A clinic is replacing a Cobalt-60 teletherapy unit with a linear accelerator, and the physics team notes that one machine's output drifts down over time while the other holds steady unless the energy setting changes. Which explanation of how these two radiation sources produce and control their beams is correct?

A Cobalt-60 teletherapy unit produces photons through spontaneous decay in a sealed source. The source emits continuously, so its activity and dose rate decrease over time according to the isotope’s half-life, even while the unit is idle and without electrical input to create radiation. A linear accelerator instead uses high-speed electrons that strike a target to produce photons, or can deliver electrons directly; radiation is produced when the beam is on. At a given energy and dose-rate setting, its output is intended to remain consistent within applicable tolerances, rather than decrease through radioactive decay. Saying that Cobalt-60 makes photons by striking a target while the accelerator contains a decaying sealed source reverses the production mechanisms. Saying both machines aim electrons at a metal target also fails: Cobalt-60 photons come from radioactive decay, not an electron beam, and the Cobalt-60 unit is not a linac at lower electron energy. Saying the accelerator fades on a half-life schedule while the Cobalt-60 output stays steady until source exchange likewise reverses the physics; only the sealed radioactive source decays on its own. An exam caveat: the specific half-life and output figures are not the stable concept being tested. A practical check is to confirm the modality named in the treatment order before planning delivery.