Which instrument is primarily used to measure exposure rate in air and is a gas-filled detector?

• A. Geiger-Müller counter.
• B. TLD.
• C. OSL dosimeter.
• D. Ionization chamber.

Answer: (D)

Measuring exposure rate in air relies on detecting the ionization produced by radiation in a known volume of air and converting that ionization into an electrical signal. An ionization chamber does exactly this: it is a gas-filled detector with an electric field. Radiation creates ion pairs in the air inside the chamber, and the applied field collects these charges, producing a current that is proportional to the rate of ionization, i.e., the exposure rate. This real-time, linear response over a wide range makes it the instrument of choice for air exposure-rate measurements and for calibrating other dosimeters.

Geiger-Müller counters are also gas-filled, but they are designed to register individual events and can suffer from dead time and energy dependence, which makes them less accurate for precise exposure-rate measurements over a broad range. TLDs and OSL dosimeters are passive, integrating dose over time and then read out later, so they do not provide real-time exposure-rate information.