-parameter concept because higher mA scales photon number without changing energy distribution, whereas higher kVp shifts bremsstrahlung upward and adds peaks. Exam caveat: with no patient present, do not confuse target-level spectrum creation with downstream hardening by filtration or attenuation, which alters quality after production but does not create bremsstrahlung versus characteristic. Practical check: at constant mAs compare half-value layer and spectra for higher kVp versus higher mA at fixed kVp to verify changed penetration and K-peaks only with kVp.