The radiation-sensitive portion of an ionization chamber is composed of:

• A. Air
• B. Tungsten
• C. Semiconductor crystal
• D. Scintillation crystal

Answer: (A)

In an ionization chamber, the key idea is that the sensitive medium is a gas. When ionizing radiation passes through the chamber, it interacts with gas molecules and creates electron–ion pairs. An electric field across the chamber collects these charges, producing a current that is proportional to the amount of ionization, and thus to the radiation dose rate. Air is a common and convenient gas choice because it’s readily available and works well for producing a measurable current without complex requirements.

Tungsten isn’t used as the sensing medium in this type of detector; it’s a dense metal often used as a target or shielding material in other contexts, not for gas ionization. A semiconductor crystal would form a solid-state detector where radiation creates electron–hole pairs in a solid, with different behavior and readout. A scintillation crystal detects radiation by emitting light upon energy deposition and requires a light detector to convert that light into a signal, which is a different detection mechanism from ionization in gas.