What device must be used for beam calibration at the hospital?

The correct answer is an ionization chamber because it is specifically designed to measure exposure to ionizing radiation, making it essential for beam calibration in clinical settings. These chambers function based on the principle that ionizing radiation creates ion pairs in the gas contained within the chamber. By measuring the electrical current produced by these ions, the ionization chamber can provide accurate dosimetric measurements.

Ionization chambers are commonly used due to their precision, stability, and the ability to be calibrated against known radiation standards. They are suitable for assessing both high and low energy beams, making them versatile for various types of calibration in radiation therapy. This makes them the standard choice for ensuring that the radiation output from treatment machines is consistent and accurate.

Other devices such as thermoluminescent dosimeters are better suited for personal dosimetry rather than beam calibration. Farmer type chambers, while also a type of ionization chamber, are a specific model primarily used in reference conditions rather than for routine calibration across a range of scenarios in a hospital. The Geiger-Muller counter is not suitable for beam calibration as it is more appropriate for detecting and measuring lower levels of radiation in environmental settings.