Which type of chamber is used for beam calibration in high energy photon beams exceeding 1 MV?

The Farmer type chamber is specifically designed for dosimetry in high energy photon beams, particularly those exceeding 1 MV. Its construction features a relatively large cavity and a well-defined geometry, which makes it suitable for accurate measurements in high-energy fields. This type of chamber is constructed with materials that ensure good energy response and stability, allowing for reliable calibration of high-energy beams.

When calibrating high-energy photon beams, factors such as energy dependence and the response of the chamber to various radiation types are critical. The Farmer chamber has a robust design that minimizes the effects of factors like recombination and enables effective collection of ionization produced in the chamber. This results in precise dosimetric measurements necessary for accurate patient treatment planning.

Other types of chambers, such as thimble type or spherical ionization chambers, may not offer the same level of accuracy or energy response needed for high-energy beams. They usually serve different applications or are better suited for lower energy measurements, which is why they are not the preferred choice for this specific purpose. The Farmer chamber's proven reliability and accuracy make it the industry standard for beam calibration in high-energy settings.