A Farmer type chamber used for measurements in a hospital must have what kind of wall?

The correct choice is an air equivalent wall for a Farmer type chamber used in clinical measurements. This is because the Farmer chamber is designed to measure the amount of ionizing radiation accurately. The wall of the chamber being air equivalent means that it has a similar atomic composition and mass density as air, which is crucial for maintaining the accuracy of the radiation dose measurements.

When the wall is air equivalent, it ensures that the response of the chamber mimics that of air, enabling it to accurately reflect the radiation being measured without introducing significant measurement errors that could arise from other materials. This property is essential when the chamber is used in different radiation fields, allowing for consistent and reliable results regardless of the type of radiation or energy levels involved.

Using materials like steel, aluminum, or lead would not provide the necessary equivalence to air and thus could distort the dose readings, as these materials have different radiation interactions compared to air. Therefore, the air equivalent wall is specifically chosen to align the chamber's response with the radiation field it is designed to measure, ensuring accurate dosimetry in a hospital setting.