What is the radiation-sensitive portion of an ionization chamber primarily composed of?

The radiation-sensitive portion of an ionization chamber is primarily composed of air. In an ionization chamber, ionizing radiation interacts with the air molecules present inside the chamber, causing ionization—i.e., the formation of charged particles (ions). These ions can then be collected and measured, providing a way to detect and quantify the amount of radiation present.

The use of air as the sensitive medium is effective because it allows for the detection of a range of radiation types, including alpha, beta, and gamma radiation. Since air is also a gas at room temperature, it can effectively expand and contract, allowing for real-time measurements of ionization without the complications that might arise with liquids or solids as the detection medium.

Other materials listed in the choices do not serve as the primary component for the radiation-sensitive portion of an ionization chamber. For instance, while lead is often used for shielding against radiation, and plastics may be used in certain detectors, they do not play the role of the sensing medium in an ionization chamber. Instead, their functions are typically more about protection or structural integrity than direct measurement of radiation.