Which instrument is used to measure collimator scatter for accelerator photon beams in air?

The correct choice for measuring collimator scatter from accelerator photon beams in air is a Farmer chamber with a cap of appropriate buildup thickness. This is due to the Farmer chamber's design, which allows for accurate measurements of dose in a clinical setting, particularly for high-energy photon beams produced by linear accelerators.

When measuring collimator scatter, it's crucial to ensure that the depth of measurement is representative of the scatter conditions experienced during actual treatment. The buildup cap provides the necessary material to ensure that electronic equilibrium conditions are met at the point of measurement. In high-energy photon beams, photons are highly penetrating and require the appropriate buildup material to produce accurate readings. Without this buildup, the readings might not reflect the true dose delivered by the beam and could be influenced by air gaps and other factors.

The ionization chamber without buildup might not provide accurate results under high-energy conditions, as it lacks the necessary material to achieve electronic equilibrium. Similarly, a Geiger-Muller counter is typically used for detecting ionizing radiation rather than providing detailed dose measurements. Thermoluminescent dosimeters, while useful for measuring doses, do not provide real-time feedback or adequate scattering measurement capabilities needed for collimator scatter assessments.