Why are Farmer chambers usually calibrated with a build-up cap at 60Co energy?

Farmer chambers are typically calibrated with a build-up cap at cobalt-60 (60Co) energy to ensure that the dosimeter accurately measures the dose of radiation. The build-up cap is integral in establishing electronic equilibrium by providing a sufficient thickness of material over the chamber's sensitive volume.

At 60Co energy, which is a commonly used source of radiation in radiotherapy, the photon interactions are more pronounced with increased depth in tissue. The build-up cap facilitates the establishment of dose equilibrium in the air surrounding the chamber. Without this cap, there could be inaccuracies in the measurement due to insufficient interaction events (scattering and absorption) occurring within the sensitive volume of the chamber.

By using a build-up cap, the walls of the chamber are effectively "thickened," allowing photons to energy deposit within the chamber volume without being significantly influenced by surface effects. This setup helps provide a reliable calibration point for dosimetry purposes, ensuring that measurements correspond accurately to the radiation dose delivered in clinical settings.